
About the Cover:
Recent Developments and Applications of Plasmonics Editorial
Special Issue on Recent Developments and Applications of Plasmonics
Yoshimasa Kawata - ,
Greg Sun - ,
Din Ping Tsai *- , and
Anatoly Zayats
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Perspectives

Comparative Analysis of Metals and Alternative Infrared Plasmonic Materials
Wen Ting Hsieh - ,
Pin Chieh Wu - ,
Jacob B. Khurgin - ,
Din Ping Tsai - ,
Ning Liu - , and
Greg Sun *
In the past decade or two, the field of nanophotonics has seen rapid development, empowered by introducing the concepts of plasmonics and metamaterials. The enabling feature behind this progress has been the use of noble metals that exhibit negative dielectric permittivities over a wide spectral range of visible and infrared wavelengths that allowed for manipulating the light on the subwavelength scale. Consequently, numerous interesting phenomena that otherwise do not exist in nature have been demonstrated in laboratories, but their transitions to practical applications have been painfully slow due to the large ohmic losses that are inherent in all metals. Doped semiconductors with lower losses have been proposed as new plasmonic materials to replace noble metals. Because the electron densities that are introduced with the artificial doping are always a few order of magnitude lower than what naturally available in metals, their plasma frequencies are shifted considerably toward longer wavelengths to the infrared (IR). This work compares these two categories of plasmonic media in structures that support either localized or propagating surface plasmon polaritons (SPPs) in mid-IR. We have found that in both cases new plasmonic materials underperform noble metals in terms of enhancing optical field in localized SPPs and reducing SPP propagation loss in plasmonic waveguides. The cause of this subpar performance is the inherently low electron density that yields a significantly reduced plasma frequency compared to noble metals. This fundamental property associated with all new plasmonic materials dictates that, while new materials do hold a number of advantages, including tunability and ability to withstand high temperatures, noble metals, even with their ohmic losses, are not likely to be replaced in the foreseeable future.
Letters

Realization of Wafer-Scale Hyperlens Device for Sub-diffractional Biomolecular Imaging
Dasol Lee - ,
Yang Doo Kim - ,
Minkyung Kim - ,
Sunae So - ,
Hak-Jong Choi - ,
Jungho Mun - ,
Duc Minh Nguyen - ,
Trevon Badloe - ,
Jong G. Ok - ,
Kyunghoon Kim - ,
Heon Lee *- , and
Junsuk Rho *
A hyperlens is a super-resolution optical imaging device based on unique hyperbolic dispersions making the sub-diffraction-limited information on objects propagate to the far-field. Here, we propose a new device consisting of a 4-inch wafer-scale spherical hyperlens array that allows high-throughput and easy-to-handle real-time biomolecular imaging. With this proposed device, we report the first experimental demonstration of real-time sub-diffraction-limited biomolecular imaging using a hyperlens. Hippocampal neuron cells are imaged using a hyperlens at a resolution down to 151 nm, much smaller than the diffraction limit of conventional imaging systems in the visible wavelength. These wafer-scale hyperlens devices have great potential for simple, compact, and low-cost integration with conventional optics and therefore a large variety of imaging applications in biology, pathology, medical science and general nanoscience.

Three-Dimensional Multipole Rotation in Spherical Silver Nanoparticles Observed by Cathodoluminescence
Zac Thollar - ,
Carl Wadell - ,
Taeko Matsukata - ,
Naoki Yamamoto - , and
Takumi Sannomiya *
A spherical metallic nanoparticle is the simplest and most frequently used example of plasmonic nanostructures. In such a highly symmetric structure the plasmon modes consist of degenerate multipoles, which cannot be separately observed by only utilizing energy resolved means. We here demonstrate nanoscale optical field mappings of degenerate multipole modes in spherical silver nanoparticles using an angle- and polarization-resolved cathodoluminescence technique combined with scanning transmission electron microscopy. By properly selecting the detection angle and polarization, the observed optical field maps of spherical silver nanoparticles exhibit dipole or quadrupole features, depending on the energy. The angle-dependent multipole patterns visualize their three-dimensional rotation, which can be explained as superposition of the degenerate modes due to the spherical symmetry.

Epitaxial VO2 Nanostructures: A Route to Large-Scale, Switchable Dielectric Metasurfaces
Filip Ligmajer - ,
Lukáš Kejík - ,
Uddhab Tiwari - ,
Meng Qiu - ,
Joyeeta Nag - ,
Martin Konečný - ,
Tomáš Šikola - ,
Wei Jin - ,
Richard F. Haglund Jr.- ,
Kannatassen Appavoo *- , and
Dang Yuan Lei *
Metasurfaces offer unparalleled functionalities for controlling the propagation and properties of electromagnetic waves. But to transfer these functions to technological applications, it is critical to render them tunable and to enable fast control by external stimuli. In most cases, this has been realized by utilizing tunable materials combined with a top-down nanostructuring process, which is often complicated and time intensive. Here we present a novel strategy for fabricating a tunable metasurface comprising epitaxially grown nanobeams of a phase transition material, vanadium dioxide. Without the need for extensive nanolithographic fabrication, we prepared a large-area (>1 cm2), deep-subwavelength (thickness of ∼λ/40) nanostructured thin film that can control light transmission with large modulation depth, exceeding 9 dB across all telecommunication wavelength bands. Furthermore, the transmission in the “on” state remains higher than 80% from near- to mid-infrared region. This renders our metasurface useful also as a phase-shifting element, which we demonstrate by carrying out cross-polarized transmission measurements. To provide insights about the relationship between metasurface morphology and its resulting optical properties, we perform full-field three-dimensional numerical simulations as a function of width, height, and edge-to-edge separation of the epitaxial VO2 nanobeams.

Visible Metasurfaces for On-Chip Polarimetry
Pin Chieh Wu *- ,
Jia-Wern Chen - ,
Chih-Wei Yin - ,
Yi-Chieh Lai - ,
Tsung Lin Chung - ,
Chun Yen Liao - ,
Bo Han Chen - ,
Kuan-Wei Lee - ,
Chin-Jung Chuang - ,
Chih-Ming Wang *- , and
Din Ping Tsai *
Measuring the polarization state of light and determining the optical properties of chiral materials are inherently complex issues because of the requirement of consequential measurements between different orthogonal states of polarization. Here, we introduce an on-chip polarimetry based on the visible metasurfaces for addressing the issue of polarization analysis with compact components. We demonstrate integrated metasurface chips can effectively determine a set of Stokes parameters covering a broad wave-band at visible light. For the proof of concept, the optical properties of chiral materials are measured using our proposed device, while experimental verifications are convincing by comparing with the data obtained from commercial ellipsometry.
Articles

Nanoparticle-Assisted STED Nanoscopy with Gold Nanospheres
Nicolai T. Urban - ,
Matthew R. Foreman - ,
Stefan W. Hell - , and
Yonatan Sivan *
We demonstrate stimulated emission depletion (STED) microscopy with 20 nm gold nanospheres coated by fluorescently doped silica. We demonstrate significantly improved spatial resolution down to 75 nm, which is the first time that hybrid NPs are used in STED imaging beyond the diffraction limit of confocal microscopy. Unlike previous demonstrations of super-resolution with metal nanoparticles with different techniques, this 3.3-fold resolution improvement was limited only by the particle size. The STED intensity required for this is almost twice lower than for conventional STED based on dye alone, and we observe no melting or displacement of the NPs at the utilized intensities. Moreover, we show that the nanoparticles can be imaged in an aqueous environment, demonstrating the relevance to bioimaging. Finally, we also show, for the first time in this context, an up to 3-fold reduction in the rate of photobleaching compared to standard dye-based STED, thus enabling sustainably brighter images.

Relaxation of Plasmon-Induced Hot Carriers
Jun G. Liu - ,
Hui Zhang - ,
Stephan Link - , and
Peter Nordlander *
Plasmon-induced hot carrier generation has attracted much recent attention due to its promising potential in photocatalysis and other light harvesting applications. Here we develop a theoretical model for hot carrier relaxation in metallic nanoparticles using a fully quantum mechanical jellium model. Following pulsed illumination, nonradiative plasmon decay results in a highly nonthermal distribution of hot electrons and holes. Using coupled master equations, we calculate the time-dependent evolution of this carrier distribution in the presence of electron–electron, electron–photon, and electron–phonon scattering. Electron–electron relaxation is shown to be the dominant scattering mechanism and results in efficient carrier multiplication where the energy of the initial hot electron–hole pair is transferred to other multiple electron–hole pair excitations of lower energies. During this relaxation, a small but finite fraction of electrons scatter into luminescent states where they can recombine radiatively with holes by emission of photons. The energy of the emitted photons is found to follow the energies of the electrons and thus redshifts monotonically during the relaxation process. When the energies of the electrons approach the Fermi level, electron–phonon interaction becomes dominant and results in heating of the nanoparticle. We generalize the model to continuous-wave excitation and show how nonlinear effects become important when the illumination intensity increases. When the temporal spacing between incident photons is shorter than the relaxation time of the hot carriers, we predict that the photoluminescence will blueshift with increasing illumination power. Finally, we discuss the effect of the photonic density of states (Purcell factor) on the luminescence spectra.

