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Abstract

The extraordinary optical properties of coupled plasmonic nanostructures make these materials potentially useful in many applications; thus, they have received enormous attention in basic and applied research. Coupled plasmon modes have been characterized predominantly using far-field spectroscopy. In near-field spectroscopy, the spectral response of local field enhancement in coupled plasmonic nanostructures remains largely unexplored, especially experimentally. Here, we investigate the coupled gold dolmen nanostructures in the near field using photoemission electron microscopy, with wavelength-tunable femtosecond laser pulses as an excitation source. The spatial evolution of near-field mapping of an individual dolmen structure with the excitation wavelength was successfully obtained. In the near field, we spatially resolved an anti-bonding mode and a bonding mode as the result of plasmon hybridization. Additionally, the quadrupole plasmon mode that could be involved in the formation of a Fano resonance was also revealed by spatially resolved near-field spectra, but it only contributed little to the total near-field enhancement. On the basis of these findings, we obtained a better understanding of the near-field properties of coupled plasmonic nanostructures, where the plasmon hybridization and the plasmonic Fano resonance were mixed.

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The optical properties of localized surface plasmon resonances (LSPRs) that occur on metallic nanoparticles (NPs) have been the subject of intense study for the past few decades. Under resonant light excitation, metallic NPs exhibit intense light absorption and scattering, and enhance local electromagnetic fields. As a result of these properties, metallic NPs that exhibit LSPRs have potential applications in many fields, including surface-enhanced Raman scattering,(1, 2) sensing,(3-5) imaging,(3, 6) plasmon-assisted photocurrent generation,(7, 8) photochemical reactions,(9-11) and artificial photosynthesis.(12-14) Recent advances in synthesis and nanofabrication allow for the preparation of metallic NPs with nanometer accuracy and complex shapes. In addition, complex plasmonic NPs that resemble NPs with small gap distance can be readily fabricated. Complex plasmonic NPs can induce coupling that may result in greater field enhancement; they also exhibit some striking properties, such as plasmon hybridization,(15, 16) Fano resonance,(17-27) electromagnetically induced transparency (EIT),(28) and plasmonic waveguiding.(29, 30)

Many of the proposed optical applications of plasmonic nanostructures rely on the near-field properties of nanostructures. Specifically, plasmonic modes with a large field enhancement are particularly beneficial for plasmon-enhanced nonlinear optical effects(1, 2, 31-34) and plasmon-enhanced photochemical reactions.(9-11) Compared to a simple individual plasmonic nanoparticle, complex coupled nanostructures have more complicated resonance line shapes in the far field (e.g., Fano resonance) and much richer near-field properties. In particular, the spatially resolved spectral response of coupled plasmonic nanostructures is in high demand. However, such experiments remain challenging because accessing the near-field spectral response in coupled plasmonic systems requires wavelength-tunable excitation and near-field imaging techniques with a high spatial resolution. To date, only a few attempts using scanning near-field optical microscopy,(21) electron energy loss spectroscopy, and cathodoluminescence (CL)(27, 35, 36) have been reported to detect the near field of complex plasmonic systems.

Recently, nonlinear photoemission electron microscopy (PEEM), using femtosecond laser pulses as the excitation light source, has been found to be promising for pinpointing the near-field properties of plasmonic modes.(37-40) PEEM records the electrons emitted from the surface of a sample in response to the absorption of incident photons. When the photon energy is lower than the work function of the material, nonlinear photoemission (PE), especially multiphoton PE, becomes possible when ultrafast laser pulses (typically femtosecond lasers) are used as excitation sources. In general, the probability of multiphoton excitation is low. However, when plasmonic nanostructures are irradiated by ultrafast laser pulses at resonance wavelengths of LSPRs, the near-field enhancement effect can significantly promote the multiphoton PE process. Because the PE intensity is correlated with the near-field electric field intensity on the sample surface in a nonlinear manner, the PEEM images of plasmonic nanostructures upon resonant excitation can be regarded as nonlinear maps of the near-field distribution. Similarly, the wavelength-dependent PE curves obtained by integrating the PE signal against the incidence wavelength can be treated as the nonlinear near-field spectra of the plasmonic nanostructures. Previously, we obtained the near-field mapping and near-field intensity enhancement spectra of simple aluminum (Al) nanorods and gold (Au) nanoblocks using this PEEM technique.(40-42)

In this study, we employ PEEM to experimentally investigate the near-field spectral response and spatial evolution of the near-field intensity distribution on more complex Au dolmen structures that consist of a planar nanorod monomer and two parallel planar nanorod dimers. The spectral properties of dolmen structures have been intensively studied, and on the basis of far-field spectroscopic measurements, the spectral properties of dolmen structures have been primarily explained by the Fano resonance as the result of interference between a spectral wide bright dipole plasmon mode and a narrow dark quadrupole mode.(17, 22, 27) However, in this study, we clarify that the spectral response of dolmen structures in both the far field and near field is primarily attributable to the bonding and anti-bonding plasmon modes that are thought to result from plasmon hybridization.(15) This attribution is supported by near-field spectra measured by PEEM. In particular, we obtained the spatially resolved near-field spectral response of dolmen structures and observed that the maximum near-field enhancement is dominated by bonding and anti-bonding modes. The high spatial resolution of PEEM allowed us to identify a quadrupole mode in the dimer part of the dolmen structure. This quadrupole mode was previously thought to account for the Fano resonance via the interference with the dipole mode and dominates the near-field enhancement; in our experimental work, it contributes only little to the near-field enhancement. Our near-field spectroscopic and mapping results can be reproduced very well using the finite-difference time-domain (FDTD) method.

Results and Discussion

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Characterization of Topography and Far-Field Spectra

The metallic nanoparticles investigated in this study were Au dolmen structures consisting of a planar nanorod monomer and a planar nanorod dimer for each dolmen, as schematically shown in Figure 1a. The dolmen structures were fabricated using standard electron-beam lithography followed by metal sputtering. The planar nanorod monomer and each nanorod in the planar nanorod dimer had dimensions of 100 × 150 and 80 × 140 nm², respectively. The edge-to-edge distance between two nanorods in the dimer part was 70 nm, and the gap size between the monomer and dimer was chosen to be 25 nm, which guaranteed a strong interaction between the monomer and dimer. The thickness of Au dolmen structures was 30 nm, with an additional 2 nm thick titanium layer as the adhesion layer. The structures were arranged in a two-dimensional square array in a 100 × 100 μm² area, with a pitch size of 1 μm; this pitch size was thought to be sufficiently large to avoid near-field interaction between the adjacent unit structures.(22)Figure 1b shows a scanning electron microscopy (SEM) image of Au dolmen structures. As evident in this figure, the Au dolmen structures are uniform, with good quality. Figure 1c shows the far-field extinction spectra collected using a Fourier transform infrared spectrometer (FTIR) equipped with an infrared microscope. In the case of vertical polarization (black curve) (i.e., where the electric field vector is parallel to the symmetry axis of the dolmen structures), a broad extinction band centered at approximately 800 nm is observed. However, in the case of horizontal polarization (red curve) (i.e., where the electric field vector is perpendicular to the symmetry axis), two extinction peaks, centered at approximately 770 and 860 nm, are observed. This observation is consistent with previous reports on the far-field optical properties of Au dolmen structures. When polarization is along the long axis of dimer nanorods, a broad dipole LSPR mode of the entire structure can be excited. For horizontal polarization, a dipole LSPR in the monomer nanorod can be excited. Due to the near-field interaction with the nearby monomer nanorod, two dipole LSPRs can also be excited, with π out of phase, in the two constituent nanorods of the dimer, forming a quadrupole-like mode. In the frame of plasmonic Fano resonance theory, the dipole mode in the monomer and the quadrupole mode in the dimer can interfere with each other, leading to the plasmonic Fano dip in the extinction spectrum.(17, 22, 27) Here, a dip at 810 nm was clearly observed. The quadrupole LSPR mode is usually thought to be located close to this dip position, as demonstrated through calculations of the charge distribution.(20) The radiant loss of the quadrupole mode is smaller; thus, it may exhibit a longer dephasing time and a stronger local field enhancement in the near field. Next, using wavelength-dependent PEEM measurements, we will determine whether the maximum near-field enhancement occurs at the so-called Fano resonance wavelength.

Near-Field Spectra

The near-field properties of Au dolmen structures were investigated using PEEM, with femtosecond laser pulses as the excitation source. Assisted by LSPRs, femtosecond laser pulses provide substantial photoemission via a nonlinear photoemission process mainly including multiphoton PE, especially when the laser wavelength was at or approached the LSPR wavelengths. As discussed in the introduction, the PEEM images of plasmonic nanostructures upon femtosecond laser irradiation at resonance condition can be treated as the nonlinear near-field intensity mapping of the structures, and a near-field PE intensity spectrum can be regarded as the nonlinear near-field LSPR response spectrum of plasmonic nanostructures. In our experiments, we employed a wavelength-tunable (720–920 nm) femtosecond laser with a pulse duration of ∼100 fs and a repetition rate of 77 MHz as the excitation source for the PEEM measurements. A schematic of the PEEM setup is shown in Figure 2a. The laser beam was focused onto the sample under normal incidence using a lens with a focal length of 600 mm. The focal spot on the sample surface was estimated to have a diameter of 120 μm. We primarily used a nominal field of view (FOV) of 1.25 μm for the PEEM measurements to ensure that only a single dolmen structure was imaged, considering that the pitch size of the nanostructure array was 1 μm. To obtain the near-field spectra, we imaged and integrated the near-field photoemission intensity over the entire FOV for the illumination wavelength range of 720 to 920 nm in 10 nm increments.

Figure 2b shows the near-field spectra of a Au dolmen structure under horizontal (red curve) and vertical (black curve) polarization, as shown in the inset sketch map. The near-field spectrum under the horizontally polarized excitation exhibits two peaks, located at approximately 760 and 850 nm. Additionally, the two peak wavelengths are approximately identical to those observed in the far-field extinction spectrum, as evident in Figure 2c. Unlike the results in a previous report on Fano resonant plasmonic heptamer structures,(20) the maximum near-field intensity is not observed around the dip position. For the vertically polarized excitation, the near-field spectrum also gives one broad band centered at approximately 820 nm, which is remarkably similar to the far-field extinction spectrum in Figure 2d. However, our study focuses only on the horizontally polarized excitation case.

Near-Field Mapping

The far-field and near-field optical properties were investigated, and we found that the near-field and far-field spectra of Au dolmen structures under horizontal polarization excitation have approximately the same shape. The near-field intensity distributions at various wavelengths are a matter of particular interest. As previously mentioned, the wavelength-dependent PEEM measurements can give the spatially resolved near-field intensity distribution directly; however, the photoemission intensity is correlated with the local electric field intensity in a nonlinear manner. We created an animation that represents the spatial evolution of near-field patterns on the Au dolmen structure under horizontal polarization excitation at various laser wavelengths ranging from 720 to 920 nm. The animation is provided in the supplementary animation. Additionally, most of the frames are displayed in Figure S1 in the Supporting Information. We observed that hot spot distributions (i.e., the near-field intensity distribution) evolve dramatically, especially when the wavelength changes within two peak wavelengths. To present the evolution of the near-field intensity distribution more clearly, in Figure 3, we show typical PEEM images for four characterized wavelengths, as marked in panel a. An additional UV light source (i.e., a mercury lamp) was used for the PEEM measurements. Within the FOV of the PEEM images, the UV light is uniform and the UV-PEEM images can provide details of the morphology of the Au nanostructures, being an important reference for determining the locations of plasmonic hot spots irradiated by femtosecond laser pulses.(40) The morphology of the Au nanostructure can be observed because of the work function contrast between Au and the substrate. Panel b shows the PEEM image when the sample was simultaneously irradiated by the femtosecond laser pulses and UV light. This simultaneous irradiation helps to outline Au dolmen structures, which are shown as red dashed lines. Note that the size of the nanostructure shown in the UV-PEEM image (panel b) seems relatively smaller than that shown in the SEM image (inset in panel a), and it mostly results from the topographical contrast rendering the slightly low collection efficiency of the photoemission from edges. This approach allowed us to investigate different near-field intensity distributions at four excitation wavelengths: 760, 780, 800, and 850 nm. First, at the two peak wavelengths (760 and 850 nm), the photoemission (near-field enhancement) from the top monomer nanorod dominates the total near-field enhancement. At the shorter-wavelength peak of 760 nm, the two upper corners are stronger. However, at the longer-wavelength peak of 850 nm, two lower corners are stronger. This interesting observation will be discussed later. Second, at the dip wavelength of 800 nm, four hot spots from the dimer nanorods (two nearest lower corners and two outside upper corners) become clear. However, the near-field enhancement at the dip wavelength is much weaker. The hot spots from the dimer are found to be strongest at the wavelength of 780 nm, which is not located at but close to the dip position.

Numerical Simulations and Discussion

To further explain our experimental observations, we performed numerical calculations of far-field extinction spectra, the near-field intensity enhancement spectra, the near-field intensity distributions, and charge distributions using the FDTD simulation method. The FDTD simulation results for both the far-field extinction spectrum and the near-field intensity spectrum upon horizontal polarization excitation are shown in Figure 4a. In this figure, the normalized integrated value (I/I₀)⁴ over an area of 320 nm × 320 nm at the interface between the dolmen structure and the substrate is shown for better comparison with the PEEM measurement results; we assumed an average four-photon photoemission process. Here, I and I₀ represent the local electromagnetic field intensity on the plane and the incident electromagnetic field intensity, respectively. We observed that, in the near field, the maximum intensity was not induced at the extinction dip wavelength from the near-field PE intensity spectrum (red curve). Instead, the peak wavelengths almost coincided with the far-field extinction spectrum (black curve), which is in good agreement with the experimental results. Figure 4b shows the calculated charge distribution at the two peak wavelengths, indicating the formation of an anti-bonding mode and a bonding mode that resulted from plasmon hybridization, which will be discussed later. Figure 4c presents the near-field electric field intensity distribution under the corresponding four characteristic wavelengths. The simulated near-field intensity distribution reproduces the experimental PEEM measurements qualitatively. In particular, the redistribution of the near-field enhancement between the upper and lower corners of the monomer rod is clearly observed when the incident wavelength is switched between the two peak wavelengths. At wavelength positions of 2 and 3, the near-field intensity distribution on the dimer rods is also reproduced well.

Figure 4. Spectra and mapping calculated by FDTD simulations. (a) Far-field extinction spectrum (black) and near-field PE intensity spectrum (red), plotting the integral of the (I/I₀)⁴ for a 320 nm × 320 nm area on the interface between the dolmen structure and the substrate. (b) FDTD calculated charge distribution at two characterized peak wavelengths, indicating two hybridized modes, namely, anti-bonding and bonding modes. (c) Calculated near-field intensity distribution under four characterized wavelengths: the shorter-peak wavelength of 759 nm (1), 788 nm (2) that gives the maximum enhancement for the dimer part, the dip wavelength of 802 nm (3), and the longer-peak wavelength of 853 nm (4), which are also marked as 1–4 in (a).

The wavelength-dependent near-field patterns observed in both the experiments and simulations are worthy of discussion. We find that it is reasonable to interpret the results in the frame of the plasmon hybridization model, according to which the strong near-field coupling between the dipole mode in the monomer and the quadrupole mode in the dimer results in two new hybridized states.(15, 22) The shorter-wavelength (higher-energy) peak is assumed to result from the anti-bonding state, whereas the longer-wavelength (lower-energy) peak results from the bonding state. This assumption is confirmed by the calculated surface charge distribution in Figure 4b. The charge distributions at the shorter-wavelength peak and longer-wavelength peak match very well with the proposed anti-bonding and bonding coupled modes, respectively. In the case of the anti-bonding state, the charge distribution in the monomer and dimer part leads to a repulsive Coulomb force between the monomer and dimer. Thus, the charges in the monomer concentrate more at the two upper corners and exhibit stronger near-field enhancement there. By contrast, the bonding state leads to an attractive Coulomb force between the monomer and dimer. Thus, under the longer-peak wavelength irradiation, the near-field enhancement is stronger at the two lower corners of the monomer. Our results provide the near-field experimental observation of the bonding and anti-bonding modes in coupled plasmonic nanostructures. Additionally, the phase in the dimer part undergoes a phase jump of π around the extinction dip wavelength, as can be found from the Figure S2 in the Supporting Information, where the calculated charge distribution at more wavelengths are presented. It is also worth noting that the plasmon hybridization between the nanorod monomer and the nanorod dimer part is very sensitive to the gap distances between the two parts, as shown in Figure S3 (experiments) and Figure S4 (simulations) in the Supporting Information, where the energy splitting between the two hybridized modes becomes smaller as the gap distances increase.