Nonradiating Silicon Nanoantenna Metasurfaces as Narrowband Absorbers
Chi-Yin Yang - ,
Jhen-Hong Yang - ,
Zih-Ying Yang - ,
Zhong-Xing Zhou - ,
Mao-Guo Sun - ,
Viktoriia E. Babicheva - , and
Kuo-Ping Chen *
High-refractive-index (HRI) nanostructures support optically induced electric dipole (ED) and magnetic dipole (MD) modes that can be used to control scattering and achieve narrowband absorption. In this work, a high-absorptance device is proposed and realized by using amorphous silicon nanoantenna (a-Si NA) arrays that suppress backward and forward scattering with engineered structures and in particular periods. The overlap of ED and MD resonances, by designing an array with a specific period and exciting lattice resonances, is experimentally demonstrated. The absorptance of a-Si NA arrays increases 3-fold in the near-infrared range in comparison to unpatterned silicon films. Nonradiating a-Si NA arrays can achieve high absorptance with a small resonance bandwidth (Q = 11.89) at a wavelength of 785 nm. The effect is observed not only due to the intrinsic loss of material but by overlapping the ED and MD resonances.

Surface-Enhanced Infrared Absorption for the Periodic Array of Indium Tin Oxide and Gold Microdiscs: Effect of in-Plane Light Diffraction
Ryosuke Kamakura - ,
Tomoki Takeishi - ,
Shunsuke Murai *- ,
Koji Fujita - , and
Katsuhisa Tanaka
Surface-enhanced infrared absorption (SEIRA) is an important phenomenon to achieve nondestructive, simplified, and in situ high-sensitivity infrared (IR) sensors. Conventionally, metal structures with nanogaps are employed to realize the high sensitivity owing to the extremely strong field enhancement in the hot spot. Although a library of surface modifiers has been developed, the manipulation of nanogaps and immobilization of target molecules in the hot spot are still complicated. In addition, target molecules immobilized at the positions other than the hot spot have relatively low sensitivity. A periodic array with pitch comparable to the wavelength of interest is an alternative structure in which the coupling of the plasmonic mode to in-plane light diffraction provides the hybrid mode accompanied by an enhanced electric field. Although the field enhancement by the hybrid mode depends on matching between localized surface plasmon resonance (LSPR) and diffraction, the contribution of the matching to SEIRA enhancement has never been clarified. In this work, we fabricated periodic arrays of indium tin oxide (ITO) and Au microdiscs (pitch: 3 μm, diameter: 2 μm) to analyze the contribution of the hybrid mode through varied LSPR and diffraction conditions. As a result, the ITO and Au arrays demonstrate a similar plasmonic–photonic hybrid mode in the mid-IR region despite the different excitation frequency of LSPR. To estimate the effect of the hybrid mode on SEIRA enhancement, the incident angular profiles of IR spectra of the polymer layer on the ITO and Au arrays were measured. The SEIRA enhancement factors for ITO and Au arrays are comparable in the IR measurement region (2200–1400 cm–1). Our results verify that the plasmonic–photonic hybrid mode is very efficient for SEIRA enhancement, and the periodic array of microdiscs is very suitable for this application.

Plasmonic Enhanced EIT and Velocity Selective Optical Pumping Measurements with Atomic Vapor
Eliran Talker - ,
Pankaj Arora - ,
Yefim Barash - ,
Liron Stern - , and
Uriel Levy *
In this work, we experimentally observe for the first time nanoscale plasmonic enhanced Electromagnetically Induced Transparency (EIT) and Velocity Selective Optical Pumping (VSOP) effects in miniaturized Integrated Quantum Plasmonic Device (IQPD) for D2 transitions in rubidium (Rb). Our device consists of a vapor cell integrated on top of a prism coated with a thin layer of metal. This configuration is known to allow efficient excitation of Surface Plasmon Resonance (SPR). The evanescent field of the surface plasmon mode interacts with the atomic media in close vicinity to the metal. In spite of the limited interaction length between SPR and Rb atoms, the signature of EIT along with VSOP signals could be clearly observed in our miniaturized IQPD under proper conditions of pump and probe intensities. A gradual decrease in the contrast of the plasmonic enhanced EIT and VSOP signals was observed as the excitation was detuned from the SPR critical angle, due to reduction in electromagnetic field enhancement, leading to a reduced interaction of the evanescent field with the atomic vapor media. Following the demonstration of these effects, we also present a detailed model revealing the mechanisms and the origin of plasmonic enhanced EIT and VSOP effects in our system. The model, which is based on the Bloch equations, is in good agreement with the observed experimental results. The obtained results are regarded as an important step in the quest for the realization of nanoscale quantum plasmonic effects and devices.

Plasmonic Hot-Carriers in Channel-Coupled Nanogap Structure for Metal–Semiconductor Barrier Modulation and Spectral-Selective Plasmonic Monitoring
Ya-Lun Ho - ,
Yi-Hsin Tai - ,
J. Kenji Clark - ,
Zhiyu Wang - ,
Pei-Kuen Wei - , and
Jean-Jacques Delaunay *
Plasmonic hot-carriers, which are induced by plasmons at metal surfaces, can be used to convert photon energy into excited carriers over a subwavelength region and provide a new means to realize photodetection within the sub-band-gap region of semiconductor materials. However, the barrier height of the metal–semiconductor junction affects the behavior of the plasmon-induced hot-carriers and limits the electrical response of photodetection. High electrical responsivity, achieved by manipulating the barrier height using plasmon-induced hot electrons, is desired to broaden the possible applications. Here we report a plasmonic channel-coupled nanogap structure, where the barrier height of the metal–semiconductor junction is altered upon the excitation of plasmon-induced hot-carriers. The structure consists of semiconductor channels and metal slabs forming nanogaps, which sustain coupled plasmons and confine light to the semiconductor–metal interfaces. In contrast to conventional Schottky barriers and ohmic contacts, in which plasmon-induced hot-carriers and the generation of electron–hole pairs by photoabsorption cause an increase in the photocurrent, the generation of plasmon-induced hot-carriers at the resonant wavelength results in an increase in the junction barrier height and a decrease in the photocurrent induced by photoabsorption. By modifying the barrier height, the plasmon resonance can be monitored from the electrical response with a high spectral resolution and a large modulation.

Epitaxial Aluminum-on-Sapphire Films as a Plasmonic Material Platform for Ultraviolet and Full Visible Spectral Regions
Chang-Wei Cheng - ,
Yun-Jhen Liao - ,
Cheng-Yen Liu - ,
Bao-Hsien Wu - ,
Soniya S. Raja - ,
Chun-Yuan Wang - ,
Xiaoqin Li - ,
Chih-Kang Shih - ,
Lih-Juann Chen *- , and
Shangjr Gwo *
In comparison to noble metals (gold and silver), aluminum is a sustainable and widely applicable plasmonic material owing to its abundance in the Earth’s crust and compatibility with the complementary metal–oxide–semiconductor (CMOS) technology for integrated devices. Aluminum (Al) has a superior performance in the ultraviolet (UV) regime with the lowest material loss and good performance in the full visible regime. Furthermore, aluminum films can remain very stable in ambient environment due to the formation of surface native oxide (alumina) acting as a passivation layer. In this work, we develop an epitaxial growth technique for forming atomically smooth aluminum films on transparent c-plane (0001) sapphire (Al-on-Sapphire, ALOSA) by molecular-beam epitaxy (MBE). The MBE-grown ALOSA films have small plasmonic losses and enable us to fabricate and utilize high-quality plasmonic nanostructures in a variety of optical configurations (reflection, transmission, and scattering). Here, the surface roughness and crystal orientation of ALOSA films are characterized by atomic force microscopy (AFM) and X-ray diffraction (XRD). Moreover, the formation of smooth native oxide layer and abrupt heterointerfaces are investigated by transmission electron microscopy (TEM). We have also measured the optical dielectric function of epitaxial aluminum films by using spectroscopic ellipsometry (SE). These results show that the structural and optical properties of epitaxial aluminum films grown by MBE are excellent compared to polycrystalline aluminum films grown by other deposition methods. To illustrate the capability of device applications for the full visible spectrum, we demonstrate clear surface plasmon polarition (SPP) interference patterns using a series of double-groove surface interferometer structures with varied groove–groove separations under white-light illumination. Finally, we show the device performance of zinc oxide (ZnO) nanowire (UV) and indium gallium nitride (InGaN) nanorod (blue and green) plasmonic lasers prepared by using the epitaxial Al films. The measured lasing thresholds are comparable with the best available data obtained on the Ag films. According to these result, we suggest that epitaxial ALOSA films are a versatile plasmonic material platform in the UV and full visible spectral regions.

Field-Effect Tunable and Broadband Epsilon-Near-Zero Perfect Absorbers with Deep Subwavelength Thickness
Aleksei Anopchenko *- ,
Long Tao - ,
Catherine Arndt - , and
Ho Wai Howard Lee *
We report perfect light absorption due to the excitation of bound and radiative p-polarized optical modes in epsilon-near-zero (ENZ) conducting oxide nanolayers with thicknesses as thin as λENZ/100. Perfect absorption in the wavelength range of 600 nm to 2 μm may be achieved for unpatterned indium tin oxide (ITO) nanolayers with an electron density of 5 × 1020 to 2 × 1021 cm–3. Multilayer stacks of ITO nanolayers with a gradient of electron densities and optimized thicknesses enable broadband perfect absorption. The postfabrication tuning, of the perfect absorption wavelength, of 32 nm is achieved in a metal-oxide-semiconductor (MOS) geometry with applied voltage of 5 V. Such ultrathin and tunable broadband perfect absorbers have many potential applications in nonlinear flat ENZ optics, thermophotovoltaics, hot-electron generation in the ENZ regime, and other fields.

High-Performance Plasmonic Nanolasers with a Nanotrench Defect Cavity for Sensing Applications
Pi-Ju Cheng - ,
Zhen-Ting Huang - ,
Jhu-Hong Li - ,
Bo-Tsun Chou - ,
Yu-Hsun Chou - ,
Wei-Cheng Lo - ,
Kuo-Ping Chen - ,
Tien-Chang Lu *- , and
Tzy-Rong Lin *
Recent developments in small footprint plasmonic nanolasers show promise for active optical sensing with potential applications in various fields, including real-time and label-free biochemical sensing, and gas detection. In this study, we demonstrate a novel hybrid plasmonic crystal nanolaser that features a ZnO nanowire placed on Al grating surfaces with a nanotrench defect nanocavity. The lasing action of gain-assisted defect nanocavity overcomes the ohmic loss parasitically in the plasmonic nanostructures. Therefore, the plasmonic nanolaser exhibits an extremely small mode volume, a narrow linewidth Δλ, and a high Purcell factor that can facilitate the strong interaction between light and matter. This can be used as a refractive index sensor and is highly sensitive to local changes in the refractive indices of ambient materials. By careful design, the near-ultraviolet nanolaser sensors have significant sensing performances of glucose solutions, revealing a high sensitivity of 249 nm/RIU and high resolution, with a figure of merit of 1132, at the resonant wavelength of 373 nm.
Letters

Terahertz Nanoimaging of Graphene
Jiawei Zhang - ,
Xinzhong Chen - ,
Scott Mills - ,
Thomas Ciavatti - ,
Ziheng Yao - ,
Ryan Mescall - ,
Hai Hu - ,
Vyacheslav Semenenko - ,
Zhe Fei - ,
Hua Li - ,
Vasili Perebeinos - ,
Hu Tao - ,
Qing Dai - ,
Xu Du - , and
Mengkun Liu *
Accessing the nonradiative near-field electromagnetic interactions with high in-plane momentum (q) is the key to achieve super resolution imaging far beyond the diffraction limit. At far-infrared and terahertz (THz) wavelengths (e.g., 300 μm = 1 terahertz = 4 meV), the study of high q response and nanoscale near-field imaging is still a nascent research field. In this work, we report on THz nanoimaging of exfoliated single and multilayer graphene flakes by using a state-of-the-art scattering-type near-field optical microscope (s-SNOM). We experimentally demonstrated that the single layer graphene is close to a perfect near-field reflector at ambient environment, comparable to that of the noble metal films at the same frequency range. Further modeling and analysis considering the nonlocal graphene conductivity indicate that the high near-field reflectivity of graphene is a rather universal behavior: graphene operates as a perfect high-q reflector at room temperature. Our work uncovers the unique high-q THz response of graphene, which is essential for future applications of graphene in nano-optics or tip-enhanced technologies.

Strong Room-Temperature Visible Photoluminescence of Amorphous Si Nanowires Prepared by Electrodeposition in Ionic Liquids
Shibin Thomas - ,
Jeremy Mallet - ,
Florie Martineau - ,
Hervé Rinnert - , and
Michael Molinari *
Visible photoluminescence at room temperature is reported from the silicon nanowires prepared by electrodeposition in ionic liquids. Nanowires with diameters 110, 200, and 400 nm have been synthesized by a template-assisted electrodeposition using ionic liquid electrolytes. The obtained nanowires are amorphous in nature and possess robustness and good structural and compositional quality. The PL measurements at room temperature reveal a strong visible light emission from the 110 nm silicon nanowires. The strong and efficient red luminescence from the nanowires is found to be size-dependent and originates from the intrinsic radiative recombination and spatial confinement of carriers in the amorphous silicon nanowires with a thin surface oxide shell. The silicon nanowires with 110 nm diameter shows an exceptionally high quantum yield >25%. These light-emitting nanowires obtained through a simple alternative growth technique could find applications in future Si-based optoelectronic devices.

Correlated Disordered Plasmonic Nanostructures Arrays for Augmented Reality
Herve Bertin - ,
Yoann Brûlé - ,
Giovanni Magno - ,
Thomas Lopez - ,
Philippe Gogol - ,
Laetitia Pradere - ,
Boris Gralak - ,
David Barat - ,
Guillaume Demésy - , and
Beatrice Dagens *
Plasmonic resonators are excellent candidates to control reflectance of functionalized substrates. Because of their subwavelength characteristic dimensions, they can even be used to modify the color of transparent glass plates without altering the transparency quality. Their spatial arrangement must be carefully chosen so that the plates do not produce nonspecular diffraction, whatever their spatial density. We compare here the response of silver nanoparticles (NPs) arrays with different NPs sizes, spatial densities, and arrangements (periodic and correlated disordered). The effects of these geometrical parameters are analyzed in detail by measuring the reflectance and transmittance spectra in visible wavelength. We show that correlated disordered gratings attenuate diffraction effects appearing at lower spatial densities while keeping similar reflectance and transmittance responses and maintaining clear transparency of the glass plate. Promising configurations for head-up displays and applications in augmented reality emerge from this study.

Nanophotonic Platforms for Enhanced Chiral Sensing
E. Mohammadi - ,
K. L. Tsakmakidis - ,
A. N. Askarpour - ,
P. Dehkhoda - ,
A. Tavakoli - , and
H. Altug *
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Chirality plays an essential role in life, providing unique functionalities to a wide range of biomolecules, chemicals, and drugs, which makes chiral sensing and analysis critically important. The wider application of chiral sensing continues to be constrained by the involved chiral signals being inherently weak. To remedy this, plasmonic and dielectric nanostructures have recently been shown to offer a viable route for enhancing weak circular dichroism (CD) effects, but most relevant studies have thus far been ad hoc, not guided by a rigorous analytical methodology. Here, we report the first analytical treatment of CD enhancement and extraction from a chiral biolayer placed on top of a nanostructured substrate. We derive closed-form expressions of the CD and its functional dependence on the background-chiroptical response, substrate thickness and chirality, as well as on the optical chirality and intensity enhancement provided by the structure. Our results provide key insights into the trade-offs that are to be accommodated in the design and conception of optimal nanophotonic structures for enhancing CD effects for chiral molecule detection. Based on our analysis, we also introduce a practical, dielectric platform for chiral sensing featuring large CD enhancements while exhibiting vanishing chiroptical background noise.