In most coupled plasmonic systems, plasmon hybridization and Fano resonance are mixed; furthermore, they are hard to be distinguished from the far-field extinction or scattering spectra. This problem is similar to the crossover between the EIT phenomena and Rabi splitting in a plasmon molecular-coupled system; a recent theoretical effort was used to distinguish EIT and Rabi splitting via excitation spectra that reflect molecular absorption spectra.(43) On the basis of the aforementioned results, we propose that near-field spectral properties can be applied to distinguish the difference of the contribution to the near-field enhancement between a Fano resonance and plasmon hybridization. Halas and co-workers investigated the near-field properties of individual Fano resonant plasmonic heptamer structures using surface-enhanced Raman scattering (SERS) and numerical calculations.(20) They observed that SERS signals are most enhanced when the wavelength of the pump laser and the Raman Stokes mode of interest overlap the Fano dip wavelength. Additionally, their calculations revealed that the wavelength of the Fano dip in the far-field scattering spectrum is very close to the wavelength of the maximum in the near-field enhancement spectrum. In a plasmonic heptamer, plasmon hybridization and Fano resonance coexist. The dipole LSPR of the central NP and the LSPRs of outer NPs are hybridized to a bonding bright (super-radiant) mode and an anti-bonding dark (subradiant) mode. Then, the interference between the bonding mode and the anti-bonding mode induces the Fano resonance.(44, 45) In such plasmonic Fano nanostructures, the spectrally narrow plasmon mode around the extinction/scattering dip position should dominate the near-field enhancement. This finding was also reported by Frimmer et al., using the CL technique.(35) However, a recent theoretical work reported that, for the plasmonic dolmen structure, the Fano resonance wavelength does not necessarily correspond to a specific point of the reflectance spectrum (for instance, a local minimum or maximum), but the maximum near-field intensity enhancement was reported at the Fano resonance position.(46) Nevertheless, experimentally, we did not observe the strongest near-field intensity enhancement at either the extinction/scattering dip position or the Fano resonance position in the case of the dolmen structures investigated here. Instead, the anti-bonding and bonding modes dominated the near-field enhancement. In the plasmonic dolmen structure, the Fano resonance was previously thought to result from the interference between the super-radiant dipole mode in the monomer and the subradiant quadrupole mode in the dimer part. In the near field, we did not find a significant contribution from Fano resonance; thus, it is reasonable to state that plasmon hybridization is dominant in the dolmen structure.

Furthermore, the high spatial resolution of PEEM measurements allows us to plot the near-field PE intensity spectra from different regions, as shown in Figure 5. In Figure 5a, the total PE intensity curve can be decomposed into three bands that dominate the PE signal in three different regions. Areas 1 and 2 contribute to the shorter-wavelength and longer-wavelength peaks in the total spectrum, respectively. However, for area 3, a small peak is found at 780 nm, corresponding to the wavelength 2 in Figure 3 and Figure 4. The simulation results agree well with their experimental counterparts. This indicates that three different modes can be identified and spatially separated. In particular, the quadrupole mode in the dimer rods is revealed. This quadrupole mode may contribute to the formation of the Fano resonance via the interference with the dipole mode in the monomer rod. However, the near-field enhancement at the Fano resonance wavelength or the quadrupole mode is lower than the two hybridized modes. From both our experimental and simulation results, it is reasonable to conclude that in the plasmonic dolmen structures, the contribution to near-field enhancement is dominated by plasmon hybridization rather than Fano resonance.

Figure 5. Spatially resolved PE intensity spectra. (a) Spatially resolved PE spectrum of the dolmen, obtained for four different regions by integrating the PE signal from selected areas. (b) Contour part of the FDTD simulation results. The corresponding selected areas are indicated by the dashed lines on the right side. From these analyses, different modes, including the quadrupole mode on the dimer part, can be identified.

It is also important to remark about the link between plasmon hybridization and Fano resonance from two aspects. On the one hand, both phenomena can be explained in the frame of the interaction or coupling between different (typically two) plasmon modes with different interaction/coupling strength. In the strong plasmon interaction/coupling region, plasmon hybridization is strong such that the hybridized modes dominate the near-field enhancement. In the weak interaction/coupling region, Fano resonance becomes strong and thus induces the largest near-field enhancement at the Fano resonance position due to its subradiant nature. On the other hand, when the widths of two plasmon resonance modes are significantly different (one is narrow and the other is broad), the plasmon interaction/coupling can be described by Fano models. When the widths of the two resonances are comparable, the plasmon interaction/coupling can be described by plasmon hybridization theory.(47) As seen from the near-field spectra shown in Figure 5a, the quadrupole mode in the dolmen structure becomes broad due to the strong near-field interaction between the monomer rod and the dimer rods. We argue that in the dolmen structure, the broadening of the quadrupole mode leads to the attenuation and even disabling of the Fano resonance, thus strengthening the plasmon hybridization, resulting in the dominant near-field enhancement in the two hybridized modes, which can be regarded as two new plasmon eigenmodes of the whole dolmen nanostructure.(48)

Conclusions

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In summary, we experimentally investigated the near-field properties of a coupled Au dolmen structure using PEEM. The near-field plasmon coupling was revealed by the spatial evolution of the near-field patterns. In particular, the evolution of near-field mapping at high spatial resolution with the excitation wavelength revealed that the coupled bonding and anti-bonding plasmon modes were hybridized from a dipole mode in the monomer part and the quadrupole mode in the dimer part. We observed that the hybridized anti-bonding and bonding modes dominated the near-field enhancement, although the quadrupole mode in the dimer part can be identified via near-field mapping and spatially resolved near-field spectra. Furthermore, the measurement of near-field spectral properties allows for distinguishing the difference of their contribution to the near-field enhancement between plasmon hybridization and Fano resonances in plasmonic dolmen structures. More generally, this methodology can be applied in evaluating other coupled plasmonic nanostructures. The results deepen our understanding of plasmon hybridization and plasmonic Fano resonances and are expected to facilitate further development of potential applications.

Methods

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Sample Fabrication and Characterization

Planar Au dolmen structures were fabricated on indium tin oxide (ITO)-coated glass substrates with an approximately 150 nm thick ITO layer using electron-beam lithography (EBL) and metal evaporation techniques. The substrate was then sequentially cleaned with acetone, methanol, and ultrapure water in an ultrasonic bath (each step was 5 min). A conventional copolymer resist (ZEP520, Zeon Chemicals) diluted with a ZEP thinner (1:1) was then spin-coated onto the substrate at 1000 rpm for 10 s and at 4000 rpm for 90 s. The substrate was subsequently prebaked on a hot plate for 2 min at 150 °C. In this study, a high-resolution EBL system (ELS-F130HM, Elionix) operated at 130 kV was used for sample fabrication. The EBL was conducted at a current of 50 pA. After the development, a 2 nm thick titanium layer was first deposited via sputtering (MPS-4000, ULVAC) as the adhesive layer, followed by deposition of a 30 nm thick Au film. Lift-off was performed by successively immersing the sample in anisole, acetone, methanol, and ultrapure water in an ultrasonic bath. Morphologies were analyzed using field-emission scanning electron microscopy (JSM-6700FT, JEOL). Far-field spectral properties were analyzed using FTIR spectroscopy (FT/IR-6000TM-M, JASCO).

PEEM Measurements

The photoemission electron microscope used in this study was a PEEM-III equipped with an energy analyzer (Elmitec GmbH). We used two light beams as excitation sources for PEEM. One beam was UV light from a mercury lamp (unpolarized continuous wave light with a cutoff energy of 4.9 eV), which was used to characterize the morphologies of metallic nanoparticles. The other one was a Ti:sapphire femtosecond laser that delivers approximately 100 fs laser pulses with a tunable central wavelength (720–920 nm) at a repetition rate of 77 MHz. The laser beam was focused onto the sample from the Au nanostructure side under normal incidence using a lens with a focal length of 600 mm. We used this laser for wavelength-dependent PEEM measurements to obtain the near-field PE intensity spectra, as well as for near-field mapping at various wavelengths. By changing the irradiation wavelengths from 720 to 920 nm step-by-step in increments of 10 nm, a series of PEEM images can be obtained. The PE signal from the entire field of view or the selected area can be integrated and plotted against the incidence wavelength; thus, the so-called near-field PE spectra can be obtained. Thus, the near-field properties, such as mapping and spectra, can be investigated via this method. PEEM images in Figure 3 and Figure S1 were taken under the field of view of 1.25 μm, with an exposure time of 2 s. The laser power was kept as 120 mW before the PEEM window and the focal lens for the wavelength-dependent measurements. The laser intensity on the sample was estimated to be ∼120 MW/cm², and considering a near-field intensity enhancement factor of 10²–10³, the maximum local electric intensity could reach 12–120 GW/cm², which is sufficient to generate multiphoton photoemission.

Numerical Simulations

Numerical simulation of the near-field properties of Au dolmen structures was performed using the FDTD Solutions software package (Lumerical, Inc.). The ITO-covered glass substrate was assumed to behave as a dielectric material with an average refractive index n = 1.55. The optical properties of Au were obtained using the data from Johnson and Christy.(49) The FDTD simulations were performed on a discrete, uniformly spaced mesh with a mesh size of 3 nm. The plane wave light source was injected onto Au dolmen structures from the structure side. In the light propagation direction, the perfectly matched layer boundary conditions were imposed, and in the plane perpendicular to the light propagation direction, the periodic boundary conditions were applied on each boundary. The simulation region in this plane is 1000 nm × 1000 nm, corresponding to one unit of the dolmen structure in the array. The extinction spectra were obtained by a transmission power monitor located at 230 nm from below the ITO surface. A power and profile monitor located at the interface of the Au nanostructure and ITO was used to calculate near-field spectra and near-field intensity distribution.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06206.

Caption for the supplementary animation; PEEM images of the single Au dolmen excited by femtosecond laser pulses with different central wavelengths, charge distribution calculated by FDTD simulations, experimental results of far-field and near-field spectra of Au dolmen structures with different gap distances, and FDTD simulation results of far-field and near-field spectra of Au dolmen structures with different gap distances (PDF)
Supplementary animation of the spatial evolution of the near-field patterns on the Au dolmen structure (AVI)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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Corresponding Author
- Hiroaki Misawa - Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0021, Japan; Department of Applied Chemistry & Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan; Email: [email protected]
Authors
- Han Yu - Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0021, Japan
- Quan Sun - Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0021, Japan
- Kosei Ueno - Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0021, Japan
- Tomoya Oshikiri - Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0021, Japan
- Atsushi Kubo - Institute of Physics, University of Tsukuba, Tsukuba 305-8571, Japan
- Yasutaka Matsuo - Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0021, Japan
Notes
The authors declare no competing financial interest.

Acknowledgment

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This study was supported by KAKENHI Grant Nos. JP23225006, JP26870014, JP15H00856, JP15H01073, JP15K04589, and JP15K17438, the Nanotechnology Platform (Hokkaido University), and Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials (Five-Star Alliance) of MEXT.

References

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This article references 49 other publications.