Plug and Play Anisotropy-Based Nanothermometers
Sebastian A. Thompson *- ,
Ignacio A. Martínez - ,
Patricia Haro-González - ,
Alejandro P. Adam - ,
Daniel Jaque - ,
Jana B. Nieder *- , and
Roberto de la Rica *
Temperature is a crucial parameter in biology, nanoelectronics, nanophotonics, and microfluidics. Optical methods excel for measuring temperature because they are noninvasive, spatially accurate, and can measure real time local changes in temperature. Among these, fluorescence anisotropy-based methods are particularly advantageous because they are less affected by changes in the probe concentration and irradiation conditions. However, at physiologically relevant temperature ranges in aqueous solution, fluorescence anisotropy contrast can only be achieved with rather large fluorescent proteins such as the green fluorescent protein (GFP), which can limit the range of applications through this method. Here, we propose a method to add thermosensitivity to any protein thereby transforming them into fluorescence anisotropy-based thermoprobes. It consists of covalently attaching a dye to the protein, which increases the rotational time of the dye-protein system compared to the free dye and confers thermosensitivity to the resulting bioconjugates. With this method we transformed bovine serum albumin, glucose oxidase and catalase into nanothermothers. This also allowed us to analyze the anisotropy signal changes occurring during the catalytic cycle of catalase, as well as their correlation with the reaction exothermicity. The potential of this method ensures applicability in extending temperature measurements to any protein-based experiments.

Neural Polarimeter and Wavemeter
Einar B. Magnusson - ,
J. P. Balthasar Mueller - ,
Michael Juhl - ,
Carlos Mendoza - , and
Kristjan Leosson *
Numerous optical devices can be conveniently described in terms of a transfer function matrix formalism. An important example is the intensity-division Stokes polarimeter where four device outputs can be related to the four parameters of the Stokes vector using a linear 4 × 4 matrix transformation. In the present paper, we demonstrate how the functionality of such devices can be substantially enhanced by increasing the number of outputs and employing deep neural networks instead of the traditional linear algebra approach to establish correlations between device outputs and inputs. Specifically, we employ a neural network calibration of a metasurface-based intensity-division Stokes polarimeter with six outputs to accurately measure the four parameters of the Stokes vector of the input light across a much wider wavelength range than is afforded by a canonical linear transfer matrix model. Furthermore, the neural network model allows the device to determine the input wavelength from the measured data. We argue that nonlinear machine learning models used to fit calibration functions in this way are able to capture physical parameters that cannot be easily described using analytically derived models and that this approach is thus poised to improve the performance of a broad variety of optical sensors.

Limiting Optical Diodes Enabled by the Phase Transition of Vanadium Dioxide
Chenghao Wan - ,
Erik H. Horak - ,
Jonathan King - ,
Jad Salman - ,
Zhen Zhang - ,
You Zhou - ,
Patrick Roney - ,
Bradley Gundlach - ,
Shriram Ramanathan - ,
Randall H. Goldsmith - , and
Mikhail A. Kats *
A limiting optical diode is an asymmetric nonlinear device that is bidirectionally transparent at low power but becomes opaque when illuminated by sufficiently intense light incident from a particular direction. We explore the use of a phase-transition material, vanadium dioxide (VO2), as an active element of limiting optical diodes. The VO2 phase transition can be triggered by optical absorption, resulting in a change in refractive index orders of magnitude larger than what can be achieved with conventional nonlinearities. As a result, a limiting optical diode based on incident-direction-dependent absorption in a VO2 layer can be very thin, and can function at low powers without field enhancement, resulting in broadband operation. We demonstrate a simple thin-film limiting optical diode comprising a transparent substrate, a VO2 film, and a semitransparent metallic layer. For sufficiently high incident intensity, our proof-of-concept device realizes broadband asymmetric transmission across the near-infrared, and is approximately ten times thinner than the free-space wavelength.

Atomic Layer GaSe/MoS2 van der Waals Heterostructure Photodiodes with Low Noise and Large Dynamic Range
Arnob Islam - ,
Jaesung Lee - , and
Philip X.-L. Feng *
We report on the demonstration of atomic layer van der Waals (vdW) heterostructure photodiodes operating in the visible regime, enabled by stacking single- to few-layer n-type molybdenum disulfide (MoS2) on top of few-layer p-type gallium selenide (GaSe) crystals. The atomic layer vdW photodiode exhibits an excellent photoresponsivity of ∼3A/W at the wavelength of 532 nm when symmetric few-layer graphene (FLG) contacts with low contact resistance are used. On the other hand, for a GaSe/MoS2 photodiode with asymmetric GaSe/FLG and MoS2/gold (Au) contacts, a very low noise equivalent power of NEP ∼ 10–14 W/Hz is obtained due to dark current reduction, which demonstrates the feasibility of detecting sub-pW (<10–12 W) level optical illumination. Further, the same photodiode exhibits a large linear dynamic range of DR ≈ 70 dB due to the remarkable photocurrent to dark current ratio. These results show that not only the p–n junction formed at the interface between p-type GaSe and n-type MoS2 but also the metal–semiconductor junction with each 2D material play a pivotal role in determining the diode characteristics and photoresponse of the vdW photodiodes.

Ultrafast Spontaneous Emission from a Slot-Antenna Coupled WSe2 Monolayer
Michael S. Eggleston - ,
Sujay B. Desai - ,
Kevin Messer - ,
Seth A. Fortuna - ,
Surabhi Madhvapathy - ,
Jun Xiao - ,
Xiang Zhang - ,
Eli Yablonovitch - ,
Ali Javey - , and
Ming C. Wu *
Optical antennas can enhance the spontaneous emission rate from nanoemitters by orders of magnitude, enabling the possibility of an ultrafast, efficient, nanoscale LED. Semiconductors would be the preferred material for such a device to allow for direct high-speed modulation. However, efficient nanoscale devices are challenging to implement because of high surface recombination typical of most III–V materials. Monolayer transition metal dichalcogenides are an attractive alternative to a III–V emitter due to their intrinsically nanoscale dimensions, direct bandgap, and near-ideal surfaces resulting in high intrinsic quantum yield. In this work, we couple a nanostrip (30 nm × 250 nm) monolayer of WSe2 to a cavity-backed optical slot antenna through a self-aligned fabrication process. Photoluminescence, scattering, and lifetime measurements are used to estimate a radiative spontaneous emission rate enhancement of 318× from WSe2 monolayers coupled to on-resonance antennas. Such a huge increase in the spontaneous emission rate results in an ultrafast radiative recombination rate and a quantum yield in nanopatterned monolayers comparable to unprocessed exfoliated flakes of WSe2.

Quenching of Infrared-Active Optical Phonons in Nanolayers of Crystalline Materials by Graphene Surface Plasmons
Peter Q. Liu *- ,
John L. Reno - , and
Igal Brener *
Optical phonons are fundamental excitations in many solid-state materials and have crucial influences on numerous material properties. Therefore, achieving extrinsic control of optical phonon properties, such as the phonon frequency, lifetime and population, may lead to new ways of tailoring various material properties relevant to key technological applications. Here, we experimentally demonstrate that infrared-active optical phonons in thin (tens of nm) layers of crystalline materials such as III–V semiconductors can be significantly quenched by a monolayer graphene. The optical phonon quenching effect is attributed to the ultrafast decay of optical phonons into resonant graphene surface plasmons at a rate which is significantly higher than the intrinsic decay rate of optical phonons due to lattice anharmonicity. Our results point to a new approach to engineering optical phonon properties and potentially other related material properties. Such an approach can be applied to a wide range of materials with infrared-active optical phonons.

Coherent Control of a Plasmonic Nanoantenna Integrated on a Silicon Chip
Alba Espinosa-Soria *- ,
Elena Pinilla-Cienfuegos - ,
Francisco J. Díaz-Fernández - ,
Amadeu Griol - ,
Javier Martí - , and
Alejandro Martínez *
Illuminating a plasmonic nanoantenna by a set of coherent light beams should tremendously modify its scattering, absorption, and polarization properties, thus, enabling all-optical dynamic manipulation. However, diffraction inherently makes coherent control of isolated subwavelength-sized nanoantennas highly challenging when illuminated from free-space. Here, we overcome this limitation by placing the nanoantenna at a subwavelength distance of the output facet of silicon waveguides that provide monolithically defined paths for multibeam coherent illumination. Inspired by coherent perfect absorption (CPA) concepts, we demonstrate experimentally modulation of the nanoantenna scattering by more than 1 order of magnitude and of the on-chip transmission by >50% over a ∼200 nm bandwidth at telecom wavelengths by changing the phase between two counter-directional coherent guided beams. Moreover, we demonstrate coherent synthesis of polarization of the radiated field by illuminating the nanoantenna from orthogonal waveguides. Our finding paves the way toward coherent manipulation of nanoantennas and all-optical processing without nonlinearities in an integrated platform.