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Nanoshell arrays have recently been found to possess ideal properties as a substrate for combining surface enhanced Raman scattering (SERS) and surface enhanced IR absorption (SEIRA) spectroscopies, with large field enhancements at the same spatial locations on the structure. For small interparticle distances, the multipolar plasmon resonances of individual nanoshells hybridize and form red shifted bands, a relatively narrow band in the near-IR (NIR) originating from quadrupolar nanoshell resonances enhancing SERS, and a very broadband in the mid-IR (MIR) arising from dipolar resonances enhancing SEIRA. The large field enhancements in the MIR and at longer wavelengths are due to the lightning-rod effect and are well described with an electrostatic model.
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Jain, P. K.; Huang, X. H.; El-Sayed, I. H.; El-Sayed, M. A. Noble Metals on the Nanoscale: Optical and Photothermal Properties and Some Applications in Imaging, Sensing, Biology, and Medicine Acc. Chem. Res. 2008, 41, 1578– 1586 DOI: 10.1021/ar7002804
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Noble Metals on the Nanoscale: Optical and Photothermal Properties and Some Applications in Imaging, Sensing, Biology, and Medicine
Jain, Prashant K.; Huang, Xiaohua; El-Sayed, Ivan H.; El-Sayed, Mostafa A.
Accounts of Chemical Research (2008), 41 (12), 1578-1586CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
A review. Noble metal nanostructures attract much interest because of their unique properties, including large optical field enhancements resulting in the strong scattering and absorption of light. The enhancement in the optical and photothermal properties of noble metal nanoparticles arises from resonant oscillation of their free electrons in the presence of light, also known as localized surface plasmon resonance (LSPR). The plasmon resonance can either radiate light (Mie scattering), a process that finds great utility in optical and imaging fields, or be rapidly converted to heat (absorption); the latter mechanism of dissipation has opened up applications in several new areas. The ability to integrate metal nanoparticles into biol. systems has had greatest impact in biol. and biomedicine. In this Account, the authors discuss the plasmonic properties of gold and silver nanostructures and present examples of how they are being utilized for biodiagnostics, biophys. studies, and medical therapy. For instance, taking advantage of the strong LSPR scattering of gold nanoparticles conjugated with specific targeting mols. allows the mol.-specific imaging and diagnosis of diseases such as cancer. The authors emphasize in particular how the unique tunability of the plasmon resonance properties of metal nanoparticles through variation of their size, shape, compn., and medium allows chemists to design nanostructures geared for specific bio-applications. The authors discuss some interesting nanostructure geometries, including nanorods, nanoshells, and nanoparticle pairs, that exhibit dramatically enhanced and tunable plasmon resonances, making them highly suitable for bio-applications. Tuning the nanostructure shape (e.g., nanoprisms, nanorods, or nanoshells) is another means of enhancing the sensitivity of the LSPR to the nanoparticle environment and, thereby, designing effective biosensing agents. Metal nanoparticle pairs or assemblies display distance-dependent plasmon resonances as a result of field coupling. A universal scaling model, relating the plasmon resonance frequency to the interparticle distance in terms of the particle size, becomes potentially useful for measuring nanoscale distances (and their changes) in biol. systems. The strong plasmon absorption and photothermal conversion of gold nanoparticles has been exploited in cancer therapy through the selective localized photothermal heating of cancer cells. For nanorods or nanoshells, the LSPR can be tuned to the near-IR region, making it possible to perform in vivo imaging and therapy. The examples of the applications of noble metal nanostructures provided herein can be readily generalized to other areas of biol. and medicine because plasmonic nanomaterials exhibit great range, versatility, and systematic tunability of their optical attributes.
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Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Biosensing with Plasmonic Nanosensors Nat. Mater. 2008, 7, 442– 453 DOI: 10.1038/nmat2162
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4
Biosensing with plasmonic nanosensors
Anker, Jeffrey N.; Hall, W. Paige; Lyandres, Olga; Shah, Nilam C.; Zhao, Jing; Van Duyne, Richard P.
Nature Materials (2008), 7 (6), 442-453CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
A review. Light incident on metallic nanoparticles can induce a collective motion of electrons that can lead to a strong amplification of the local electromagnetic field. As reviewed here, these plasmonic resonances have important applications in biosensing where they push resoln. and sensitivity towards the single-mol. detection limit. Recent developments have greatly improved the sensitivity of optical sensors based on metal nanoparticle arrays and single nanoparticles. The authors introduce the localized surface plasmon resonance (LSPR) sensor and describe how its exquisite sensitivity to size, shape and environment can be harnessed to detect mol. binding events and changes in mol. conformation. The authors then describe recent progress in three areas representing the most significant challenges: pushing sensitivity towards the single-mol. detection limit, combining LSPR with complementary mol. identification techniques such as surface-enhanced Raman spectroscopy, and practical development of sensors and instrumentation for routine use and high-throughput detection. This review highlights several exceptionally promising research directions and discusses how diverse applications of plasmonic nanoparticles can be integrated in the near future.
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Lal, S.; Link, S.; Halas, N. J. Nano-Optics from Sensing to Waveguiding Nat. Photonics 2007, 1, 641– 648 DOI: 10.1038/nphoton.2007.223
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5
Nano-optics from sensing to waveguiding
Lal, Surbhi; Link, Stephan; Halas, Naomi J.
Nature Photonics (2007), 1 (11), 641-648CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
A review. The design and realization of metallic nanostructures with tunable plasmon resonances was greatly advanced by combining a wealth of nanofabrication techniques with advances in computational electromagnetic design. Plasmonics - a rapidly emerging subdiscipline of nanophotonics- is aimed at exploiting both localized and propagating surface plasmons for technol. important applications, specifically in sensing and waveguiding.
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Kawata, S.; Inouye, Y.; Verma, P. Plasmonics for Near-Field Nano-Imaging and Superlensing Nat. Photonics 2009, 3, 388– 394 DOI: 10.1038/nphoton.2009.111
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Plasmonics for near-field nano-imaging and superlensing
Kawata, Satoshi; Inouye, Yasushi; Verma, Prabhat
Nature Photonics (2009), 3 (7), 388-394CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
A review. Diffraction of light prevents optical microscopes from having spatial resoln. beyond a value comparable to the wavelength of the probing light. This essentially means that visible light cannot image nanomaterials. Here the authors review the mechanism for going beyond this diffraction limit and discuss how manipulation of light by surface plasmons propagating along the metal surface can help to achieve this. The interesting behavior of light under the influence of plasmons not only allows superlensing, in which perfect imaging is possible through a flat thin metal film, but can also provide nano-imaging of practical samples by using a localized surface plasmon mode at the tip of a metallic nanoprobe. The authors also discuss the current research status and some intriguing future possibilities.
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Ueno, K.; Misawa, H. Plasmon-Enhanced Photocurrent Generation and Water Oxidation from Visible to Near-Infrared Wavelengths NPG Asia Mater. 2013, 5, e61 DOI: 10.1038/am.2013.42
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Plasmon-enhanced photocurrent generation and water oxidation from visible to near-infrared wavelengths
Ueno, Kosei; Misawa, Hiroaki
NPG Asia Materials (2013), 5 (Sept.), e61CODEN: NAMPCE; ISSN:1884-4057. (Nature Publishing Group)
A review. This paper presents recent studies of plasmon-enhanced photoelec. conversion and H2O oxidn. by visible and near-IR light irradn. Since the discovery of the Honda-Fujishima effect in 1972, significant efforts were devoted to lengthening the light-energy conversion wavelength. In this context, plasmonic photoelec. conversion was recently demonstrated at visible-to-near-IR wavelengths without deteriorating photoelec. conversion by employing TiO2 (TiO2) single-crystal photoelectrodes, in which Au nanorods are elaborately arrayed on the surface. A KClO4 aq. soln. was employed as an electrolyte soln. without addnl. electron donors; thus, H2O mols. provided the electrons. The stoichiometric evolution of O and H2O2 as a result of the 4- or 2-electron oxidn. of H2O mols., resp., was accomplished with near-IR light irradn. using the plasmonic optical antenna effect. As there is very little overpotential for H2O oxidn., these results constitute a significant advancement in this field. This photoelec. conversion system could potentially be employed in artificial photosynthesis systems that exceed the photosynthetic capabilities of plants by allowing for photoconversion over a wide range of wavelengths.
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Shi, X.; Ueno, K.; Takabayashi, N.; Misawa, H. Plasmon-Enhanced Photocurrent Generation and Water Oxidation with a Gold Nanoisland-Loaded Titanium Dioxide Photoelectrode J. Phys. Chem. C 2013, 117, 2494– 2499 DOI: 10.1021/jp3064036
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Plasmon-Enhanced Photocurrent Generation and Water Oxidation with a Gold Nanoisland-Loaded Titanium Dioxide Photoelectrode
Shi, Xu; Ueno, Kosei; Takabayashi, Naoki; Misawa, Hiroaki
Journal of Physical Chemistry C (2013), 117 (6), 2494-2499CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
Metallic nanoparticles showing localized surface plasmon resonance (LSPR) are efficient elements in the localization of light to nanometer-scale regions and enhance the light-matter interaction. The authors show that gold nanoisland (Au-NI)-loaded titanium dioxide (TiO2) photoelectrodes exhibited plasmon-enhanced photocurrent generation in the visible wavelength region, and the photocurrent action spectrum was corresponding to the LSPR band. The photocurrent enhancement may result from the plasmon-assisted electron transfer reaction from Au-NI to TiO2. A hole with high oxidn. ability was left at the TiO2 surface states near the Au-NIs/TiO2 interface, which has the potential for photocatalytic water oxidn. The photocurrent generation efficiency of Au-NIs/TiO2 photoelectrode is highly dependent on the annealing temp. for the prepn. of Au-NIs. High-resoln. transmission electron microscopy and electron energy-loss spectroscopy analyses show that the interfacial structure between Au-NI and TiO2 plays a crucial role in the photocurrent generation efficiency and photocatalytic ability.
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Ueno, K.; Juodkazis, S.; Shibuya, T.; Yokota, Y.; Mizeikis, V.; Sasaki, K.; Misawa, H. Nanoparticle Plasmon-Assisted Two-Photon Polymerization Induced by Incoherent Excitation Source J. Am. Chem. Soc. 2008, 130, 6928– 6929 DOI: 10.1021/ja801262r
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Nanoparticle Plasmon-Assisted Two-Photon Polymerization Induced by Incoherent Excitation Source
Ueno, Kosei; Juodkazis, Saulius; Shibuya, Toshiyuki; Yokota, Yukie; Mizeikis, Vygantas; Sasaki, Keiji; Misawa, Hiroaki
Journal of the American Chemical Society (2008), 130 (22), 6928-6929CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
We demonstrate the possibility to achieve optical triggering of photochem. reactions via two-photon absorption using incoherent light sources. This is accomplished by the use of arrays of gold nanoparticles, specially tailored with high precision to obtain high near-field intensity enhancement.
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Gao, S. Y.; Ueno, K.; Misawa, H. Plasmonic Antenna Effects on Photochemical Reactions Acc. Chem. Res. 2011, 44, 251– 260 DOI: 10.1021/ar100117w
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Plasmonic Antenna Effects on Photochemical Reactions
Gao, Shuyan; Ueno, Kosei; Misawa, Hiroaki
Accounts of Chemical Research (2011), 44 (4), 251-260CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
A review with 45 refs. Efficient solar energy conversion has been vigorously pursued since the 1970s, but its large-scale(coating process) implementation hinges on the availability of high-efficiency modules. For max. efficiency, it is important to absorb most of the incoming radiation, which necessitates both efficient photoexcitation and minimal electron-hole recombination. To date, researchers have primarily focused on the latter difficulty: finding a strategy to effectively sep. photoinduced electrons and holes. Very few reports have been devoted to broadband sunlight absorption and photoexcitation. However, the currently available photovoltaic cells, such as metallic glasses silicon, and even single-crystal silicon and sensitized solar cells, cannot respond to the wide range of the solar spectrum. The photoelec. conversion characteristics of solar cells generally decrease in the IR wavelength range. Thus, the fraction of the solar spectrum absorbed is relatively poor. In addn., the large mismatch between the diffraction limit of light and the absorption cross-section makes the probability of interactions between photons and cell materials quite low, which greatly limits photoexcitation efficiency. Therefore, there is a pressing need for research aimed at finding conditions that lead to highly efficient photoexcitation over a wide spectrum of sunlight, particularly in the visible to near-IR wavelengths. As characterized in the emerging field of plasmonics, metallic nanostructures are endowed with optical antenna effects. These plasmonic antenna effects provide a promising platform for artificially sidestepping the diffraction limit of light and strongly enhancing absorption cross-sections. Moreover, they can efficiently excite photochem. reactions between photons and mols. close to an optical antenna through the local field enhancement. This technol. has the potential to induce highly efficient photoexcitation between photons and mols. over a wide spectrum of sunlight, from visible to near-IR wavelengths. In this Account, we describe our recent work in using metallic nanostructures to assist photochem. reactions for augmenting photoexcitation efficiency. These studies investigate the optical antenna effects of coupled plasmonic gold nanoblocks, which were fabricated with electron-beam lithog. and a lift-off technique to afford high resoln. and nanometric accuracy. The two-photon photoluminescence of gold and the resulting nonlinear photopolymn. on gold nanoblocks substantiate the existence of enhanced optical field domains. Local two-photon photochem. reactions due to weak incoherent light sources were identified. The optical antenna effects support the unprecedented realization of (i) direct photocarrier injection from the gold nanorods into TiO2 and (ii) efficient and stable photocurrent generation in the absence of electron donors from visible (450 nm) to near-IR (1300 nm) wavelengths.
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Brongersma, M. L.; Halas, N. J.; Nordlander, P. Plasmon-Induced Hot Carrier Science and Technology Nat. Nanotechnol. 2015, 10, 25– 34 DOI: 10.1038/nnano.2014.311
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Plasmon-induced hot carrier science and technology
Brongersma, Mark L.; Halas, Naomi J.; Nordlander, Peter
Nature Nanotechnology (2015), 10 (1), 25-34CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
A review. The discovery of the photoelec. effect by Heinrich Hertz in 1887 set the foundation for over 125 years of hot carrier science and technol. In the early 1900s it played a crit. role in the development of quantum mechanics, but even today the unique properties of these energetic, hot carriers offer new and exciting opportunities for fundamental research and applications. Measurement of the kinetic energy and momentum of photoejected hot electrons can provide valuable information on the electronic structure of materials. The heat generated by hot carriers can be harvested to drive a wide range of phys. and chem. processes. Their kinetic energy can be used to harvest solar energy or create sensitive photodetectors and spectrometers. Photoejected charges can also be used to elec. dope two-dimensional materials. Plasmon excitations in metallic nanostructures can be engineered to enhance and provide valuable control over the emission of hot carriers. This Review discusses recent advances in the understanding and application of plasmon-induced hot carrier generation and highlights some of the exciting new directions for the field.
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Zhong, Y. Q.; Ueno, K.; Mori, Y.; Shi, X.; Oshikiri, T.; Murakoshi, K.; Inoue, H.; Misawa, H. Plasmon-Assisted Water Splitting Using Two Sides of the Same SrTiO₃ Single-Crystal Substrate: Conversion of Visible Light to Chemical Energy Angew. Chem., Int. Ed. 2014, 53, 10350– 10354 DOI: 10.1002/anie.201404926
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12
Plasmon-Assisted Water Splitting Using Two Sides of the Same SrTiO3 Single-Crystal Substrate: Conversion of Visible Light to Chemical Energy
Zhong, Yuqing; Ueno, Kosei; Mori, Yuko; Shi, Xu; Oshikiri, Tomoya; Murakoshi, Kei; Inoue, Haruo; Misawa, Hiroaki
Angewandte Chemie, International Edition (2014), 53 (39), 10350-10354CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
A plasmon-induced water splitting system that operates under irradn. by visible light was successfully developed; the system is based on the use of both sides of the same strontium titanate (SrTiO3) single-crystal substrate. The water splitting system contains two soln. chambers to sep. hydrogen (H2) and oxygen (O2). To promote water splitting, a chem. bias was applied by regulating the pH values of the chambers. The quantity of H2 evolved from the surface of platinum, which was used as a redn. co-catalyst, was twice the quantity of O2 evolved from an Au-nanostructured surface. Thus, the stoichiometric evolution of H2 and O2 was clearly demonstrated. The hydrogen-evolution action spectrum closely corresponds to the plasmon resonance spectrum, indicating that the plasmon-induced charge sepn. at the Au/SrTiO3 interface promotes water oxidn. and the subsequent redn. of a proton on the backside of the SrTiO3 substrate. The chem. bias is significantly reduced by plasmonic effects, which indicates the possibility of constructing an artificial photosynthesis system with low energy consumption.
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Oshikiri, T.; Ueno, K.; Misawa, H. Plasmon-Induced Ammonia Synthesis through Nitrogen Photofixation with Visible Light Irradiation Angew. Chem., Int. Ed. 2014, 53, 9802– 9805 DOI: 10.1002/anie.201404748
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Plasmon-Induced Ammonia Synthesis through Nitrogen Photofixation with Visible Light Irradiation
Oshikiri, Tomoya; Ueno, Kosei; Misawa, Hiroaki
Angewandte Chemie, International Edition (2014), 53 (37), 9802-9805CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
We have successfully developed a plasmon-induced technique for ammonia synthesis that responds to visible light through a strontium titanate (SrTiO3) photoelectrode loaded with gold (Au) nanoparticles. The photoelectrochem. reaction cell was divided into two chambers to sep. the oxidized (anodic side) and reduced (cathodic side) products. To promote NH3 formation, a chem. bias was applied by regulating the pH value of these compartments, and ethanol was added to the anodic chamber as a sacrificial donor. The quantity of NH3 formed at the ruthenium surface, which was used as a co-catalyst for SrTiO3, increases linearly as a function of time under irradn. with visible light at wavelengths longer than 550 nm. The NH3 formation action spectrum approx. corresponds to the plasmon resonance spectrum. We deduced that plasmon-induced charge sepn. at the Au/SrTiO3 interface promotes oxidn. at the anodic chamber and subsequent nitrogen redn. on the cathodic side.