Gapless Broadband Terahertz Emission from a Germanium Photoconductive Emitter
A. Singh - ,
A. Pashkin - ,
S. Winnerl - ,
M. Helm - , and
H. Schneider *
Photoconductive terahertz (THz) emitters have been fulfilling many demands required for table-top THz time-domain spectroscopy up to 3–4 THz. In contrast to the widely used photoconductive materials such as GaAs and InGaAs, Ge is a nonpolar semiconductor characterized by a gapless transmission in the THz region due to absence of one-phonon absorption. We present here the realization of a Ge-based photoconductive THz emitter with a smooth broadband spectrum extending up to 13 THz and compare its performance with a GaAs-based analogue. We show that the spectral bandwidth of the Ge emitter is limited mainly by the laser pulse width (65 fs) and, thus, can be potentially extended to even much higher THz frequencies.

Mode-Hopping Phenomena in the InGaN-Based Core–Shell Nanorod Array Collective Lasing
Chia-Yen Huang *- ,
Tzu-Ying Tai - ,
Jing-Jie Lin - ,
Tsu-Chi Chang - ,
Che-Yu Liu - ,
Tien-Chang Lu - ,
Yuh-Renn Wu - , and
Hao-Chung Kuo
A two-dimensional monolithic InGaN-based core–shell nanorod array was excited under a varying temperature. We observed an abrupt dominant mode hopping from 3.29 to 3.41 eV in the collective lasing as the temperature decreased from T = 240 to 225 K. Photoluminescence under a near-threshold pumping density revealed the splitting of spontaneous emission between the core and the shell with a decreasing temperature. The bandgap evolution of GaN and InGaN showed opposite trends due to the interaction of temperature effects and bandgap renormalization effects. Theoretical simulation revealed the differences of gain spectra evolution between room temperature and low temperature due to the difference in the carrier dynamics. In the optically coupled nanorod array, the dominant lasing mode was not only determined by its structure, but also strongly influenced by external operating conditions. Rather than singular microcavity lasers, the coupled nanorod array demonstrated a broader parameter space for on-chip wavelength manipulation.

Anapole-Enhanced Intrinsic Raman Scattering from Silicon Nanodisks
Denis G. Baranov *- ,
Ruggero Verre - ,
Pawel Karpinski - , and
Mikael Käll *
Enhancement of inelastic light emission processes through resonant excitation usually correlates with enhanced scattering of the excitation light, as is for example typically the case for surface-enhanced fluorescence and Raman scattering from plasmonic nanostructures. Here, we demonstrate an unusual case where a reverse correlation is instead observed, that is, we measure a multifold enhancement of Raman emission along with suppressed elastic scattering. The system enabling this peculiar effect is composed of silicon nanodisks excited in the so-called anapole state, for which electric and toroidal dipoles interfere destructively in the far-field, thereby preventing elastic scattering, while the optical fields in the core of the silicon particles are enhanced, thus, amplifying light–matter interaction and Raman scattering at the Stokes-shifted emission wavelength. Our results demonstrate an unusual relation between resonances in elastic and inelastic scattering from nanostructures and suggest a route toward background-free frequency conversion devices.

Long-Range Resonant Energy Transfer Using Optical Topological Transitions in Metamaterials
Rahul Deshmukh - ,
Svend-Age Biehs - ,
Emaad Khwaja - ,
Tal Galfsky - ,
Girish S. Agarwal - , and
Vinod M. Menon *
The control and enhancement of resonance energy transfer is highly desirable for a variety of applications ranging from solar cells to spectroscopic rulers. However, the process of direct resonance energy transfer is distance dependent and limited to ∼10 nm for typical donor–acceptor pairs. Here we demonstrate long-range (∼160 nm) direct energy transfer between donor quantum dots and acceptor dye molecules through the use of an optical topological transition (OTT) in a metamaterial. The OTT in a metamaterial, modifies the density of states between the donor and acceptor, resulting in the long-range energy transfer with transfer efficiency of ∼32%. Theoretical calculation based on master-equation formalism is used to model the system and is found to be in good agreement with the experimental observation. The use of OTTs in metamaterials to enhance and control energy transfer process can have wide array of potential applications ranging from organic solar cells to quantum entanglement.

Lateral Heterogeneous Integration of Quantum Cascade Lasers
Yang Yang *- ,
Andrew Paulsen - ,
David Burghoff - ,
John L. Reno - , and
Qing Hu
Broadband terahertz radiation potentially has extensive applications, ranging from personal health care to industrial quality control and security screening. While traditional methods for broadband terahertz generation rely on bulky and expensive mode-locked lasers, frequency combs based on quantum cascade lasers (QCLs) can provide an alternative compact, high power, wideband terahertz source. QCL frequency combs incorporating a heterogeneous gain medium design can obtain even greater spectral range by having multiple lasing transitions at different frequencies. However, despite their greater spectral coverage, the comparatively low gain from such gain media lowers the maximum operating temperature and power. Lateral heterogeneous integration offers the ability to cover an extensive spectral range while maintaining the competitive performance offered from each homogeneous gain media. Here, we present the first lateral heterogeneous design for broadband terahertz generation: by combining two different homogeneous gain media, we have achieved a two-color frequency comb spaced by 1.5 THz.

Thin-Film Architectures with High Spectral Selectivity for Thermophotovoltaic Cells
Tobias Burger - ,
Dejiu Fan - ,
Kyusang Lee - ,
Stephen R. Forrest - , and
Andrej Lenert *
Thermophotovoltaic (TPV) systems are a promising technology for distributed conversion of high-temperature heat to electricity. To achieve high conversion efficiency, the transport of sub-bandgap radiation between the thermal emitter and PV cell should be suppressed. This can be achieved by recycling sub-bandgap radiation back to the emitter using a spectrally selective cell. However, conventional TPV cells exhibit limited sub-bandgap reflectance. Here we demonstrate thin-film In0.53Ga0.47As-based structures with high spectral selectivity, including record-high average sub-bandgap reflectance (96%). Selectivity is enabled by short optical paths through a high-quality material fabricated using epitaxial lift-off, high-reflectance back surfaces, and optimized interference. In addition, we use a parallel-plate TPV model to evaluate the impact of specific structural features on performance and to optimize the cell architecture. We show that a dielectric spacer between InGaAs and the Au back surface is an important feature that enables a predicted TPV efficiency above 50% (with a power output of 2.1 W/cm2), significantly higher than current TPV devices. This work provides guidelines for the design of high-efficiency, low-cost TPV generators.

High Thermo-Optic Coefficient of Silicon Oxycarbide Photonic Waveguides
Faisal Ahmed Memon *- ,
Francesco Morichetti - , and
Andrea Melloni
In this Letter, we report on the observation of a high thermo-optic coefficient (TOC) of silicon oxycarbide (SiOC) films deposited by reactive RF magnetron sputtering for integrated photonic waveguides. In the 1550 nm wavelength range, the measured TOC of SiOC is as large as 2.5 × 10–4 RIU °C–1, which is about 30 times larger than that of silica and almost twice that of silicon. Thin films of SiOC have been integrated in germanium-doped silica and silicon oxynitride conventional waveguide technology, achieving a 10× and 3× enhancement of the waveguide effective TOC, respectively. These results demonstrate the potential of SiOC for the realization of highly efficient phase actuators and low-power-consumption thermally tunable photonic integrated platforms.

Facile and Dynamic Color-Tuning Approach for Organic Light-Emitting Diodes Using Anisotropic Filters
Joshua N. Arthur - ,
David P. Forrestal - ,
Maria A. Woodruff - ,
Ajay K. Pandey - , and
Soniya D. Yambem *
Organic light-emitting diodes (OLEDs) typically have broad emission spectra, and tuning of OLED emission is often required for use in displays, lighting, and biomedical applications. In this study, we investigate a facile and dynamic color-tuning approach for the fine-tuning of OLED emission by using the anisotropic property of squaraine dye as an external thin film filter. We compared the results of the proposed tuning method with conventional methods of emission tuning, using the microcavity effect and varying emissive layer thickness. Our experiments and optical simulations show that the anisotropic filtering effect, caused by graded absorption of the squaraine layer, effectively removes the tail emission at longer wavelengths and has the highest range of color tuning as compared to the conventional methods. Our results demonstrate the potential of anisotropic filters as a simple alternative method for OLED emission tuning.