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Oshikiri, T.; Ueno, K.; Misawa, H. Selective Dinitrogen Conversion to Ammonia Using Water and Visible Light via Plasmon-Induced Charge Separation Angew. Chem., Int. Ed. 2016, 55, 3942– 3946 DOI: 10.1002/anie.201511189
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15
Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P. A Hybridization Model for the Plasmon Response of Complex Nanostructures Science 2003, 302, 419– 422 DOI: 10.1126/science.1089171
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15
A hybridization model for the plasmon response of complex nanostructures
Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P.
Science (Washington, DC, United States) (2003), 302 (5644), 419-422CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
The authors present a simple and intuitive picture, an electromagnetic analog of MO theory, that describes the plasmon response of complex nanostructures of arbitrary shape. The authors' model can be understood as the interaction or hybridization of elementary plasmons supported by nanostructures of elementary geometries. As an example, the approach is applied to the important case of a four-layer concentric nanoshell, where the hybridization of the plasmons of the inner and outer nanoshells dets. the resonant frequencies of the multilayer nanostructure.
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Sonnefraud, Y.; Koh, A. L.; McComb, D. W.; Maier, S. A. Nanoplasmonics: Engineering and Observation of Localized Plasmon Modes Las. Photon. Rev. 2012, 6, 277– 295 DOI: 10.1002/lpor.201100027
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There is no corresponding record for this reference.
17
Verellen, N.; Sonnefraud, Y.; Sobhani, H.; Hao, F.; Moshchalkov, V. V.; Van Dorpe, P.; Nordlander, P.; Maier, S. A. Fano Resonances in Individual Coherent Plasmonic Nanocavities Nano Lett. 2009, 9, 1663– 1667 DOI: 10.1021/nl9001876
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Fano Resonances in Individual Coherent Plasmonic Nanocavities
Verellen, Niels; Sonnefraud, Yannick; Sobhani, Heidar; Hao, Feng; Moshchalkov, Victor V.; Van Dorpe, Pol; Nordlander, Peter; Maier, Stefan A.
Nano Letters (2009), 9 (4), 1663-1667CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
The authors observe the appearance of Fano resonances in the optical response of plasmonic nanocavities due to the coherent coupling between their superradiant and subradiant plasmon modes. Two reduced-symmetry nanostructures probed via confocal spectroscopy, a dolmen-style slab arrangement and a ring/disk dimer, clearly exhibit the strong polarization and geometry dependence expected for this behavior at the individual nanostructure level, confirmed by full-field electrodynamic anal. of each structure. In each case, multiple Fano resonances occur as structure size is increased.
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Fan, J. A.; Wu, C. H.; Bao, K.; Bao, J. M.; Bardhan, R.; Halas, N. J.; Manoharan, V. N.; Nordlander, P.; Shvets, G.; Capasso, F. Self-Assembled Plasmonic Nanoparticle Clusters Science 2010, 328, 1135– 1138 DOI: 10.1126/science.1187949
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Self-Assembled Plasmonic Nanoparticle Clusters
Fan, Jonathan A.; Wu, Chihhui; Bao, Kui; Bao, Jiming; Bardhan, Rizia; Halas, Naomi J.; Manoharan, Vinothan N.; Nordlander, Peter; Shvets, Gennady; Capasso, Federico
Science (Washington, DC, United States) (2010), 328 (5982), 1135-1138CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
The self-assembly of colloids is an alternative to top-down processing that enables the fabrication of nanostructures. We show that self-assembled clusters of metal-dielec. spheres are the basis for nanophotonic structures. By tailoring the no. and position of spheres in close-packed clusters, plasmon modes exhibiting strong magnetic and Fano-like resonances emerge. The use of identical spheres simplifies cluster assembly and facilitates the fabrication of highly sym. structures. Dielec. spacers are used to tailor the interparticle spacing in these clusters to be approx. 2 nm. These types of chem. synthesized nanoparticle clusters can be generalized to other two- and three-dimensional structures and can serve as building blocks for new metamaterials.
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Bao, Y. J.; Hu, Z. J.; Li, Z. W.; Zhu, X.; Fang, Z. Y. Magnetic Plasmonic Fano Resonance at Optical Frequency Small 2015, 11, 2177– 2181 DOI: 10.1002/smll.201402989
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19
Magnetic Plasmonic Fano Resonance at Optical Frequency
Bao, Yanjun; Hu, Zhijian; Li, Ziwei; Zhu, Xing; Fang, Zheyu
Small (2015), 11 (18), 2177-2181CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
Plasmonic Fano resonances are typically understood and studied assuming elec. mode hybridization. A purely magnetic plasmon Fano resonance can be realized at optical frequency with Au split ring hexamer nanostructure excited by an azimuthally polarized incident light. Collective magnetic plasmon modes induced by the circular elec. field within the hexamer and each of the split ring can be controlled and effectively hybridized by designing the size and orientation of each ring unit. With simulated results reproducing the expt., the suggested configuration with narrow line-shape magnetic Fano resonance has significant potential applications in low-loss sensing and may serves as suitable elementary building blocks for optical metamaterials.
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Ye, J.; Wen, F. F.; Sobhani, H.; Lassiter, J. B.; Van Dorpe, P.; Nordlander, P.; Halas, N. J. Plasmonic Nanoclusters: Near Field Properties of the Fano Resonance Interrogated with SERS Nano Lett. 2012, 12, 1660– 1667 DOI: 10.1021/nl3000453
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Plasmonic Nanoclusters: Near Field Properties of the Fano Resonance Interrogated with SERS
Ye, Jian; Wen, Fangfang; Sobhani, Heidar; Lassiter, J. Britt; Dorpe, Pol Van; Nordlander, Peter; Halas, Naomi J.
Nano Letters (2012), 12 (3), 1660-1667CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
While the far field properties of Fano resonances are well-known, clusters of plasmonic nanoparticles also possess Fano resonances with unique and spatially complex near field properties. Here we examine the near field properties of individual Fano resonant plasmonic clusters using surface-enhanced Raman scattering (SERS) both from mols. distributed randomly on the structure and from dielec. nanoparticles deposited at specific locations within the cluster. Cluster size, geometry, and interparticle spacing all modify the near field properties of the Fano resonance. For mols., the spatially dependent SERS response obtained from near field calcns. correlates well with the relative SERS intensities obsd. for individual clusters and for specific Stokes modes of a para-mercaptoaniline adsorbate. In all cases, the largest SERS enhancement is found when both the excitation and the Stokes shifted wavelengths overlap the Fano resonances. In contrast, for SERS from carbon nanoparticles we find that the dielec. screening introduced by the nanoparticle can drastically redistribute the field enhancement assocd. with the Fano resonance and lead to a significantly modified SERS response compared to what would be anticipated from the bare nanocluster.
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Alonso-Gonzalez, P.; Schnell, M.; Sarriugarte, P.; Sobhani, H.; Wu, C. H.; Arju, N.; Khanikaev, A.; Golmar, F.; Albella, P.; Arzubiaga, L.; Casanova, F.; Hueso, L. E.; Nordlander, P.; Shvets, G.; Hillenbrand, R. Real-Space Mapping of Fano Interference in Plasmonic Metamolecules Nano Lett. 2011, 11, 3922– 3926 DOI: 10.1021/nl2021366
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Real-Space Mapping of Fano Interference in Plasmonic Metamolecules
Alonso-Gonzalez, Pablo; Schnell, Martin; Sarriugarte, Paulo; Sobhani, Heidar; Wu, Chihhui; Arju, Nihal; Khanikaev, Alexander; Golmar, Federico; Albella, Pablo; Arzubiaga, Libe; Casanova, Felix; Hueso, Luis E.; Nordlander, Peter; Shvets, Gennady; Hillenbrand, Rainer
Nano Letters (2011), 11 (9), 3922-3926CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
An unprecedented control of the spectral response of plasmonic nanoantennas has recently been achieved by designing structures that exhibit Fano resonances. This new insight is paving the way for a variety of applications, such as biochem. sensing and surface-enhanced Raman spectroscopy. Scattering-type near-field optical microscopy was used to map the spatial field distribution of Fano modes in IR plasmonic systems. The interference of narrow (dark) and broad (bright) plasmonic resonances was obsd. in real space, yielding intensity and phase toggling between different portions of the plasmonic metamols. when either their geometric sizes or the illumination wavelength is varied.
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Yan, C.; Martin, O. J. F. Periodicity-Induced Symmetry Breaking in a Fano Lattice: Hybridization and Tight-Binding Regimes ACS Nano 2014, 8, 11860– 11868 DOI: 10.1021/nn505642n
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Periodicity-Induced Symmetry Breaking in a Fano Lattice: Hybridization and Tight-Binding Regimes
Yan, Chen; Martin, Olivier J. F.
ACS Nano (2014), 8 (11), 11860-11868CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
The authors study exptl. and theor. the role of periodicity on the optical response of dolmen plasmonic arrays that exhibit a Fano line shape. Contrary to previous works on single nanostructures, this study deals with the in-plane near-field coupling between adjacent unit cells. By making an analogy to the electronic properties of atoms in the tight-binding model, specific behaviors of photonic states are studied numerically as a function of the structural asymmetry for different coupling directions. These predictions are verified exptl. with dark-field measurements on nanostructure arrays which exhibit high tunability and fine control of their spectral features as a function of the lattice consts. These effects, originated from symmetry-breaking and selective excitation of the subradiant mode, provide addnl. degree of freedom for tuning the spectral response and can be used for the sensitive detection of local perturbations. This study provides a general understanding of the near-field interactions in Fano resonant lattices that can be used for the design of plasmonic nanostructures and planar metamaterials.
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Luk’yanchuk, B.; Zheludev, N. I.; Maier, S. A.; Halas, N. J.; Nordlander, P.; Giessen, H.; Chong, C. T. The Fano Resonance in Plasmonic Nanostructures and Metamaterials Nat. Mater. 2010, 9, 707– 715 DOI: 10.1038/nmat2810
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23
The Fano resonance in plasmonic nanostructures and metamaterials
Luk'yanchuk, Boris; Zheludev, Nikolay I.; Maier, Stefan A.; Halas, Naomi J.; Nordlander, Peter; Giessen, Harald; Chong, Chong Tow
Nature Materials (2010), 9 (9), 707-715CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
A review. Since its discovery, the asym. Fano resonance was a characteristic feature of interacting quantum systems. The shape of this resonance is distinctively different from that of conventional sym. resonance curves. Recently, the Fano resonance was found in plasmonic nanoparticles, photonic crystals, and electromagnetic metamaterials. The steep dispersion of the Fano resonance profile promises applications in sensors, lasing, switching, and nonlinear and slow-light devices.
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Fang, Z. Y.; Cai, J. Y.; Yan, Z. B.; Nordlander, P.; Halas, N. J.; Zhu, X. Removing a Wedge from a Metallic Nanodisk Reveals a Fano Resonance Nano Lett. 2011, 11, 4475– 4479 DOI: 10.1021/nl202804y
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Removing a Wedge from a Metallic Nanodisk Reveals a Fano Resonance
Fang, Zheyu; Cai, Junyi; Yan, Zhongbo; Nordlander, Peter; Halas, Naomi J.; Zhu, Xing
Nano Letters (2011), 11 (10), 4475-4479CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
A wide variety of complex, multicomponent plasmonic nanostructures possess Fano resonances. Here the authors introduce a remarkably simple planar nanostructure, a single metallic nanodisk with a missing wedge-shaped slice, that supports a Fano resonance. In this geometry, the Fano line shape arises from the coupling between a hybridized plasmon resonance of the disk and a narrower quadrupolar mode supported by the edge of the missing wedge slice. As a consequence, both disk size and wedge angle control the properties of the resonance. A semianal. description of plasmon hybridization proves useful for analyzing the resulting line shape.
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Gallinet, B.; Martin, O. J. F. Influence of Electromagnetic Interactions on the Line Shape of Plasmonic Fano Resonances ACS Nano 2011, 5, 8999– 9008 DOI: 10.1021/nn203173r
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Influence of Electromagnetic Interactions on the Line Shape of Plasmonic Fano Resonances
Gallinet, Benjamin; Martin, Olivier J. F.
ACS Nano (2011), 5 (11), 8999-9008CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
The optical properties of plasmonic nanostructures supporting Fano resonances are studied with an electromagnetic theory. Contrary to the original work of Fano, this theory includes losses in the materials composing the system. A more general formula is obtained for the response of the system and general conclusions for the detn. of the resonance parameters are drawn. These predictions are verified with surface integral numerical calcns. in a broad variety of plasmonic nanostructures including dolmens, oligomers, and gratings. This work presents a robust and consistent anal. of plasmonic Fano resonances and enables the control of their line shape based on Maxwell's equations. The insights into the phys. understanding of Fano resonances gained this way will be of great interest for the design of plasmonic systems with specific spectral responses for applications such as sensing and optical metamaterials.
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Rahmani, M.; Luk’yanchuk, B.; Hong, M. H. Fano Resonance in Novel Plasmonic Nanostructures Las. Photon. Rev. 2013, 7, 329– 349 DOI: 10.1002/lpor.201200021
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26
Fano resonance in novel plasmonic nanostructures
Rahmani, Mohsen; Luk'yanchuk, Boris; Hong, Minghui
Laser & Photonics Reviews (2013), 7 (3), 329-349CODEN: LPRAB8; ISSN:1863-8880. (Wiley-VCH Verlag GmbH & Co. KGaA)
A review. Recently, a large no. of exptl. and theor. works have revealed a variety of plasmonic nanostructures with the capabilities of Fano resonance (FR) generation. Among these structures, plasmonic oligomers consisting of packed metallic nanoelements with certain configurations have been of significant interest. Oligomers can exhibit FR independently of the polarization direction based on dipole-dipole antiparallel modes without the need to excite challenging high-order modes. The purpose of this review article is to provide an overview of recent achievements on FR of plasmonic nanostructures in recent years. Meanwhile, more attention is given to the optical properties of plasmonic oligomers due to the high potential of such structures in optical spectra engineering.
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Coenen, T.; Schoen, D. T.; Mann, S. A.; Rodriguez, S. R. K.; Brenny, B. J. M.; Polman, A.; Brongersma, M. L. Nanoscale Spatial Coherent Control over the Modal Excitation of a Coupled Plasmonic Resonator System Nano Lett. 2015, 15, 7666– 7670 DOI: 10.1021/acs.nanolett.5b03614
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Liu, N.; Langguth, L.; Weiss, T.; Kastel, J.; Fleischhauer, M.; Pfau, T.; Giessen, H. Plasmonic Analogue of Electromagnetically Induced Transparency at the Drude Damping Limit Nat. Mater. 2009, 8, 758– 762 DOI: 10.1038/nmat2495
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Plasmonic analogue of electromagnetically induced transparency at the Drude damping limit
Liu, Na; Langguth, Lutz; Weiss, Thomas; Kaestel, Juergen; Fleischhauer, Michael; Pfau, Tilman; Giessen, Harald
Nature Materials (2009), 8 (9), 758-762CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
In at. physics, the coherent coupling of a broad and a narrow resonance leads to quantum interference and provides the general recipe for electromagnetically induced transparency (EIT). A sharp resonance of nearly perfect transmission can arise within a broad absorption profile. These features show remarkable potential for slow light, novel sensors and low-loss metamaterials. In nanophotonics, plasmonic structures enable large field strengths within small mode vols. Therefore, combining EIT with nanoplasmonics would pave the way towards ultracompact sensors with extremely high sensitivity. Here, the authors exptl. demonstrate a nanoplasmonic analog of EIT using a stacked optical metamaterial. A dipole antenna with a large radiatively broadened linewidth is coupled to an underlying quadrupole antenna, of which the narrow linewidth is solely limited by the fundamental nonradiative Drude damping. In accordance with EIT theory, the authors achieve a very narrow transparency window with high modulation depth owing to nearly complete suppression of radiative losses.
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Chen, H. Y.; He, C. L.; Wang, C. Y.; Lin, M. H.; Mitsui, D.; Eguchi, M.; Teranishi, T.; Gwo, S. Far-Field Optical Imaging of a Linear Array of Coupled Gold Nanocubes: Direct Visualization of Dark Plasmon Propagating Modes ACS Nano 2011, 5, 8223– 8229 DOI: 10.1021/nn2029007
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Far-Field Optical Imaging of a Linear Array of Coupled Gold Nanocubes: Direct Visualization of Dark Plasmon Propagating Modes
Chen, Hung-Ying; He, Chieh-Lun; Wang, Chun-Yuan; Lin, Meng-Hsien; Mitsui, Daisuke; Eguchi, Miharu; Teranishi, Toshiharu; Gwo, Shang-Jr
ACS Nano (2011), 5 (10), 8223-8229CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
Plasmonic nanoantenna arrays hold great promise for diffraction-unlimited light localization, confinement, and transport. Linear plasmonic nanoantenna arrays composed of colloidal Au nanocubes precisely assembled using a nanomanipulation technique are reported. Direct evidence is shown of dark propagating modes in the plasmon coupling regime, allowing for transport of guided plasmon waves without far-field radiation losses. The possibility of plasmon dispersion engineering in coupled Au nanocube chains is demonstrated. By assembling a nanocube chain with 2 sections of coupled nanocubes of different intercube sepns., the effect of a band-pass nanofilter is produced.
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Solis, D.; Willingham, B.; Nauert, S. L.; Slaughter, L. S.; Olson, J.; Swanglap, P.; Paul, A.; Chang, W. S.; Link, S. Electromagnetic Energy Transport in Nanoparticle Chains via Dark Plasmon Modes Nano Lett. 2012, 12, 1349– 1353 DOI: 10.1021/nl2039327
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Electromagnetic Energy Transport in Nanoparticle Chains via Dark Plasmon Modes
Solis, David, Jr.; Willingham, Britain; Nauert, Scott L.; Slaughter, Liane S.; Olson, Jana; Swanglap, Pattanawit; Paul, Aniruddha; Chang, Wei-Shun; Link, Stephan
Nano Letters (2012), 12 (3), 1349-1353CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
Using light to exchange information offers large bandwidths and high speeds, but the miniaturization of optical components is limited by diffraction. Converting light into electron waves in metals allows 1 to overcome this problem. However, metals are lossy at optical frequencies and large-area fabrication of nanometer-sized structures by conventional top-down methods can be cost-prohibitive. The authors show electromagnetic energy transport with Au nanoparticles that were assembled into close-packed linear chains. The small interparticle distances enabled strong electromagnetic coupling causing the formation of low-loss subradiant plasmons, which facilitated energy propagation over many micrometers. Electrodynamic calcns. confirmed the dark nature of the propagating mode and showed that disorder in the nanoparticle arrangement enhances energy transport, demonstrating the viability of using bottom-up nanoparticle assemblies for ultracompact opto-electronic devices.
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Kauranen, M.; Zayats, A. V. Nonlinear Plasmonics Nat. Photonics 2012, 6, 737– 748 DOI: 10.1038/nphoton.2012.244
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31
Nonlinear plasmonics
Kauranen, Martti; Zayats, Anatoly V.
Nature Photonics (2012), 6 (11), 737-748CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
A review. When light interacts with metal nanostructures, it can couple to free-electron excitations near the metal surface. The electromagnetic resonances assocd. with these surface plasmons depend on the details of the nanostructure, opening up opportunities for controlling light confinement on the nanoscale. The resulting strong electromagnetic fields allow weak nonlinear processes, which depend superlinearly on the local field, to be significantly enhanced. In addn. to providing enhanced nonlinear effects with ultrafast response times, plasmonic nanostructures allow nonlinear optical components to be scaled down in size. In this Review, we discuss the principles of nonlinear plasmonic effects and present an overview of their main applications, including frequency conversion, switching and modulation of optical signals, and soliton effects.
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Schumacher, T.; Kratzer, K.; Molnar, D.; Hentschel, M.; Giessen, H.; Lippitz, M. Nanoantenna-Enhanced Ultrafast Nonlinear Spectroscopy of a Single Gold Nanoparticle Nat. Commun. 2011, 2, 333 DOI: 10.1038/ncomms1334
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33
Celebrano, M.; Wu, X. F.; Baselli, M.; Großmann, S.; Biagioni, P.; Locatelli, A.; De Angelis, C.; Cerullo, G.; Osellame, R.; Hecht, B.; Duo, L.; Ciccacci, F.; Finazzi, M. Mode Matching in Multiresonant Plasmonic Nanoantennas for Enhanced Second Harmonic Generation Nat. Nanotechnol. 2015, 10, 412– 417 DOI: 10.1038/nnano.2015.69
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Mode matching in multiresonant plasmonic nanoantennas for enhanced second harmonic generation
Celebrano, Michele; Wu, Xiaofei; Baselli, Milena; Grossmann, Swen; Biagioni, Paolo; Locatelli, Andrea; De Angelis, Costantino; Cerullo, Giulio; Osellame, Roberto; Hecht, Bert; Duo, Lamberto; Ciccacci, Franco; Finazzi, Marco
Nature Nanotechnology (2015), 10 (5), 412-417CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
Doubly-resonant single-cryst. Au nanostructures with no axial symmetry displaying spatial mode overlap at both the excitation and 2nd harmonic wavelengths are described. The combination of these features allows the attainment of a nonlinear coeff. for 2nd harmonic generation of ∼5 × 10-10 W-1, enabling a 2nd harmonic photon yield >3 × 106 photons per s. Theor. estns. point toward the use of the nonlinear plasmonic nanoantennas as efficient platforms for label-free mol. sensing.
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Aouani, H.; Rahmani, M.; Navarro-Cia, M.; Maier, S. A. Third-Harmonic-Upconversion Enhancement from a Single Semiconductor Nanoparticle Coupled to a Plasmonic Antenna Nat. Nanotechnol. 2014, 9, 290– 294 DOI: 10.1038/nnano.2014.27
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34
Third-harmonic-upconversion enhancement from a single semiconductor nanoparticle coupled to a plasmonic antenna
Aouani, Heykel; Rahmani, Mohsen; Navarro-Cia, Miguel; Maier, Stefan A.
Nature Nanotechnology (2014), 9 (4), 290-294CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
The ability to convert low-energy quanta into a quantum of higher energy is of great interest for a variety of applications, including bioimaging, drug delivery and photovoltaics. Although high conversion efficiencies can be achieved using macroscopic nonlinear crystals, upconverting light at the nanometer scale remains challenging because the subwavelength scale of materials prevents the exploitation of phase-matching processes. Light-plasmon interactions that occur in nanostructured noble metals have offered alternative opportunities for nonlinear upconversion of IR light, but conversion efficiency rates remain extremely low due to the weak penetration of the exciting fields into the metal. Here, 3rd-harmonic generation from an individual semiconductor In Sn oxide nanoparticle is significantly enhanced when coupled within a plasmonic Au dimer. The plasmonic dimer acts as a receiving optical antenna, confining the incident far-field radiation into a near field localized at its gap; the In Sn oxide nanoparticle located at the plasmonic dimer gap acts as a localized nonlinear transmitter upconverting three incident photons at frequency ω into a photon at frequency 3ω. This hybrid nanodevice provides 3rd-harmonic-generation enhancements of up to 106-fold compared with an isolated In Sn oxide nanoparticle, with an effective 3rd-order susceptibility up to 3.5 × 103 nm V-2 and conversion efficiency of 0.0007%. Also the upconverted 3rd-harmonic emission can be exploited to probe the near-field intensity at the plasmonic dimer gap.
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Frimmer, M.; Coenen, T.; Koenderink, A. F. Signature of a Fano Resonance in a Plasmonic Metamolecule’s Local Density of Optical States Phys. Rev. Lett. 2012, 108, 077404 DOI: 10.1103/PhysRevLett.108.077404
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35
Signature of a Fano resonance in a plasmonic metamolecule's local density of optical states
Frimmer, Martin; Coenen, Toon; Koenderink, A. Femius
Physical Review Letters (2012), 108 (7), 077404/1-077404/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
We present measurements on plasmonic metamols. under local excitation using cathodoluminescence which show a spatial redistribution of the local d. of optical states at the same frequency where a sharp spectral Fano feature in extinction has been obsd. Our anal. model shows that both near- and far-field effects arise due to interference of the same two eigenmodes of the system. We present quant. insights both in a bare state, and in a dressed state picture that describe Fano interference either as near-field amplitude transfer between coupled bare states, or as interference of uncoupled eigenmodes in the far field. We identify the same eigenmode causing a dip in extinction to strongly enhance the radiative local d. of optical states, making it a promising candidate for spontaneous emission control.
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Lassiter, J. B.; Sobhani, H.; Knight, M. W.; Mielczarek, W. S.; Nordlander, P.; Halas, N. J. Designing and Deconstructing the Fano Lineshape in Plasmonic Nanoclusters Nano Lett. 2012, 12, 1058– 1062 DOI: 10.1021/nl204303d
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36
Designing and Deconstructing the Fano Lineshape in Plasmonic Nanoclusters
Lassiter, J. Britt; Sobhani, Heidar; Knight, Mark W.; Mielczarek, Witold S.; Nordlander, Peter; Halas, Naomi J.
Nano Letters (2012), 12 (2), 1058-1062CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
By varying the relative dimensions of the central and peripheral disks of a plasmonic nanocluster, the depth of its Fano resonance can be systematically modified; spectral windows where the scattering cross section of the nanocluster is negligible can be obtained. But electron-beam excitation of the plasmon modes at specific locations within the nanocluster yields cathodoluminescence spectra with no Fano resonance. By examg. the selection rules for plasmon excitation in the context of a coupled oscillator picture, the authors provide an intuitive explanation of this behavior based on the plasmon modes obsd. for optical and electron-beam excitation in this family of nanostructures.
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Kubo, A.; Onda, K.; Petek, H.; Sun, Z. J.; Jung, Y. S.; Kim, H. K. Femtosecond Imaging of Surface Plasmon Dynamics in a Nanostructured Silver Film Nano Lett. 2005, 5, 1123– 1127 DOI: 10.1021/nl0506655
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37
Femtosecond Imaging of Surface Plasmon Dynamics in a Nanostructured Silver Film
Kubo, Atsushi; Onda, Ken; Petek, Hrvoje; Sun, Zhijun; Jung, Yun S.; Kim, Hong Koo
Nano Letters (2005), 5 (6), 1123-1127CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
Light interacting with nanostructured metals excites the collective charge d. fluctuations known as surface plasmons (SP). Through excitation of the localized SP eigenmodes incident light is trapped on the nanometer spatial and femtosecond temporal scales and its field is enhanced. Here we demonstrate the imaging and quantum control of SP dynamics in a nanostructured silver film. By inducing and imaging the nonlinear two-photon photoemission from the sample with a pair of identical 10-fs laser pulses while scanning the pulse delay, we record a movie of SP fields at a rate of 330-as/frame.
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Schertz, F.; Schmelzeisen, M.; Mohammadi, R.; Kreiter, M.; Elmers, H. J.; Schönhense, G. Near Field of Strongly Coupled Plasmons: Uncovering Dark Modes Nano Lett. 2012, 12, 1885– 1890 DOI: 10.1021/nl204277y
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38
Near Field of Strongly Coupled Plasmons: Uncovering Dark Modes
Schertz, Florian; Schmelzeisen, Marcus; Mohammadi, Reza; Kreiter, Maximilian; Elmers, Hans-Joachim; Schoenhense, Gerd
Nano Letters (2012), 12 (4), 1885-1890CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
Strongly coupled plasmons in a system of individual Au nanoparticles placed at sub-nm distance to a Au film (nanoparticle-on-plane, NPOP) are studied using 2 complementary single particle spectroscopy techniques. Optical scattering spectroscopy exclusively detects plasmon modes that couple to the far field via their dipole moment (bright modes). By using photoemission electron microscopy (PEEM), the authors detect in the identical NPOPs near-field modes that do not couple to the scattered far field (dark modes) and are characterized by a strongly enhanced nonlinear electron emission process. Both far- and near-field spectroscopy were carried out for identical individual nanostructures interacting via a sub-nm gap. Strongly resonant electron emission occurs at excitation wavelengths far off-resonant in the scattering spectra.
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Douillard, L.; Charra, F. Photoemission Electron Microscopy, A Tool for Plasmonics J. Electron Spectrosc. Relat. Phenom. 2013, 189, 24– 29 DOI: 10.1016/j.elspec.2013.03.013
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39
Photoemission electron microscopy, a tool for plasmonics
Douillard, L.; Charra, F.
Journal of Electron Spectroscopy and Related Phenomena (2013), 189 (Suppl.), 24-29CODEN: JESRAW; ISSN:0368-2048. (Elsevier B.V.)
A review. A key challenge to plasmonics is the development of exptl. tools allowing access to the spatial distribution of the optical near field at the nanometer scale. A recent approach for mapping the near optical field is the use of the photoemission electron microscopy PEEM. Indeed, photoemission can be strongly enhanced upon excitation of surface plasmons. By collecting the photoemitted electrons, two-dimensional intensity maps reflecting the actual distribution of the optical near-field are obtained. In the following a brief overview of the possibilities of the photoemission electron microscopy as a tool for plasmonics is given. Main focuses will be set on exptl. results regarding the mapping of the near optical field at nanometer scale, the investigation of the spatio-temporal dynamics of plasmon-polariton waves and the manipulation at will of the near field.
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Sun, Q.; Ueno, K.; Yu, H.; Kubo, A.; Matsuo, Y.; Misawa, H. Direct Imaging of the Near Field and Dynamics of Surface Plasmon Resonance on Gold Nanostructures Using Photoemission Electron Microscopy Light: Sci. Appl. 2013, 2, e118 DOI: 10.1038/lsa.2013.74
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Direct imaging of the near field and dynamics of surface plasmon resonance on gold nanostructures using photoemission electron microscopy
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Light: Science & Applications (2013), 2 (Dec.), e118CODEN: LSAIAZ; ISSN:2047-7538. (Nature Publishing Group)
Localized surface plasmon resonance (LSPR) can be supported by metallic nanoparticles and engineered nanostructures. An understanding of the spatially resolved near-field properties and dynamics of LSPR is important, but remains exptl. challenging. We report exptl. studies toward this aim using photoemission electron microscopy (PEEM) with high spatial resoln. of sub-10 nm. Various engineered gold nanostructure arrays (such as rods, nanodisk-like particles and dimers) are investigated via PEEM using near-IR (NIR) femtosecond laser pulses as the excitation source. When the LSPR wavelengths overlap the spectrum of the femtosecond pulses, the LSPR is efficiently excited and promotes multiphoton photoemission, which is correlated with the local intensity of the metallic nanoparticles in the near field. Thus, the local field distribution of the LSPR on different Au nanostructures can be directly explored and discussed using the PEEM images. In addn., the dynamics of the LSPR is studied by combining interferometric time-resolved pump-probe technique and PEEM. Detailed information on the oscillation and dephasing of the LSPR field can be obtained. The results identify PEEM as a powerful tool for accessing the near-field mapping and dynamic properties of plasmonic nanostructures.
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Sun, Q.; Yu, H.; Ueno, K.; Kubo, A.; Matsuo, Y.; Misawa, H. Dissecting the Few-Femtosecond Dephasing Time of Dipole and Quadrupole Modes in Gold Nanoparticles Using Polarized Photoemission Electron Microscopy ACS Nano 2016, 10, 3835– 3842 DOI: 10.1021/acsnano.6b00715
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ACS Nano (2016), 10 (3), 3835-3842CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
Dipole and quadrupole modes are the 2 lowest orders of localized surface plasmon resonance (LSPR) eigenmodes in metallic nanoparticles. Of these 2 modes, the quadrupole mode is forbidden for sym. metallic nanoparticles excited by linearly polarized light at normal incidence. Excitation of the quadrupole mode in sym. Au nanoblocks shined with s-polarized light at oblique incidence was demonstrated. The near-field LSPR in Au nanoblocks using photoemission electron microscopy (PEEM) was probed, and at oblique incidence, the dipole and quadrupole modes can be selectively excited, in terms of near-field enhancement, by manipulating the light polarization state. By time-resolved PEEM measurements, exptl. the quadrupole mode in sym. Au nanoblocks has longer dephasing time than that of the dipole mode.
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Lecarme, O.; Sun, Q.; Ueno, K.; Misawa, H. Robust and Versatile Light Absorption at Near-Infrared Wavelengths by Plasmonic Aluminum Nanorods ACS Photonics 2014, 1, 538– 546 DOI: 10.1021/ph500096q
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ACS Photonics (2014), 1 (6), 538-546CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
The authors study the far-field and near-field properties of Al nanorods fabricated by electron beam lithog. and exhibiting plasmonic resonance in the near-IR region. First, plasmonic modes within nanorod arrays can be tuned by geometrical parameters, allowing 1 to control the system transparency. Next, the light absorption in this structure is closely examd., and Al has great potential due to its unique interband transition at 800 nm. The roles of the dielec. confinement and the coupling between plasmonic resonance and the interband transition are particularly emphasized, as their adjustment can be used to switch from highly scattering particles to absorbing particles without a significant modification of the plasmonic resonance position. Finally, the authors image the plasmon-generated local field distribution in the Al nanostructures and observe, for the 1st time, the effect of the interband transition on the near-field behavior. The effect of the dielec. confinement is also numerically studied, as it is shown to play a significant role in near-field enhancement.
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Lassiter, J. B.; Sobhani, H.; Fan, J. A.; Kundu, J.; Capasso, F.; Nordlander, P.; Halas, N. J. Fano Resonances in Plasmonic Nanoclusters: Geometrical and Chemical Tunability Nano Lett. 2010, 10, 3184– 3189 DOI: 10.1021/nl102108u
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Nano Letters (2010), 10 (8), 3184-3189CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
Clusters of plasmonic nanoparticles and nanostructures support Fano resonances. This spectral feature, produced by the interference between bright and dark modes of the nanoparticle cluster, is strongly dependent upon both geometry and local dielec. environment. This permits a highly sensitive tunability of the Fano dip in both wavelength and amplitude by varying cluster dimensions, geometry, and relative size of the individual nanocluster components. Plasmonic nanoclusters show an unprecedented sensitivity to dielec. environment with a local surface plasmon resonance figure of merit of 5.7, the highest yet reported for localized surface plasmon resonance sensing in a finite nanostructure.
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Nano Letters (2010), 10 (7), 2721-2726CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
The transition from isolated to collective optical modes in plasmonic oligomers is demonstrated. The resonant behavior of planar plasmonic hexamers and heptamers with gradually decreasing the interparticle gap sepn. was studied. A pronounced Fano resonance is obsd. in the plasmonic heptamer for sepns. <60 nm. The spectral characteristics change drastically upon removal of the central nanoparticle. The work paves the road toward complex hierarchical plasmonic oligomers with tailored optical properties.
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The interaction between plasmonic resonances, sharp modes, and light in nanoscale plasmonic systems often leads to Fano interference effects. This occurs because the plasmonic excitations are usually spectrally broad and the characteristic narrow asym. Fano line-shape results upon interaction with spectrally sharper modes. By considering the plasmonic resonance in the Fano model, as opposed to previous flat continuum approaches, a simple and exact expression for the line-shape can be found. This allows the role of the width and energy of the plasmonic resonance to be properly understood. Fano resonances measured on an array of Au nanoantennas covered with PMMA, as well as the hybridization of dark with bright plasmons in nanocavities, are reproduced with a simple exact formula and without any fitting parameters.
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Physical Review B: Solid State (1972), 6 (12), 4370-9CODEN: PLRBAQ; ISSN:0556-2805.
The optical consts. n and k were obtained for Cu, Ag, and Au from reflection and transmission measurements on vacuum-evapd. thin films at room temp., in the spectral range 0.5-6.5 eV. The film-thickness range was 185-500 Å. Three optical measurements were inverted to obtain the film thickness d as well as n and k. The estd. error in d was ±2 Å, and that in n, k was <0.02 over most of the spectral range. The results in the film-thickness range 250-500 Å were independent of thickness, and were unchanged after vacuum annealing or aging in air. The free-electron optical effective masses and relaxation times derived from the results in the near ir agreed satisfactorily with previous values. The interband contribution to the imaginary part of the dielec. const. was obtained by subtracting the free-electron contribution. Some recent theor. calcns. were compared with the results for Cu and Au. In addn., some other recent expts. are crit. compared with the present results.
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Abstract
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Figure 1
Figure 1. Characterization of topography and far-field spectra. (a) Sketch map of the Au dolmen structure that consists of a planar nanorod monomer and a planar nanorod dimer. Design parameters of the structure are as follows: L1 = 150 nm, W1 = 100 nm, G = 25 nm, L2 = 140 nm, W2 = 80 nm, D = 70 nm, H = 30 nm. (b) SEM image of the Au dolmen array, with a pitch size of 1 μm; the scale bar is 200 nm. (c) Far-field extinction spectra collected using different linearly polarized light: vertical light irradiation (parallel to the symmetry axis (black curve)) and horizontal polarized light irradiation (perpendicular to the symmetry axis (red curve)).
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Figure 2
Figure 2. Near-field spectra from PEEM measurements. (a) Schematic of the PEEM setup. (b) Near-field spectra of the Au dolmen under horizontal polarization (red curve) and vertical polarization (black curve) using wavelength-dependent PEEM measurements with a wavelength-tunable laser. (c,d) Comparison between the near-field spectrum and far-field extinction spectrum under horizontal polarization and vertical polarization, respectively.