1D photonic crystal strain sensors
Tsan-Wen Lu *- ,
Chia-Cheng Wu - , and
Po-Tsung Lee *
We introduce a photonic crystal nanocavity consisting of one-dimensional periodic nanorods embedded in a deformable polydimethylsiloxane substrate, which exhibits a high-quality factor and a large wavelength response to the applied strain. By further investigating its wavelength response to nonaxial planar strains, we propose and experimentally demonstrate a sensor unit that can precisely identify different planar strains, including their application direction, type, and value. This sensor prototype with small footprint and feasible coupling with optical waveguides could provide additional flexibility for strain analysis in a wide range of fields.

Far Infrared Synchrotron Near-Field Nanoimaging and Nanospectroscopy
Omar Khatib - ,
Hans A. Bechtel *- ,
Michael C. Martin - ,
Markus B. Raschke *- , and
G. Lawrence Carr *
Scattering scanning near-field optical microscopy (s-SNOM) has emerged as a powerful imaging and spectroscopic tool for investigating nanoscale heterogeneities in biology, quantum matter, and electronic and photonic devices. However, many materials are defined by a wide range of fundamental molecular and quantum states at far-infrared (FIR) resonant frequencies currently not accessible by s-SNOM. Here we show ultrabroadband FIR s-SNOM nanoimaging and spectroscopy by combining synchrotron infrared radiation with a novel fast and low-noise copper-doped germanium (Ge:Cu) photoconductive detector. This approach of FIR synchrotron infrared nanospectroscopy (SINS) extends the wavelength range of s-SNOM to 31 μm (320 cm–1, 9.7 THz), exceeding conventional limits by an octave to lower energies. We demonstrate this new nanospectroscopic window by measuring elementary excitations of exemplary functional materials, including surface phonon polariton waves and optical phonons in oxides and layered ultrathin van der Waals materials, skeletal and conformational vibrations in molecular systems, and the highly tunable plasmonic response of graphene.

Gas-Phase Microresonator-Based Comb Spectroscopy without an External Pump Laser
Mengjie Yu *- ,
Yoshitomo Okawachi - ,
Chaitanya Joshi - ,
Xingchen Ji - ,
Michal Lipson - , and
Alexander L. Gaeta
We present a novel approach to realize microresonator-comb-based high resolution spectroscopy that combines a fiber-laser cavity with a microresonator. Although the spectral resolution of a chip-based comb source is typically limited by the free spectral range (FSR) of the microresonator, we overcome this limit by tuning the 200 GHz repetition-rate comb over one FSR via control of an integrated heater. Our dual-cavity scheme allows for self-starting comb generation without the need for conventional pump-cavity detuning while achieving a spectral resolution equal to the comb line width. We measure broadband molecular absorption spectra of acetylene by interleaving 800 spectra taken at 250 MHz per spectral step using a 60 GHz coarse-resolution spectrometer and exploit advances of an integrated heater, which can locally and rapidly change the refractive index of a microresonator with low electrical consumption (0.9 GHz/mW), which is orders of magnitude lower than a fiber-based comb. This approach offers a path toward a simple, robust, and low-power consumption CMOS-compatible platform capable of remote sensing.

10-Fold-Stack Multilayer-Grown Nanomembrane GaAs Solar Cells
Boju Gai - ,
Yukun Sun - ,
Huandong Chen - ,
Minjoo Larry Lee *- , and
Jongseung Yoon *
Multilayer-grown nanomembrane GaAs represents an enabling materials platform for cost-efficient III–V photovoltaics. Herein we present for the first time 10-fold-stack ultrathin (emitter + base: 300 nm) GaAs solar cells. Photovoltaic performance of 10-fold-stack GaAs solar cells exhibited promising uniformity, with only slight efficiency degradation, where comparatively poor short-wavelength response was mainly responsible for the slightly reduced performance in early grown materials. Secondary ion mass spectrometry revealed the concentration of p-type dopant has been changed due to the out-diffusion of beryllium, while the extent of diffusion increasingly diminished in early grown stacks because of the reduced concentration gradient as well as the decrease of beryllium diffusivity at longer annealing times. It is therefore concluded that the performance degradation in 10-fold-stack GaAs solar cells does not develop continuously throughout the growth, but instead becomes spontaneously saturated at longer growth times, providing promising outlook for the practical application of multilayer epitaxy toward cost-competitive GaAs solar cells.

Selective Pump Focusing on Individual Laser Modes in Microcavities
Jae-Hyuck Choi - ,
Sehwan Chang - ,
Kyoung-Ho Kim - ,
Wonjun Choi - ,
Soon-Jae Lee - ,
Jung Min Lee - ,
Min-Soo Hwang - ,
Jungkil Kim - ,
Seungwon Jeong - ,
Min-Kyo Seo - ,
Wonshik Choi - , and
Hong-Gyu Park *
We demonstrate selective pump focusing for highly isolated single-mode lasers in microdisk and microring cavities, and achieve lasing action from a microdisk cavity underneath a scattering medium. The spatial profile of the pumping light evolves by an iterative feedback process and is optimized to maximize the field overlap with a selected cavity mode. The high order of mode selectivity and high resolving power are obtained in a multimode cavity in the presence of significant modal overlaps. As a result of the adaptive optical pumping, we successfully achieve the efficient energy transfer to a microdisk underneath a random scattering medium and observe lasing action through the scattering medium. We believe that our selective pumping procedure will pave the way for the development of low-threshold, single-mode nanolasers embedded in various materials.
Articles

Silicon Thin-Film Solar Cells Approaching the Geometric Light-Trapping Limit: Surface Texture Inspired by Self-Assembly Processes
Asman Tamang - ,
Hitoshi Sai - ,
Vladislav Jovanov - ,
Koji Matsubara - , and
Dietmar Knipp *
A new device design of microcrystalline silicon thin-film solar cell allows for approaching the geometric light-trapping limit. The solar cell is based on triangular textured surfaces in combination with optimized front and back contacts with very low optical losses. In comparison to crystalline silicon solar cells with record energy conversion efficiency the material usage of the thin-film solar cells is reduced to 1–2%, while exhibiting the potential to achieve short circuit current densities of more than 80% of their counterparts. The short circuit current density of the thin-film solar cells is approaching the geometric light-trapping limit commonly known as the Yablonovitch limit under perpendicular incidence. The design of the solar cell is described considering the electrical and optical properties of the textured solar cell.

Sub-nanometer Thin Oxide Film Sensing with Localized Surface Phonon Polaritons
Rodrigo Berte - ,
Christopher R. Gubbin - ,
Virginia D. Wheeler - ,
Alexander J. Giles - ,
Vincenzo Giannini - ,
Stefan A. Maier - ,
Simone De Liberato - , and
Joshua D. Caldwell *
Chemical sensing methods based on surface polaritonic resonances stem from their intense near fields and resultant sensitivity to changes in local refractive index. Polar dielectric crystals (e.g., SiC, hBN) support surface phonon polaritons (SPhPs) from the mid-infrared to terahertz range with mode volumes and quality factors exceeding the best case scenario attained by plasmonic counterparts, making them strong candidates for resonant surface-enhanced infrared spectroscopy. We report on the behavior of SPhP resonances of SiC nanopillars following the incorporation of sub-nano- and nanometric coatings of Al2O3 and ZrO2 obtained by atomic layer deposition. Concurrent anomalous red- and blue-shifts of SPhP resonances were observed upon deposition of sub-nanometric Al2O3 films, with shift direction dictated by the mode position relative to the ordinary longitudinal optic phonon of Al2O3. These concurrent shifts, which are attributed to coupling to the Berreman mode of the Al2O3 layer, persist for thicker films and are correctly predicted by numerical calculations employing the measured Al2O3 permittivity. Deposition of ZrO2, whose phonon resonances are detuned from the SPhPs, also led to anomalous blue-shifts of transverse and longitudinal SPhP resonances around 900 cm–1 for films up to ∼1.5 nm, reversing to the canonical red-shift for thicker layers. These anomalous shifts were not reproduced numerically using the measured ZrO2 permittivity and suggest the existence of a localized surface state, which when modeled as a simple Lorentz oscillator, provides semiquantitative agreement with experimental results. In addition, predicted shifts for thicker ZrO2 layers may thus provide a tool for real-time monitoring of ultrathin film growth.