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Figure 3
Figure 3. Near-field mapping obtained from PEEM measurements. (a) Near-field spectrum of a single Au dolmen structure, obtained using wavelength-dependent PEEM measurements. The inset shows an SEM image of a single dolmen structure investigated using PEEM. (b) PEEM image of Au dolmen structures simultaneously irradiated with UV light and femtosecond laser pulses, with a central wavelength of 800 nm. The PEEM image of a Au dolmen structure under UV light excitation reveals the morphology of the structure. Using PEEM images collected under simultaneous irradiation with UV light and laser pulses, the exact locations of hot spots were confirmed. (c–f) PEEM images collected under irradiation with femtosecond laser pulses at four different wavelengths: the shorter-peak wavelength of 760 nm (c), the dip wavelength of 800 nm (e), and the longer-peak wavelength of 850 nm (f) and additional 780 nm (d) with the maximum enhancement from the dimer part. These wavelengths are also marked as 1–4 in (a). The red dashed lines in (b–f) plot the dolmen structure geometry. The scale bar in all images is 100 nm.
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Figure 4
Figure 4. Spectra and mapping calculated by FDTD simulations. (a) Far-field extinction spectrum (black) and near-field PE intensity spectrum (red), plotting the integral of the (I/I₀)⁴ for a 320 nm × 320 nm area on the interface between the dolmen structure and the substrate. (b) FDTD calculated charge distribution at two characterized peak wavelengths, indicating two hybridized modes, namely, anti-bonding and bonding modes. (c) Calculated near-field intensity distribution under four characterized wavelengths: the shorter-peak wavelength of 759 nm (1), 788 nm (2) that gives the maximum enhancement for the dimer part, the dip wavelength of 802 nm (3), and the longer-peak wavelength of 853 nm (4), which are also marked as 1–4 in (a).
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Figure 5
Figure 5. Spatially resolved PE intensity spectra. (a) Spatially resolved PE spectrum of the dolmen, obtained for four different regions by integrating the PE signal from selected areas. (b) Contour part of the FDTD simulation results. The corresponding selected areas are indicated by the dashed lines on the right side. From these analyses, different modes, including the quadrupole mode on the dimer part, can be identified.
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References
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This article references 49 other publications.
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3. 3
  Jain, P. K.; Huang, X. H.; El-Sayed, I. H.; El-Sayed, M. A. Noble Metals on the Nanoscale: Optical and Photothermal Properties and Some Applications in Imaging, Sensing, Biology, and Medicine Acc. Chem. Res. 2008, 41, 1578– 1586 DOI: 10.1021/ar7002804
  [ACS Full Text ], [CAS], Google Scholar
  3
  Noble Metals on the Nanoscale: Optical and Photothermal Properties and Some Applications in Imaging, Sensing, Biology, and Medicine
  Jain, Prashant K.; Huang, Xiaohua; El-Sayed, Ivan H.; El-Sayed, Mostafa A.
  Accounts of Chemical Research (2008), 41 (12), 1578-1586CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
  A review. Noble metal nanostructures attract much interest because of their unique properties, including large optical field enhancements resulting in the strong scattering and absorption of light. The enhancement in the optical and photothermal properties of noble metal nanoparticles arises from resonant oscillation of their free electrons in the presence of light, also known as localized surface plasmon resonance (LSPR). The plasmon resonance can either radiate light (Mie scattering), a process that finds great utility in optical and imaging fields, or be rapidly converted to heat (absorption); the latter mechanism of dissipation has opened up applications in several new areas. The ability to integrate metal nanoparticles into biol. systems has had greatest impact in biol. and biomedicine. In this Account, the authors discuss the plasmonic properties of gold and silver nanostructures and present examples of how they are being utilized for biodiagnostics, biophys. studies, and medical therapy. For instance, taking advantage of the strong LSPR scattering of gold nanoparticles conjugated with specific targeting mols. allows the mol.-specific imaging and diagnosis of diseases such as cancer. The authors emphasize in particular how the unique tunability of the plasmon resonance properties of metal nanoparticles through variation of their size, shape, compn., and medium allows chemists to design nanostructures geared for specific bio-applications. The authors discuss some interesting nanostructure geometries, including nanorods, nanoshells, and nanoparticle pairs, that exhibit dramatically enhanced and tunable plasmon resonances, making them highly suitable for bio-applications. Tuning the nanostructure shape (e.g., nanoprisms, nanorods, or nanoshells) is another means of enhancing the sensitivity of the LSPR to the nanoparticle environment and, thereby, designing effective biosensing agents. Metal nanoparticle pairs or assemblies display distance-dependent plasmon resonances as a result of field coupling. A universal scaling model, relating the plasmon resonance frequency to the interparticle distance in terms of the particle size, becomes potentially useful for measuring nanoscale distances (and their changes) in biol. systems. The strong plasmon absorption and photothermal conversion of gold nanoparticles has been exploited in cancer therapy through the selective localized photothermal heating of cancer cells. For nanorods or nanoshells, the LSPR can be tuned to the near-IR region, making it possible to perform in vivo imaging and therapy. The examples of the applications of noble metal nanostructures provided herein can be readily generalized to other areas of biol. and medicine because plasmonic nanomaterials exhibit great range, versatility, and systematic tunability of their optical attributes.
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4. 4
  Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Biosensing with Plasmonic Nanosensors Nat. Mater. 2008, 7, 442– 453 DOI: 10.1038/nmat2162
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  4
  Biosensing with plasmonic nanosensors
  Anker, Jeffrey N.; Hall, W. Paige; Lyandres, Olga; Shah, Nilam C.; Zhao, Jing; Van Duyne, Richard P.
  Nature Materials (2008), 7 (6), 442-453CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
  A review. Light incident on metallic nanoparticles can induce a collective motion of electrons that can lead to a strong amplification of the local electromagnetic field. As reviewed here, these plasmonic resonances have important applications in biosensing where they push resoln. and sensitivity towards the single-mol. detection limit. Recent developments have greatly improved the sensitivity of optical sensors based on metal nanoparticle arrays and single nanoparticles. The authors introduce the localized surface plasmon resonance (LSPR) sensor and describe how its exquisite sensitivity to size, shape and environment can be harnessed to detect mol. binding events and changes in mol. conformation. The authors then describe recent progress in three areas representing the most significant challenges: pushing sensitivity towards the single-mol. detection limit, combining LSPR with complementary mol. identification techniques such as surface-enhanced Raman spectroscopy, and practical development of sensors and instrumentation for routine use and high-throughput detection. This review highlights several exceptionally promising research directions and discusses how diverse applications of plasmonic nanoparticles can be integrated in the near future.
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5. 5
  Lal, S.; Link, S.; Halas, N. J. Nano-Optics from Sensing to Waveguiding Nat. Photonics 2007, 1, 641– 648 DOI: 10.1038/nphoton.2007.223
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  5
  Nano-optics from sensing to waveguiding
  Lal, Surbhi; Link, Stephan; Halas, Naomi J.
  Nature Photonics (2007), 1 (11), 641-648CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
  A review. The design and realization of metallic nanostructures with tunable plasmon resonances was greatly advanced by combining a wealth of nanofabrication techniques with advances in computational electromagnetic design. Plasmonics - a rapidly emerging subdiscipline of nanophotonics- is aimed at exploiting both localized and propagating surface plasmons for technol. important applications, specifically in sensing and waveguiding.
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6. 6
  Kawata, S.; Inouye, Y.; Verma, P. Plasmonics for Near-Field Nano-Imaging and Superlensing Nat. Photonics 2009, 3, 388– 394 DOI: 10.1038/nphoton.2009.111
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  6
  Plasmonics for near-field nano-imaging and superlensing
  Kawata, Satoshi; Inouye, Yasushi; Verma, Prabhat
  Nature Photonics (2009), 3 (7), 388-394CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
  A review. Diffraction of light prevents optical microscopes from having spatial resoln. beyond a value comparable to the wavelength of the probing light. This essentially means that visible light cannot image nanomaterials. Here the authors review the mechanism for going beyond this diffraction limit and discuss how manipulation of light by surface plasmons propagating along the metal surface can help to achieve this. The interesting behavior of light under the influence of plasmons not only allows superlensing, in which perfect imaging is possible through a flat thin metal film, but can also provide nano-imaging of practical samples by using a localized surface plasmon mode at the tip of a metallic nanoprobe. The authors also discuss the current research status and some intriguing future possibilities.
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7. 7
  Ueno, K.; Misawa, H. Plasmon-Enhanced Photocurrent Generation and Water Oxidation from Visible to Near-Infrared Wavelengths NPG Asia Mater. 2013, 5, e61 DOI: 10.1038/am.2013.42
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  7
  Plasmon-enhanced photocurrent generation and water oxidation from visible to near-infrared wavelengths
  Ueno, Kosei; Misawa, Hiroaki
  NPG Asia Materials (2013), 5 (Sept.), e61CODEN: NAMPCE; ISSN:1884-4057. (Nature Publishing Group)
  A review. This paper presents recent studies of plasmon-enhanced photoelec. conversion and H2O oxidn. by visible and near-IR light irradn. Since the discovery of the Honda-Fujishima effect in 1972, significant efforts were devoted to lengthening the light-energy conversion wavelength. In this context, plasmonic photoelec. conversion was recently demonstrated at visible-to-near-IR wavelengths without deteriorating photoelec. conversion by employing TiO2 (TiO2) single-crystal photoelectrodes, in which Au nanorods are elaborately arrayed on the surface. A KClO4 aq. soln. was employed as an electrolyte soln. without addnl. electron donors; thus, H2O mols. provided the electrons. The stoichiometric evolution of O and H2O2 as a result of the 4- or 2-electron oxidn. of H2O mols., resp., was accomplished with near-IR light irradn. using the plasmonic optical antenna effect. As there is very little overpotential for H2O oxidn., these results constitute a significant advancement in this field. This photoelec. conversion system could potentially be employed in artificial photosynthesis systems that exceed the photosynthetic capabilities of plants by allowing for photoconversion over a wide range of wavelengths.
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8. 8
  Shi, X.; Ueno, K.; Takabayashi, N.; Misawa, H. Plasmon-Enhanced Photocurrent Generation and Water Oxidation with a Gold Nanoisland-Loaded Titanium Dioxide Photoelectrode J. Phys. Chem. C 2013, 117, 2494– 2499 DOI: 10.1021/jp3064036
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  8
  Plasmon-Enhanced Photocurrent Generation and Water Oxidation with a Gold Nanoisland-Loaded Titanium Dioxide Photoelectrode
  Shi, Xu; Ueno, Kosei; Takabayashi, Naoki; Misawa, Hiroaki
  Journal of Physical Chemistry C (2013), 117 (6), 2494-2499CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  Metallic nanoparticles showing localized surface plasmon resonance (LSPR) are efficient elements in the localization of light to nanometer-scale regions and enhance the light-matter interaction. The authors show that gold nanoisland (Au-NI)-loaded titanium dioxide (TiO2) photoelectrodes exhibited plasmon-enhanced photocurrent generation in the visible wavelength region, and the photocurrent action spectrum was corresponding to the LSPR band. The photocurrent enhancement may result from the plasmon-assisted electron transfer reaction from Au-NI to TiO2. A hole with high oxidn. ability was left at the TiO2 surface states near the Au-NIs/TiO2 interface, which has the potential for photocatalytic water oxidn. The photocurrent generation efficiency of Au-NIs/TiO2 photoelectrode is highly dependent on the annealing temp. for the prepn. of Au-NIs. High-resoln. transmission electron microscopy and electron energy-loss spectroscopy analyses show that the interfacial structure between Au-NI and TiO2 plays a crucial role in the photocurrent generation efficiency and photocatalytic ability.
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9. 9
  Ueno, K.; Juodkazis, S.; Shibuya, T.; Yokota, Y.; Mizeikis, V.; Sasaki, K.; Misawa, H. Nanoparticle Plasmon-Assisted Two-Photon Polymerization Induced by Incoherent Excitation Source J. Am. Chem. Soc. 2008, 130, 6928– 6929 DOI: 10.1021/ja801262r
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  9
  Nanoparticle Plasmon-Assisted Two-Photon Polymerization Induced by Incoherent Excitation Source
  Ueno, Kosei; Juodkazis, Saulius; Shibuya, Toshiyuki; Yokota, Yukie; Mizeikis, Vygantas; Sasaki, Keiji; Misawa, Hiroaki
  Journal of the American Chemical Society (2008), 130 (22), 6928-6929CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  We demonstrate the possibility to achieve optical triggering of photochem. reactions via two-photon absorption using incoherent light sources. This is accomplished by the use of arrays of gold nanoparticles, specially tailored with high precision to obtain high near-field intensity enhancement.
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10. 10
  Gao, S. Y.; Ueno, K.; Misawa, H. Plasmonic Antenna Effects on Photochemical Reactions Acc. Chem. Res. 2011, 44, 251– 260 DOI: 10.1021/ar100117w
  [ACS Full Text ], [CAS], Google Scholar
  10
  Plasmonic Antenna Effects on Photochemical Reactions
  Gao, Shuyan; Ueno, Kosei; Misawa, Hiroaki
  Accounts of Chemical Research (2011), 44 (4), 251-260CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
  A review with 45 refs. Efficient solar energy conversion has been vigorously pursued since the 1970s, but its large-scale(coating process) implementation hinges on the availability of high-efficiency modules. For max. efficiency, it is important to absorb most of the incoming radiation, which necessitates both efficient photoexcitation and minimal electron-hole recombination. To date, researchers have primarily focused on the latter difficulty: finding a strategy to effectively sep. photoinduced electrons and holes. Very few reports have been devoted to broadband sunlight absorption and photoexcitation. However, the currently available photovoltaic cells, such as metallic glasses silicon, and even single-crystal silicon and sensitized solar cells, cannot respond to the wide range of the solar spectrum. The photoelec. conversion characteristics of solar cells generally decrease in the IR wavelength range. Thus, the fraction of the solar spectrum absorbed is relatively poor. In addn., the large mismatch between the diffraction limit of light and the absorption cross-section makes the probability of interactions between photons and cell materials quite low, which greatly limits photoexcitation efficiency. Therefore, there is a pressing need for research aimed at finding conditions that lead to highly efficient photoexcitation over a wide spectrum of sunlight, particularly in the visible to near-IR wavelengths. As characterized in the emerging field of plasmonics, metallic nanostructures are endowed with optical antenna effects. These plasmonic antenna effects provide a promising platform for artificially sidestepping the diffraction limit of light and strongly enhancing absorption cross-sections. Moreover, they can efficiently excite photochem. reactions between photons and mols. close to an optical antenna through the local field enhancement. This technol. has the potential to induce highly efficient photoexcitation between photons and mols. over a wide spectrum of sunlight, from visible to near-IR wavelengths. In this Account, we describe our recent work in using metallic nanostructures to assist photochem. reactions for augmenting photoexcitation efficiency. These studies investigate the optical antenna effects of coupled plasmonic gold nanoblocks, which were fabricated with electron-beam lithog. and a lift-off technique to afford high resoln. and nanometric accuracy. The two-photon photoluminescence of gold and the resulting nonlinear photopolymn. on gold nanoblocks substantiate the existence of enhanced optical field domains. Local two-photon photochem. reactions due to weak incoherent light sources were identified. The optical antenna effects support the unprecedented realization of (i) direct photocarrier injection from the gold nanorods into TiO2 and (ii) efficient and stable photocurrent generation in the absence of electron donors from visible (450 nm) to near-IR (1300 nm) wavelengths.
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11. 11
  Brongersma, M. L.; Halas, N. J.; Nordlander, P. Plasmon-Induced Hot Carrier Science and Technology Nat. Nanotechnol. 2015, 10, 25– 34 DOI: 10.1038/nnano.2014.311
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  11
  Plasmon-induced hot carrier science and technology
  Brongersma, Mark L.; Halas, Naomi J.; Nordlander, Peter
  Nature Nanotechnology (2015), 10 (1), 25-34CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
  A review. The discovery of the photoelec. effect by Heinrich Hertz in 1887 set the foundation for over 125 years of hot carrier science and technol. In the early 1900s it played a crit. role in the development of quantum mechanics, but even today the unique properties of these energetic, hot carriers offer new and exciting opportunities for fundamental research and applications. Measurement of the kinetic energy and momentum of photoejected hot electrons can provide valuable information on the electronic structure of materials. The heat generated by hot carriers can be harvested to drive a wide range of phys. and chem. processes. Their kinetic energy can be used to harvest solar energy or create sensitive photodetectors and spectrometers. Photoejected charges can also be used to elec. dope two-dimensional materials. Plasmon excitations in metallic nanostructures can be engineered to enhance and provide valuable control over the emission of hot carriers. This Review discusses recent advances in the understanding and application of plasmon-induced hot carrier generation and highlights some of the exciting new directions for the field.
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12. 12
  Zhong, Y. Q.; Ueno, K.; Mori, Y.; Shi, X.; Oshikiri, T.; Murakoshi, K.; Inoue, H.; Misawa, H. Plasmon-Assisted Water Splitting Using Two Sides of the Same SrTiO₃ Single-Crystal Substrate: Conversion of Visible Light to Chemical Energy Angew. Chem., Int. Ed. 2014, 53, 10350– 10354 DOI: 10.1002/anie.201404926
  [Crossref], [CAS], Google Scholar
  12
  Plasmon-Assisted Water Splitting Using Two Sides of the Same SrTiO3 Single-Crystal Substrate: Conversion of Visible Light to Chemical Energy
  Zhong, Yuqing; Ueno, Kosei; Mori, Yuko; Shi, Xu; Oshikiri, Tomoya; Murakoshi, Kei; Inoue, Haruo; Misawa, Hiroaki
  Angewandte Chemie, International Edition (2014), 53 (39), 10350-10354CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A plasmon-induced water splitting system that operates under irradn. by visible light was successfully developed; the system is based on the use of both sides of the same strontium titanate (SrTiO3) single-crystal substrate. The water splitting system contains two soln. chambers to sep. hydrogen (H2) and oxygen (O2). To promote water splitting, a chem. bias was applied by regulating the pH values of the chambers. The quantity of H2 evolved from the surface of platinum, which was used as a redn. co-catalyst, was twice the quantity of O2 evolved from an Au-nanostructured surface. Thus, the stoichiometric evolution of H2 and O2 was clearly demonstrated. The hydrogen-evolution action spectrum closely corresponds to the plasmon resonance spectrum, indicating that the plasmon-induced charge sepn. at the Au/SrTiO3 interface promotes water oxidn. and the subsequent redn. of a proton on the backside of the SrTiO3 substrate. The chem. bias is significantly reduced by plasmonic effects, which indicates the possibility of constructing an artificial photosynthesis system with low energy consumption.