Controlling the Plasmonic Properties of Ultrathin TiN Films at the Atomic Level
Deesha Shah - ,
Alessandra Catellani - ,
Harsha Reddy - ,
Nathaniel Kinsey - ,
Vladimir Shalaev - ,
Alexandra Boltasseva - , and
Arrigo Calzolari *
By combining first-principles theoretical calculations and experimental optical and structural characterization such as spectroscopic ellipsometry, X-ray spectroscopy, and electron microscopy, we study the dielectric permittivity and plasmonic properties of ultrathin TiN films at an atomistic level. Our theoretical results indicate a remarkably persistent metallic character of ultrathin TiN films and a progressive red shift of the plasmon energy as the thickness of the film is reduced, which is consistent with previous experimental studies. The microscopic origin of this trend is interpreted in terms of the characteristic two-band electronic structure of the system. Surface oxidation and substrate strain are also investigated to explain the deviation of the optical properties from the ideal case. This paves the way to the realization of ultrathin TiN films with tailorable and tunable plasmonic properties in the visible range for applications in ultrathin metasurfaces and nonlinear optics.

Axial Inhomogeneity of Mg-Doped GaN Rods: A Strong Correlation among Componential, Electrical, and Optical Analyses
Sunghan Choi - ,
Hyun Gyu Song - ,
Yang-Seok Yoo - ,
Chulwon Lee - ,
Kie Young Woo - ,
Eunhyung Lee - ,
Sungwon David Roh - , and
Yong-Hoon Cho *
We systematically characterized the inhomogeneous doping properties along the c-axis of Mg-doped p-GaN microrods. Axial variation of doping concentration and electrical resistance on the p-GaN rod were measured by time-of-flight secondary-ion-mass-spectrometry and four-point probe measurements, respectively. Defects-related optical information was obtained from photoluminescence spectra together with Raman experiments revealing the change of crystal quality and strain along the rod. On the basis of a correlation of these analyses, we confirmed that Mg concentration decreased along the axial direction of the rod, leading to increasing electrical resistance. This axial Mg concentration change was revealed by green luminescence because the intensity of green luminescence sensitively varied with the doping density in both high-doping and low-doping rods. Interestingly, all the resistances at the highly doped rods were higher than the lowly doped rods due to overall mobility degradation at the high-doping rods caused by a scattering effect of increased Mg impurities and strain. All analyses provided complementary information on the p-type doping process and contribute to understanding the p-doping properties of GaN rod based photonic devices. Furthermore, our axially resolved optical spectroscopic (photoluminescence and Raman) methods can provide a facile, fast, and nondestructive way to estimate the axial doping and conductivity inhomogeneity of a Mg-doped p-GaN rod without having complex, time-consuming, and destructive structural and electrical measurements.

Plasmon 3D Electron Tomography and Local Electric-Field Enhancement of Engineered Plasmonic Nanoantennas
B. S. Archanjo *- ,
T. L. Vasconcelos - ,
B. S. Oliveira - ,
C. Song - ,
F. I. Allen - ,
C. A. Achete - , and
P. Ercius
Plasmonic nanoantennas are pushing the limits of optical imaging resolution capabilities in near-field scanning optical microscopy (NSOM). Accordingly, these techniques are driving the basic understanding of photonic and optoelectronic nanoscale devices with applications in sensing, energy conversion, solid-state lighting, and information technology. Imaging the localized surface plasmon resonance (LSPR) at the nanoscale is a key to understanding the optical responses of a given tip geometry in order to engineer better plasmonic nanoantennas for near-field experiments. In recent years the advancement of focused ion beam technology provides the ability to directly modify plasmonic structures with nanometer resolution. Also, scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS) is an established technique allowing imaging of LSPR. Specifically, the combination of these two techniques provides spectrally sensitive two-dimensional (2D) image information to better visualize and understand LSPR on the nanometer scale. This can be combined with electron tomography to provide the three-dimensional LSPR distribution. Here we demonstrate the fabrication of Au nanopyramids using helium ion microscopy, and analyze the LSPR in 3D reconstructions produced by total variation (TV)-norm minimization of a set of 2D STEM-EELS maps. Additionally, a boundary element simulation method was used to verify the experimentally observed nanopyramid LSPR modes. Finally, we show that the point-spread-functions (PSF) of LSPR mode hot spots in nanopyramids differ to local electric-field enhancement under optical excitation making direct comparison to NSOM experimental resolution difficult. However, the STEM-EELS results show how LSPR modes are influenced by the tip characteristics, which can inform the development of new nanoantenna designs.

Toward Practical Carrier Multiplication: Donor/Acceptor Codoped Si Nanocrystals in SiO2
Nguyen Xuan Chung - ,
Rens Limpens *- ,
Chris de Weerd - ,
Arnon Lesage - ,
Minoru Fujii - , and
Tom Gregorkiewicz *
Carrier multiplication (CM) is an interesting fundamental phenomenon with application potential in optoelectronics and photovoltaics, and it has been shown to be promoted by quantum confinement effects in nanostructures. However, mostly due to the short lifetimes of additional electron–hole (e-h) pairs generated by CM, major improvements of quantum dot devices that exploit CM are limited. Here we investigate CM in SiO2 solid state dispersions of phosphorus and boron codoped Si nanocrystals (NCs): an exotic variant of Si NCs whose photoluminescence (PL) emission energy, the optical bandgap, is significantly red-shifted in comparison to undoped Si NCs. By combining the results obtained by ultrafast induced absorption (IA) with PL quantum yield (PL QY) measurements, we demonstrate CM with a long (around 100 μs) lifetime of the additional e-h pairs created by the process, similar as previously reported for undoped Si NCs, but with a significantly lower CM threshold energy. This constitutes a significant step toward the practical implementation of Si-based NCs in optoelectronic devices: we demonstrate efficient CM at the energy bandgap optimal for photovoltaic conversion.

Direct Particle Tracking Observation and Brownian Dynamics Simulations of a Single Nanoparticle Optically Trapped by a Plasmonic Nanoaperture
Zhe Xu - ,
Wuzhou Song - , and
Kenneth B. Crozier *
Optical trapping using plasmonic nanoapertures has proven to be an effective means for the contactless manipulation of nanometer-sized particles under low optical intensities. These particles have included polystyrene and silica nanospheres, proteins, coated quantum dots and magnetic nanoparticles. Here we employ fluorescence microscopy to directly observe the optical trapping process, tracking the position of a polystyrene nanosphere (20 nm diameter) trapped in water by a double nanohole (DNH) aperture in a gold film. We show that position distribution in the plane of the film has an elliptical shape. Comprehensive simulations are performed to gain insight into the trapping process, including of the distributions of the electric field, temperature, fluid velocity, optical force, and potential energy. These simulations are combined with stochastic Brownian diffusion to directly model the dynamics of the trapping process, that is, particle trajectories. We anticipate that the combination of direct particle tracking experiments with Brownian motion simulations will be valuable tool for the better understanding of fundamental mechanisms underlying nanostructure-based trapping. It could thus be helpful in the development of the future novel optical trapping devices.

Rapid Voltage Sensing with Single Nanorods via the Quantum Confined Stark Effect
Omri Bar-Elli - ,
Dan Steinitz - ,
Gaoling Yang - ,
Ron Tenne - ,
Anastasia Ludwig - ,
Yung Kuo - ,
Antoine Triller - ,
Shimon Weiss - , and
Dan Oron *
This publication is Open Access under the license indicated. Learn More
Properly designed colloidal semiconductor quantum dots (QDs) have already been shown to exhibit high sensitivity to external electric fields via the quantum confined Stark effect (QCSE). Yet, detection of the characteristic spectral shifts associated with the effect of the QCSE has traditionally been painstakingly slow, dramatically limiting the sensitivity of these QD sensors to fast transients. We experimentally demonstrate a new detection scheme designed to achieve shot-noise-limited sensitivity to emission wavelength shifts in QDs, showing feasibility for their use as local electric field sensors on the millisecond time scale. This regime of operation is already potentially suitable for detection of single action potentials in neurons at a high spatial resolution.

Space–Time Metamaterials
Andrei Rogov - and
Evgenii Narimanov *
Despite more than a decade of active research, the fundamental problem of material loss remains a major obstacle in fulfilling the promise of the recently emerged fields of metamaterials and plasmonics to bring in revolutionary practical applications. In the present work, we demonstrate that the problem of strong material absorption that is inherent to plasmonic systems and metamaterials based on plasmonic components, can be addressed by utilizing the time dimension. By matching the pulse profile to the actual response of a lossy metamaterial, this approach allows to offset the effect of the material absorption. The existence of the corresponding solution relies on the fundamental property of causality, that relates the absorption in the medium to the variations in the frequency-dependent time delay introduced by the material via the Kramers–Kronig relations. We demonstrate that the proposed space-time approach can be applied to a broad range of metamaterial-based and plasmonic systems, from hyperbolic media to metal optics and new plasmonic materials.