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13. 13
  Oshikiri, T.; Ueno, K.; Misawa, H. Plasmon-Induced Ammonia Synthesis through Nitrogen Photofixation with Visible Light Irradiation Angew. Chem., Int. Ed. 2014, 53, 9802– 9805 DOI: 10.1002/anie.201404748
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  13
  Plasmon-Induced Ammonia Synthesis through Nitrogen Photofixation with Visible Light Irradiation
  Oshikiri, Tomoya; Ueno, Kosei; Misawa, Hiroaki
  Angewandte Chemie, International Edition (2014), 53 (37), 9802-9805CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  We have successfully developed a plasmon-induced technique for ammonia synthesis that responds to visible light through a strontium titanate (SrTiO3) photoelectrode loaded with gold (Au) nanoparticles. The photoelectrochem. reaction cell was divided into two chambers to sep. the oxidized (anodic side) and reduced (cathodic side) products. To promote NH3 formation, a chem. bias was applied by regulating the pH value of these compartments, and ethanol was added to the anodic chamber as a sacrificial donor. The quantity of NH3 formed at the ruthenium surface, which was used as a co-catalyst for SrTiO3, increases linearly as a function of time under irradn. with visible light at wavelengths longer than 550 nm. The NH3 formation action spectrum approx. corresponds to the plasmon resonance spectrum. We deduced that plasmon-induced charge sepn. at the Au/SrTiO3 interface promotes oxidn. at the anodic chamber and subsequent nitrogen redn. on the cathodic side.
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14. 14
  Oshikiri, T.; Ueno, K.; Misawa, H. Selective Dinitrogen Conversion to Ammonia Using Water and Visible Light via Plasmon-Induced Charge Separation Angew. Chem., Int. Ed. 2016, 55, 3942– 3946 DOI: 10.1002/anie.201511189
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  There is no corresponding record for this reference.
15. 15
  Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P. A Hybridization Model for the Plasmon Response of Complex Nanostructures Science 2003, 302, 419– 422 DOI: 10.1126/science.1089171
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  15
  A hybridization model for the plasmon response of complex nanostructures
  Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P.
  Science (Washington, DC, United States) (2003), 302 (5644), 419-422CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
  The authors present a simple and intuitive picture, an electromagnetic analog of MO theory, that describes the plasmon response of complex nanostructures of arbitrary shape. The authors' model can be understood as the interaction or hybridization of elementary plasmons supported by nanostructures of elementary geometries. As an example, the approach is applied to the important case of a four-layer concentric nanoshell, where the hybridization of the plasmons of the inner and outer nanoshells dets. the resonant frequencies of the multilayer nanostructure.
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16. 16
  Sonnefraud, Y.; Koh, A. L.; McComb, D. W.; Maier, S. A. Nanoplasmonics: Engineering and Observation of Localized Plasmon Modes Las. Photon. Rev. 2012, 6, 277– 295 DOI: 10.1002/lpor.201100027
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  There is no corresponding record for this reference.
17. 17
  Verellen, N.; Sonnefraud, Y.; Sobhani, H.; Hao, F.; Moshchalkov, V. V.; Van Dorpe, P.; Nordlander, P.; Maier, S. A. Fano Resonances in Individual Coherent Plasmonic Nanocavities Nano Lett. 2009, 9, 1663– 1667 DOI: 10.1021/nl9001876
  [ACS Full Text ], [CAS], Google Scholar
  17
  Fano Resonances in Individual Coherent Plasmonic Nanocavities
  Verellen, Niels; Sonnefraud, Yannick; Sobhani, Heidar; Hao, Feng; Moshchalkov, Victor V.; Van Dorpe, Pol; Nordlander, Peter; Maier, Stefan A.
  Nano Letters (2009), 9 (4), 1663-1667CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
  The authors observe the appearance of Fano resonances in the optical response of plasmonic nanocavities due to the coherent coupling between their superradiant and subradiant plasmon modes. Two reduced-symmetry nanostructures probed via confocal spectroscopy, a dolmen-style slab arrangement and a ring/disk dimer, clearly exhibit the strong polarization and geometry dependence expected for this behavior at the individual nanostructure level, confirmed by full-field electrodynamic anal. of each structure. In each case, multiple Fano resonances occur as structure size is increased.
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18. 18
  Fan, J. A.; Wu, C. H.; Bao, K.; Bao, J. M.; Bardhan, R.; Halas, N. J.; Manoharan, V. N.; Nordlander, P.; Shvets, G.; Capasso, F. Self-Assembled Plasmonic Nanoparticle Clusters Science 2010, 328, 1135– 1138 DOI: 10.1126/science.1187949
  [Crossref], [PubMed], [CAS], Google Scholar
  18
  Self-Assembled Plasmonic Nanoparticle Clusters
  Fan, Jonathan A.; Wu, Chihhui; Bao, Kui; Bao, Jiming; Bardhan, Rizia; Halas, Naomi J.; Manoharan, Vinothan N.; Nordlander, Peter; Shvets, Gennady; Capasso, Federico
  Science (Washington, DC, United States) (2010), 328 (5982), 1135-1138CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
  The self-assembly of colloids is an alternative to top-down processing that enables the fabrication of nanostructures. We show that self-assembled clusters of metal-dielec. spheres are the basis for nanophotonic structures. By tailoring the no. and position of spheres in close-packed clusters, plasmon modes exhibiting strong magnetic and Fano-like resonances emerge. The use of identical spheres simplifies cluster assembly and facilitates the fabrication of highly sym. structures. Dielec. spacers are used to tailor the interparticle spacing in these clusters to be approx. 2 nm. These types of chem. synthesized nanoparticle clusters can be generalized to other two- and three-dimensional structures and can serve as building blocks for new metamaterials.
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19. 19
  Bao, Y. J.; Hu, Z. J.; Li, Z. W.; Zhu, X.; Fang, Z. Y. Magnetic Plasmonic Fano Resonance at Optical Frequency Small 2015, 11, 2177– 2181 DOI: 10.1002/smll.201402989
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  19
  Magnetic Plasmonic Fano Resonance at Optical Frequency
  Bao, Yanjun; Hu, Zhijian; Li, Ziwei; Zhu, Xing; Fang, Zheyu
  Small (2015), 11 (18), 2177-2181CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Plasmonic Fano resonances are typically understood and studied assuming elec. mode hybridization. A purely magnetic plasmon Fano resonance can be realized at optical frequency with Au split ring hexamer nanostructure excited by an azimuthally polarized incident light. Collective magnetic plasmon modes induced by the circular elec. field within the hexamer and each of the split ring can be controlled and effectively hybridized by designing the size and orientation of each ring unit. With simulated results reproducing the expt., the suggested configuration with narrow line-shape magnetic Fano resonance has significant potential applications in low-loss sensing and may serves as suitable elementary building blocks for optical metamaterials.
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20. 20
  Ye, J.; Wen, F. F.; Sobhani, H.; Lassiter, J. B.; Van Dorpe, P.; Nordlander, P.; Halas, N. J. Plasmonic Nanoclusters: Near Field Properties of the Fano Resonance Interrogated with SERS Nano Lett. 2012, 12, 1660– 1667 DOI: 10.1021/nl3000453
  [ACS Full Text ], [CAS], Google Scholar
  20
  Plasmonic Nanoclusters: Near Field Properties of the Fano Resonance Interrogated with SERS
  Ye, Jian; Wen, Fangfang; Sobhani, Heidar; Lassiter, J. Britt; Dorpe, Pol Van; Nordlander, Peter; Halas, Naomi J.
  Nano Letters (2012), 12 (3), 1660-1667CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
  While the far field properties of Fano resonances are well-known, clusters of plasmonic nanoparticles also possess Fano resonances with unique and spatially complex near field properties. Here we examine the near field properties of individual Fano resonant plasmonic clusters using surface-enhanced Raman scattering (SERS) both from mols. distributed randomly on the structure and from dielec. nanoparticles deposited at specific locations within the cluster. Cluster size, geometry, and interparticle spacing all modify the near field properties of the Fano resonance. For mols., the spatially dependent SERS response obtained from near field calcns. correlates well with the relative SERS intensities obsd. for individual clusters and for specific Stokes modes of a para-mercaptoaniline adsorbate. In all cases, the largest SERS enhancement is found when both the excitation and the Stokes shifted wavelengths overlap the Fano resonances. In contrast, for SERS from carbon nanoparticles we find that the dielec. screening introduced by the nanoparticle can drastically redistribute the field enhancement assocd. with the Fano resonance and lead to a significantly modified SERS response compared to what would be anticipated from the bare nanocluster.
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21. 21
  Alonso-Gonzalez, P.; Schnell, M.; Sarriugarte, P.; Sobhani, H.; Wu, C. H.; Arju, N.; Khanikaev, A.; Golmar, F.; Albella, P.; Arzubiaga, L.; Casanova, F.; Hueso, L. E.; Nordlander, P.; Shvets, G.; Hillenbrand, R. Real-Space Mapping of Fano Interference in Plasmonic Metamolecules Nano Lett. 2011, 11, 3922– 3926 DOI: 10.1021/nl2021366
  [ACS Full Text ], [CAS], Google Scholar
  21
  Real-Space Mapping of Fano Interference in Plasmonic Metamolecules
  Alonso-Gonzalez, Pablo; Schnell, Martin; Sarriugarte, Paulo; Sobhani, Heidar; Wu, Chihhui; Arju, Nihal; Khanikaev, Alexander; Golmar, Federico; Albella, Pablo; Arzubiaga, Libe; Casanova, Felix; Hueso, Luis E.; Nordlander, Peter; Shvets, Gennady; Hillenbrand, Rainer
  Nano Letters (2011), 11 (9), 3922-3926CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
  An unprecedented control of the spectral response of plasmonic nanoantennas has recently been achieved by designing structures that exhibit Fano resonances. This new insight is paving the way for a variety of applications, such as biochem. sensing and surface-enhanced Raman spectroscopy. Scattering-type near-field optical microscopy was used to map the spatial field distribution of Fano modes in IR plasmonic systems. The interference of narrow (dark) and broad (bright) plasmonic resonances was obsd. in real space, yielding intensity and phase toggling between different portions of the plasmonic metamols. when either their geometric sizes or the illumination wavelength is varied.
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22. 22
  Yan, C.; Martin, O. J. F. Periodicity-Induced Symmetry Breaking in a Fano Lattice: Hybridization and Tight-Binding Regimes ACS Nano 2014, 8, 11860– 11868 DOI: 10.1021/nn505642n
  [ACS Full Text ], [CAS], Google Scholar
  22
  Periodicity-Induced Symmetry Breaking in a Fano Lattice: Hybridization and Tight-Binding Regimes
  Yan, Chen; Martin, Olivier J. F.
  ACS Nano (2014), 8 (11), 11860-11868CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  The authors study exptl. and theor. the role of periodicity on the optical response of dolmen plasmonic arrays that exhibit a Fano line shape. Contrary to previous works on single nanostructures, this study deals with the in-plane near-field coupling between adjacent unit cells. By making an analogy to the electronic properties of atoms in the tight-binding model, specific behaviors of photonic states are studied numerically as a function of the structural asymmetry for different coupling directions. These predictions are verified exptl. with dark-field measurements on nanostructure arrays which exhibit high tunability and fine control of their spectral features as a function of the lattice consts. These effects, originated from symmetry-breaking and selective excitation of the subradiant mode, provide addnl. degree of freedom for tuning the spectral response and can be used for the sensitive detection of local perturbations. This study provides a general understanding of the near-field interactions in Fano resonant lattices that can be used for the design of plasmonic nanostructures and planar metamaterials.
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23. 23
  Luk’yanchuk, B.; Zheludev, N. I.; Maier, S. A.; Halas, N. J.; Nordlander, P.; Giessen, H.; Chong, C. T. The Fano Resonance in Plasmonic Nanostructures and Metamaterials Nat. Mater. 2010, 9, 707– 715 DOI: 10.1038/nmat2810
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  23
  The Fano resonance in plasmonic nanostructures and metamaterials
  Luk'yanchuk, Boris; Zheludev, Nikolay I.; Maier, Stefan A.; Halas, Naomi J.; Nordlander, Peter; Giessen, Harald; Chong, Chong Tow
  Nature Materials (2010), 9 (9), 707-715CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
  A review. Since its discovery, the asym. Fano resonance was a characteristic feature of interacting quantum systems. The shape of this resonance is distinctively different from that of conventional sym. resonance curves. Recently, the Fano resonance was found in plasmonic nanoparticles, photonic crystals, and electromagnetic metamaterials. The steep dispersion of the Fano resonance profile promises applications in sensors, lasing, switching, and nonlinear and slow-light devices.
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24. 24
  Fang, Z. Y.; Cai, J. Y.; Yan, Z. B.; Nordlander, P.; Halas, N. J.; Zhu, X. Removing a Wedge from a Metallic Nanodisk Reveals a Fano Resonance Nano Lett. 2011, 11, 4475– 4479 DOI: 10.1021/nl202804y
  [ACS Full Text ], [CAS], Google Scholar
  24
  Removing a Wedge from a Metallic Nanodisk Reveals a Fano Resonance
  Fang, Zheyu; Cai, Junyi; Yan, Zhongbo; Nordlander, Peter; Halas, Naomi J.; Zhu, Xing
  Nano Letters (2011), 11 (10), 4475-4479CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
  A wide variety of complex, multicomponent plasmonic nanostructures possess Fano resonances. Here the authors introduce a remarkably simple planar nanostructure, a single metallic nanodisk with a missing wedge-shaped slice, that supports a Fano resonance. In this geometry, the Fano line shape arises from the coupling between a hybridized plasmon resonance of the disk and a narrower quadrupolar mode supported by the edge of the missing wedge slice. As a consequence, both disk size and wedge angle control the properties of the resonance. A semianal. description of plasmon hybridization proves useful for analyzing the resulting line shape.
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25. 25
  Gallinet, B.; Martin, O. J. F. Influence of Electromagnetic Interactions on the Line Shape of Plasmonic Fano Resonances ACS Nano 2011, 5, 8999– 9008 DOI: 10.1021/nn203173r
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  25
  Influence of Electromagnetic Interactions on the Line Shape of Plasmonic Fano Resonances
  Gallinet, Benjamin; Martin, Olivier J. F.
  ACS Nano (2011), 5 (11), 8999-9008CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  The optical properties of plasmonic nanostructures supporting Fano resonances are studied with an electromagnetic theory. Contrary to the original work of Fano, this theory includes losses in the materials composing the system. A more general formula is obtained for the response of the system and general conclusions for the detn. of the resonance parameters are drawn. These predictions are verified with surface integral numerical calcns. in a broad variety of plasmonic nanostructures including dolmens, oligomers, and gratings. This work presents a robust and consistent anal. of plasmonic Fano resonances and enables the control of their line shape based on Maxwell's equations. The insights into the phys. understanding of Fano resonances gained this way will be of great interest for the design of plasmonic systems with specific spectral responses for applications such as sensing and optical metamaterials.
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26. 26
  Rahmani, M.; Luk’yanchuk, B.; Hong, M. H. Fano Resonance in Novel Plasmonic Nanostructures Las. Photon. Rev. 2013, 7, 329– 349 DOI: 10.1002/lpor.201200021
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  26
  Fano resonance in novel plasmonic nanostructures
  Rahmani, Mohsen; Luk'yanchuk, Boris; Hong, Minghui
  Laser & Photonics Reviews (2013), 7 (3), 329-349CODEN: LPRAB8; ISSN:1863-8880. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A review. Recently, a large no. of exptl. and theor. works have revealed a variety of plasmonic nanostructures with the capabilities of Fano resonance (FR) generation. Among these structures, plasmonic oligomers consisting of packed metallic nanoelements with certain configurations have been of significant interest. Oligomers can exhibit FR independently of the polarization direction based on dipole-dipole antiparallel modes without the need to excite challenging high-order modes. The purpose of this review article is to provide an overview of recent achievements on FR of plasmonic nanostructures in recent years. Meanwhile, more attention is given to the optical properties of plasmonic oligomers due to the high potential of such structures in optical spectra engineering.
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27. 27
  Coenen, T.; Schoen, D. T.; Mann, S. A.; Rodriguez, S. R. K.; Brenny, B. J. M.; Polman, A.; Brongersma, M. L. Nanoscale Spatial Coherent Control over the Modal Excitation of a Coupled Plasmonic Resonator System Nano Lett. 2015, 15, 7666– 7670 DOI: 10.1021/acs.nanolett.5b03614
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28. 28
  Liu, N.; Langguth, L.; Weiss, T.; Kastel, J.; Fleischhauer, M.; Pfau, T.; Giessen, H. Plasmonic Analogue of Electromagnetically Induced Transparency at the Drude Damping Limit Nat. Mater. 2009, 8, 758– 762 DOI: 10.1038/nmat2495
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  28
  Plasmonic analogue of electromagnetically induced transparency at the Drude damping limit
  Liu, Na; Langguth, Lutz; Weiss, Thomas; Kaestel, Juergen; Fleischhauer, Michael; Pfau, Tilman; Giessen, Harald
  Nature Materials (2009), 8 (9), 758-762CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
  In at. physics, the coherent coupling of a broad and a narrow resonance leads to quantum interference and provides the general recipe for electromagnetically induced transparency (EIT). A sharp resonance of nearly perfect transmission can arise within a broad absorption profile. These features show remarkable potential for slow light, novel sensors and low-loss metamaterials. In nanophotonics, plasmonic structures enable large field strengths within small mode vols. Therefore, combining EIT with nanoplasmonics would pave the way towards ultracompact sensors with extremely high sensitivity. Here, the authors exptl. demonstrate a nanoplasmonic analog of EIT using a stacked optical metamaterial. A dipole antenna with a large radiatively broadened linewidth is coupled to an underlying quadrupole antenna, of which the narrow linewidth is solely limited by the fundamental nonradiative Drude damping. In accordance with EIT theory, the authors achieve a very narrow transparency window with high modulation depth owing to nearly complete suppression of radiative losses.
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29. 29
  Chen, H. Y.; He, C. L.; Wang, C. Y.; Lin, M. H.; Mitsui, D.; Eguchi, M.; Teranishi, T.; Gwo, S. Far-Field Optical Imaging of a Linear Array of Coupled Gold Nanocubes: Direct Visualization of Dark Plasmon Propagating Modes ACS Nano 2011, 5, 8223– 8229 DOI: 10.1021/nn2029007
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  29
  Far-Field Optical Imaging of a Linear Array of Coupled Gold Nanocubes: Direct Visualization of Dark Plasmon Propagating Modes
  Chen, Hung-Ying; He, Chieh-Lun; Wang, Chun-Yuan; Lin, Meng-Hsien; Mitsui, Daisuke; Eguchi, Miharu; Teranishi, Toshiharu; Gwo, Shang-Jr
  ACS Nano (2011), 5 (10), 8223-8229CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  Plasmonic nanoantenna arrays hold great promise for diffraction-unlimited light localization, confinement, and transport. Linear plasmonic nanoantenna arrays composed of colloidal Au nanocubes precisely assembled using a nanomanipulation technique are reported. Direct evidence is shown of dark propagating modes in the plasmon coupling regime, allowing for transport of guided plasmon waves without far-field radiation losses. The possibility of plasmon dispersion engineering in coupled Au nanocube chains is demonstrated. By assembling a nanocube chain with 2 sections of coupled nanocubes of different intercube sepns., the effect of a band-pass nanofilter is produced.
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30. 30
  Solis, D.; Willingham, B.; Nauert, S. L.; Slaughter, L. S.; Olson, J.; Swanglap, P.; Paul, A.; Chang, W. S.; Link, S. Electromagnetic Energy Transport in Nanoparticle Chains via Dark Plasmon Modes Nano Lett. 2012, 12, 1349– 1353 DOI: 10.1021/nl2039327
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  30
  Electromagnetic Energy Transport in Nanoparticle Chains via Dark Plasmon Modes
  Solis, David, Jr.; Willingham, Britain; Nauert, Scott L.; Slaughter, Liane S.; Olson, Jana; Swanglap, Pattanawit; Paul, Aniruddha; Chang, Wei-Shun; Link, Stephan
  Nano Letters (2012), 12 (3), 1349-1353CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
  Using light to exchange information offers large bandwidths and high speeds, but the miniaturization of optical components is limited by diffraction. Converting light into electron waves in metals allows 1 to overcome this problem. However, metals are lossy at optical frequencies and large-area fabrication of nanometer-sized structures by conventional top-down methods can be cost-prohibitive. The authors show electromagnetic energy transport with Au nanoparticles that were assembled into close-packed linear chains. The small interparticle distances enabled strong electromagnetic coupling causing the formation of low-loss subradiant plasmons, which facilitated energy propagation over many micrometers. Electrodynamic calcns. confirmed the dark nature of the propagating mode and showed that disorder in the nanoparticle arrangement enhances energy transport, demonstrating the viability of using bottom-up nanoparticle assemblies for ultracompact opto-electronic devices.
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31. 31
  Kauranen, M.; Zayats, A. V. Nonlinear Plasmonics Nat. Photonics 2012, 6, 737– 748 DOI: 10.1038/nphoton.2012.244
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  Nonlinear plasmonics
  Kauranen, Martti; Zayats, Anatoly V.
  Nature Photonics (2012), 6 (11), 737-748CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
  A review. When light interacts with metal nanostructures, it can couple to free-electron excitations near the metal surface. The electromagnetic resonances assocd. with these surface plasmons depend on the details of the nanostructure, opening up opportunities for controlling light confinement on the nanoscale. The resulting strong electromagnetic fields allow weak nonlinear processes, which depend superlinearly on the local field, to be significantly enhanced. In addn. to providing enhanced nonlinear effects with ultrafast response times, plasmonic nanostructures allow nonlinear optical components to be scaled down in size. In this Review, we discuss the principles of nonlinear plasmonic effects and present an overview of their main applications, including frequency conversion, switching and modulation of optical signals, and soliton effects.
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32. 32
  Schumacher, T.; Kratzer, K.; Molnar, D.; Hentschel, M.; Giessen, H.; Lippitz, M. Nanoantenna-Enhanced Ultrafast Nonlinear Spectroscopy of a Single Gold Nanoparticle Nat. Commun. 2011, 2, 333 DOI: 10.1038/ncomms1334
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33. 33
  Celebrano, M.; Wu, X. F.; Baselli, M.; Großmann, S.; Biagioni, P.; Locatelli, A.; De Angelis, C.; Cerullo, G.; Osellame, R.; Hecht, B.; Duo, L.; Ciccacci, F.; Finazzi, M. Mode Matching in Multiresonant Plasmonic Nanoantennas for Enhanced Second Harmonic Generation Nat. Nanotechnol. 2015, 10, 412– 417 DOI: 10.1038/nnano.2015.69
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  Mode matching in multiresonant plasmonic nanoantennas for enhanced second harmonic generation
  Celebrano, Michele; Wu, Xiaofei; Baselli, Milena; Grossmann, Swen; Biagioni, Paolo; Locatelli, Andrea; De Angelis, Costantino; Cerullo, Giulio; Osellame, Roberto; Hecht, Bert; Duo, Lamberto; Ciccacci, Franco; Finazzi, Marco
  Nature Nanotechnology (2015), 10 (5), 412-417CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
  Doubly-resonant single-cryst. Au nanostructures with no axial symmetry displaying spatial mode overlap at both the excitation and 2nd harmonic wavelengths are described. The combination of these features allows the attainment of a nonlinear coeff. for 2nd harmonic generation of ∼5 × 10-10 W-1, enabling a 2nd harmonic photon yield >3 × 106 photons per s. Theor. estns. point toward the use of the nonlinear plasmonic nanoantennas as efficient platforms for label-free mol. sensing.
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34. 34
  Aouani, H.; Rahmani, M.; Navarro-Cia, M.; Maier, S. A. Third-Harmonic-Upconversion Enhancement from a Single Semiconductor Nanoparticle Coupled to a Plasmonic Antenna Nat. Nanotechnol. 2014, 9, 290– 294 DOI: 10.1038/nnano.2014.27
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  34
  Third-harmonic-upconversion enhancement from a single semiconductor nanoparticle coupled to a plasmonic antenna
  Aouani, Heykel; Rahmani, Mohsen; Navarro-Cia, Miguel; Maier, Stefan A.
  Nature Nanotechnology (2014), 9 (4), 290-294CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
  The ability to convert low-energy quanta into a quantum of higher energy is of great interest for a variety of applications, including bioimaging, drug delivery and photovoltaics. Although high conversion efficiencies can be achieved using macroscopic nonlinear crystals, upconverting light at the nanometer scale remains challenging because the subwavelength scale of materials prevents the exploitation of phase-matching processes. Light-plasmon interactions that occur in nanostructured noble metals have offered alternative opportunities for nonlinear upconversion of IR light, but conversion efficiency rates remain extremely low due to the weak penetration of the exciting fields into the metal. Here, 3rd-harmonic generation from an individual semiconductor In Sn oxide nanoparticle is significantly enhanced when coupled within a plasmonic Au dimer. The plasmonic dimer acts as a receiving optical antenna, confining the incident far-field radiation into a near field localized at its gap; the In Sn oxide nanoparticle located at the plasmonic dimer gap acts as a localized nonlinear transmitter upconverting three incident photons at frequency ω into a photon at frequency 3ω. This hybrid nanodevice provides 3rd-harmonic-generation enhancements of up to 106-fold compared with an isolated In Sn oxide nanoparticle, with an effective 3rd-order susceptibility up to 3.5 × 103 nm V-2 and conversion efficiency of 0.0007%. Also the upconverted 3rd-harmonic emission can be exploited to probe the near-field intensity at the plasmonic dimer gap.
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35. 35
  Frimmer, M.; Coenen, T.; Koenderink, A. F. Signature of a Fano Resonance in a Plasmonic Metamolecule’s Local Density of Optical States Phys. Rev. Lett. 2012, 108, 077404 DOI: 10.1103/PhysRevLett.108.077404
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  35
  Signature of a Fano resonance in a plasmonic metamolecule's local density of optical states
  Frimmer, Martin; Coenen, Toon; Koenderink, A. Femius
  Physical Review Letters (2012), 108 (7), 077404/1-077404/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
  We present measurements on plasmonic metamols. under local excitation using cathodoluminescence which show a spatial redistribution of the local d. of optical states at the same frequency where a sharp spectral Fano feature in extinction has been obsd. Our anal. model shows that both near- and far-field effects arise due to interference of the same two eigenmodes of the system. We present quant. insights both in a bare state, and in a dressed state picture that describe Fano interference either as near-field amplitude transfer between coupled bare states, or as interference of uncoupled eigenmodes in the far field. We identify the same eigenmode causing a dip in extinction to strongly enhance the radiative local d. of optical states, making it a promising candidate for spontaneous emission control.
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36. 36
  Lassiter, J. B.; Sobhani, H.; Knight, M. W.; Mielczarek, W. S.; Nordlander, P.; Halas, N. J. Designing and Deconstructing the Fano Lineshape in Plasmonic Nanoclusters Nano Lett. 2012, 12, 1058– 1062 DOI: 10.1021/nl204303d
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  36
  Designing and Deconstructing the Fano Lineshape in Plasmonic Nanoclusters
  Lassiter, J. Britt; Sobhani, Heidar; Knight, Mark W.; Mielczarek, Witold S.; Nordlander, Peter; Halas, Naomi J.
  Nano Letters (2012), 12 (2), 1058-1062CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
  By varying the relative dimensions of the central and peripheral disks of a plasmonic nanocluster, the depth of its Fano resonance can be systematically modified; spectral windows where the scattering cross section of the nanocluster is negligible can be obtained. But electron-beam excitation of the plasmon modes at specific locations within the nanocluster yields cathodoluminescence spectra with no Fano resonance. By examg. the selection rules for plasmon excitation in the context of a coupled oscillator picture, the authors provide an intuitive explanation of this behavior based on the plasmon modes obsd. for optical and electron-beam excitation in this family of nanostructures.
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37. 37
  Kubo, A.; Onda, K.; Petek, H.; Sun, Z. J.; Jung, Y. S.; Kim, H. K. Femtosecond Imaging of Surface Plasmon Dynamics in a Nanostructured Silver Film Nano Lett. 2005, 5, 1123– 1127 DOI: 10.1021/nl0506655
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  37
  Femtosecond Imaging of Surface Plasmon Dynamics in a Nanostructured Silver Film
  Kubo, Atsushi; Onda, Ken; Petek, Hrvoje; Sun, Zhijun; Jung, Yun S.; Kim, Hong Koo
  Nano Letters (2005), 5 (6), 1123-1127CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
  Light interacting with nanostructured metals excites the collective charge d. fluctuations known as surface plasmons (SP). Through excitation of the localized SP eigenmodes incident light is trapped on the nanometer spatial and femtosecond temporal scales and its field is enhanced. Here we demonstrate the imaging and quantum control of SP dynamics in a nanostructured silver film. By inducing and imaging the nonlinear two-photon photoemission from the sample with a pair of identical 10-fs laser pulses while scanning the pulse delay, we record a movie of SP fields at a rate of 330-as/frame.
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38. 38
  Schertz, F.; Schmelzeisen, M.; Mohammadi, R.; Kreiter, M.; Elmers, H. J.; Schönhense, G. Near Field of Strongly Coupled Plasmons: Uncovering Dark Modes Nano Lett. 2012, 12, 1885– 1890 DOI: 10.1021/nl204277y
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  38
  Near Field of Strongly Coupled Plasmons: Uncovering Dark Modes
  Schertz, Florian; Schmelzeisen, Marcus; Mohammadi, Reza; Kreiter, Maximilian; Elmers, Hans-Joachim; Schoenhense, Gerd
  Nano Letters (2012), 12 (4), 1885-1890CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
  Strongly coupled plasmons in a system of individual Au nanoparticles placed at sub-nm distance to a Au film (nanoparticle-on-plane, NPOP) are studied using 2 complementary single particle spectroscopy techniques. Optical scattering spectroscopy exclusively detects plasmon modes that couple to the far field via their dipole moment (bright modes). By using photoemission electron microscopy (PEEM), the authors detect in the identical NPOPs near-field modes that do not couple to the scattered far field (dark modes) and are characterized by a strongly enhanced nonlinear electron emission process. Both far- and near-field spectroscopy were carried out for identical individual nanostructures interacting via a sub-nm gap. Strongly resonant electron emission occurs at excitation wavelengths far off-resonant in the scattering spectra.
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39. 39
  Douillard, L.; Charra, F. Photoemission Electron Microscopy, A Tool for Plasmonics J. Electron Spectrosc. Relat. Phenom. 2013, 189, 24– 29 DOI: 10.1016/j.elspec.2013.03.013
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  39
  Photoemission electron microscopy, a tool for plasmonics
  Douillard, L.; Charra, F.
  Journal of Electron Spectroscopy and Related Phenomena (2013), 189 (Suppl.), 24-29CODEN: JESRAW; ISSN:0368-2048. (Elsevier B.V.)
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  Localized surface plasmon resonance (LSPR) can be supported by metallic nanoparticles and engineered nanostructures. An understanding of the spatially resolved near-field properties and dynamics of LSPR is important, but remains exptl. challenging. We report exptl. studies toward this aim using photoemission electron microscopy (PEEM) with high spatial resoln. of sub-10 nm. Various engineered gold nanostructure arrays (such as rods, nanodisk-like particles and dimers) are investigated via PEEM using near-IR (NIR) femtosecond laser pulses as the excitation source. When the LSPR wavelengths overlap the spectrum of the femtosecond pulses, the LSPR is efficiently excited and promotes multiphoton photoemission, which is correlated with the local intensity of the metallic nanoparticles in the near field. Thus, the local field distribution of the LSPR on different Au nanostructures can be directly explored and discussed using the PEEM images. In addn., the dynamics of the LSPR is studied by combining interferometric time-resolved pump-probe technique and PEEM. Detailed information on the oscillation and dephasing of the LSPR field can be obtained. The results identify PEEM as a powerful tool for accessing the near-field mapping and dynamic properties of plasmonic nanostructures.
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  Dipole and quadrupole modes are the 2 lowest orders of localized surface plasmon resonance (LSPR) eigenmodes in metallic nanoparticles. Of these 2 modes, the quadrupole mode is forbidden for sym. metallic nanoparticles excited by linearly polarized light at normal incidence. Excitation of the quadrupole mode in sym. Au nanoblocks shined with s-polarized light at oblique incidence was demonstrated. The near-field LSPR in Au nanoblocks using photoemission electron microscopy (PEEM) was probed, and at oblique incidence, the dipole and quadrupole modes can be selectively excited, in terms of near-field enhancement, by manipulating the light polarization state. By time-resolved PEEM measurements, exptl. the quadrupole mode in sym. Au nanoblocks has longer dephasing time than that of the dipole mode.
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  The authors study the far-field and near-field properties of Al nanorods fabricated by electron beam lithog. and exhibiting plasmonic resonance in the near-IR region. First, plasmonic modes within nanorod arrays can be tuned by geometrical parameters, allowing 1 to control the system transparency. Next, the light absorption in this structure is closely examd., and Al has great potential due to its unique interband transition at 800 nm. The roles of the dielec. confinement and the coupling between plasmonic resonance and the interband transition are particularly emphasized, as their adjustment can be used to switch from highly scattering particles to absorbing particles without a significant modification of the plasmonic resonance position. Finally, the authors image the plasmon-generated local field distribution in the Al nanostructures and observe, for the 1st time, the effect of the interband transition on the near-field behavior. The effect of the dielec. confinement is also numerically studied, as it is shown to play a significant role in near-field enhancement.
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  Clusters of plasmonic nanoparticles and nanostructures support Fano resonances. This spectral feature, produced by the interference between bright and dark modes of the nanoparticle cluster, is strongly dependent upon both geometry and local dielec. environment. This permits a highly sensitive tunability of the Fano dip in both wavelength and amplitude by varying cluster dimensions, geometry, and relative size of the individual nanocluster components. Plasmonic nanoclusters show an unprecedented sensitivity to dielec. environment with a local surface plasmon resonance figure of merit of 5.7, the highest yet reported for localized surface plasmon resonance sensing in a finite nanostructure.
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  The transition from isolated to collective optical modes in plasmonic oligomers is demonstrated. The resonant behavior of planar plasmonic hexamers and heptamers with gradually decreasing the interparticle gap sepn. was studied. A pronounced Fano resonance is obsd. in the plasmonic heptamer for sepns. <60 nm. The spectral characteristics change drastically upon removal of the central nanoparticle. The work paves the road toward complex hierarchical plasmonic oligomers with tailored optical properties.
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  The interaction between plasmonic resonances, sharp modes, and light in nanoscale plasmonic systems often leads to Fano interference effects. This occurs because the plasmonic excitations are usually spectrally broad and the characteristic narrow asym. Fano line-shape results upon interaction with spectrally sharper modes. By considering the plasmonic resonance in the Fano model, as opposed to previous flat continuum approaches, a simple and exact expression for the line-shape can be found. This allows the role of the width and energy of the plasmonic resonance to be properly understood. Fano resonances measured on an array of Au nanoantennas covered with PMMA, as well as the hybridization of dark with bright plasmons in nanocavities, are reproduced with a simple exact formula and without any fitting parameters.
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  The optical consts. n and k were obtained for Cu, Ag, and Au from reflection and transmission measurements on vacuum-evapd. thin films at room temp., in the spectral range 0.5-6.5 eV. The film-thickness range was 185-500 Å. Three optical measurements were inverted to obtain the film thickness d as well as n and k. The estd. error in d was ±2 Å, and that in n, k was <0.02 over most of the spectral range. The results in the film-thickness range 250-500 Å were independent of thickness, and were unchanged after vacuum annealing or aging in air. The free-electron optical effective masses and relaxation times derived from the results in the near ir agreed satisfactorily with previous values. The interband contribution to the imaginary part of the dielec. const. was obtained by subtracting the free-electron contribution. Some recent theor. calcns. were compared with the results for Cu and Au. In addn., some other recent expts. are crit. compared with the present results.
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06206.
- Caption for the supplementary animation; PEEM images of the single Au dolmen excited by femtosecond laser pulses with different central wavelengths, charge distribution calculated by FDTD simulations, experimental results of far-field and near-field spectra of Au dolmen structures with different gap distances, and FDTD simulation results of far-field and near-field spectra of Au dolmen structures with different gap distances (PDF)
- Supplementary animation of the spatial evolution of the near-field patterns on the Au dolmen structure (AVI)
- nn6b06206_si_001.pdf (1.46 MB)
- nn6b06206_si_002.avi (299.07 kb)
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Exploring Coupled Plasmonic Nanostructures in the Near Field by Photoemission Electron Microscopy

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Abstract

Results and Discussion

Characterization of Topography and Far-Field Spectra

Figure 1

Near-Field Spectra

Figure 2

Near-Field Mapping

Figure 3

Numerical Simulations and Discussion

Figure 4

Figure 5

Conclusions

Methods

Sample Fabrication and Characterization

PEEM Measurements

Numerical Simulations

Supporting Information

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Author Information

Acknowledgment

References

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Abstract

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

References

Supporting Information

Supporting Information

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