Photothermal Heating of Plasmonic Nanoantennas: Influence on Trapped Particle Dynamics and Colloid Distribution
Steven Jones *- ,
Daniel Andrén - ,
Pawel Karpinski - , and
Mikael Käll *
Plasmonic antennas are well-known and extremely powerful platforms for optical spectroscopy, sensing, and manipulation of molecules and nanoparticles. However, resistive antenna losses, resulting in highly localized photothermal heat generation, may significantly compromise their applicability. Here we investigate how the interplay between plasmon-enhanced optical and thermal forces affects the dynamics of nanocolloids diffusing in close proximity to gold bowtie nanoantennas. The study is based on an anti-Stokes thermometry technique that can measure the internal antenna temperature with an accuracy of ∼5 K over an extended temperature range. We argue that Kapitza resistances have a significant impact on the local thermal landscape, causing an interface temperature discontinuity of up to ∼20% of the total photothermal temperature increase of the antenna studied. We then use the bowties as plasmonic optical tweezers and quantify how the antenna temperature influences the motion and distribution of nearby fluorescent colloids. We find that colloidal particle motion within the plasmonic trap is primarily dictated by a competition between enhanced optical forces and enhanced heating, resulting in a surprising insensitivity to the specific resonance properties of the antenna. Furthermore, we find that thermophoretic forces inhibit diffusion of particles toward the antenna and drive the formation of a thermal depletion shell that extends several microns. The study highlights the importance of thermal management at the nanoscale and points to both neglected problems and new opportunities associated with plasmonic photothermal effects in the context of nanoscale manipulation and analysis.

Near-Field Coupling Effects in Mie-Resonant Photonic Structures and All-Dielectric Metasurfaces
Sergey Lepeshov *- and
Yuri Kivshar
We reveal that strong near-field coupling effects can be observed for dissimilar Mie-resonant dielectric meta-atoms and demonstrate that both properties and functionalities of high-index all-dielectric photonic structures can be controlled by engineering their geometry and changing the distance between meta-atoms thus enhancing the effective magnetic response. We describe dielectric dimers, quadrumers, and metasurfaces with a staggered structure of optically induced magnetic moments (the so-called ”optical antiferromagnetism”) and also demonstrate that a strong toroidal response can be introduced in metasurfaces by engineering asymmetric nanoparticle quadrumers as building blocks for novel designs in all-dielectric resonant meta-optics.

High Responsivity and Detectivity Graphene-Silicon Majority Carrier Tunneling Photodiodes with a Thin Native Oxide Layer
Hong-Ki Park - and
Jaewu Choi *
A photocurrent amplifier operable at low bias voltages with high responsivity and detectivity is highly demanding for various optoelectronic applications. This study shows majority carrier graphene-native oxide-silicon (GOS) photocurrent amplifiers complying with the demands. The photocurrent amplification is primarily attributed to the photoinduced Schottky barrier height (SBH) lowering for majority carriers. The unavoidably formed thin native oxide layer between graphene and silicon during the wet graphene transfer process plays significant roles in lowering of the dark leakage current as well as photoinduced SBH lowering. As a result, the photocurrent to dark current ratio is as high as ∼12.7 at the optical power density of 1.45 mW cm–2. These GOS devices show a high responsivity of 5.5 AW–1 at an optical power (458 nm in wavelength) of 15 μWcm–2, which corresponds to ∼1400% quantum efficiency. Further the response speed is as fast as a few ten-microseconds. Thus, these GOS majority carrier photodiodes show the highest detectivity (2.35 × 1010 cm Hz1/2 W1–) among previously reported graphene-silicon photodiodes. However, the responsivity decreases with the optical power density due to the increasing recombination rate through the interface states proportional to the optical power density.

Impeding Exciton–Exciton Annihilation in Monolayer WS2 by Laser Irradiation
Yongjun Lee - ,
Ganesh Ghimire - ,
Shrawan Roy - ,
Youngbum Kim - ,
Changwon Seo - ,
A. K. Sood - ,
Joon I. Jang - , and
Jeongyong Kim *
Monolayer (1L) transition metal dichalcogenides (TMDs) are two-dimensional direct-bandgap semiconductors with promising applications of quantum light emitters. Recent studies have shown that intrinsically low quantum yields (QYs) of 1L-TMDs can be greatly improved by chemical treatments. However, nonradiative exciton–exciton annihilation (EEA) appears to significantly limit light emission of 1L-TMDs at a nominal density of photoexcited excitons due to strong Coulomb interaction. Here we show that the EEA rate constant (γ) can be reduced by laser irradiation treatment in mechanically exfoliated monolayer tungsten disulfide (1L-WS2), causing significantly improved light emission at the saturating optical pumping level. Time-resolved photoluminescence (PL) measurements showed that γ reduced from 0.66 ± 0.15 cm2/s to 0.20 ± 0.05 cm2/s simply using our laser irradiation. The laser-irradiated region exhibited lower PL response at low excitation levels, however at the high excitation level displayed 3× higher PL intensity and QY than the region without laser treatment. The shorter PL lifetime and lower PL response at low excitation levels suggested that laser irradiation increased the density of sulfur vacancies of 1L-WS2, but we attribute these induced defects, adsorbed by oxygen in air, to the origin for reduced EEA by hindering exciton diffusion. Our laser irradiation was likewise effective for reducing EEA and increasing PL of chemically treated 1L-WS2 with a high QY, exhibiting the general applicability of our method. Our results suggest that exciton–exciton interaction in 1L-TMDs may be conveniently controlled by the laser treatment, which may lead to unsaturated exciton emission at high excitation levels.

Continuous Frequency Tuning with near Constant Output Power in Coupled Y-Branched Terahertz Quantum Cascade Lasers with Photonic Lattice
Iman Kundu *- ,
Paul Dean - ,
Alexander Valavanis - ,
Joshua R. Freeman - ,
Mark C. Rosamond - ,
Lianhe Li - ,
Yingjun Han - ,
Edmund H. Linfield - , and
A. Giles Davies
This publication is Open Access under the license indicated. Learn More
We demonstrate continuous frequency tuning in terahertz quantum cascade lasers with double metal waveguides using a Y-branched coupler. Two THz QCLs placed side-by-side couple by evanescent fields across the air gap between them. Each QCL waveguide comprises a 48-μm-wide coupler and S-bend section, which are connected to an 88-μm-wide Y-branch through an impedance matching tapered section. Photonic lattices are patterned on top of the coupler section in each QCL using focused ion-beam milling to control the spectral characteristics. The waveguide design used for individual QCL sections is optimized using finite element modeling and the spectral characteristics are modeled using a transfer matrix model. Continuous frequency tuning of ∼19 GHz is demonstrated while maintaining an output power of ∼4.2–4.8 mW and a heat sink temperature of 50 K. The tuning is controlled electrically through Stark shift and cavity pulling effects by driving both QCLs simultaneously and represents the widest electrically controlled continuous tuning performance from a THz QCL without significant change in output power.

Organic Photodiodes with an Extended Responsivity Using Ultrastrong Light–Matter Coupling
Elad Eizner *- ,
Julien Brodeur - ,
Fábio Barachati - ,
Aravindan Sridharan - , and
Stéphane Kéna-Cohen *
In organic photodiodes (OPDs), light is absorbed by excitons that dissociate to generate photocurrent. Here, we demonstrate a novel type of OPD in which light is absorbed by polaritons, hybrid light–matter states. We demonstrate polariton OPDs operating in the ultrastrong coupling regime at visible and infrared wavelengths. These devices can be engineered to show narrow responsivity with a very weak angle-dependence. More importantly, they can be tuned to operate in a spectral range outside that of the bare exciton absorption. Remarkably, we show that the responsivity of a polariton OPD can be pushed to near-infrared wavelengths, where few organic absorbers are available, with external quantum efficiencies exceeding those of our control OPD.

Two-Dimensional Multimode Terahertz Random Lasing with Metal Pillars
Yongquan Zeng - ,
Guozhen Liang - ,
Bo Qiang - ,
Kedi Wu - ,
Jing Tao - ,
Xiaonan Hu - ,
Lianhe Li - ,
Alexander Giles Davies - ,
Edmund Harold Linfield - ,
Hou Kun Liang - ,
Ying Zhang - ,
Yidong Chong - , and
Qi Jie Wang *
Random lasers employing multiple scattering and interference processes in highly disordered media have been studied for several decades. However, it remains a challenge to achieve a broadband multimode random laser with high scattering efficiency, particularly at long wavelengths. Here, we develop a new class of strongly multimode random lasers in the terahertz (THz) frequency range in which optical feedback is provided by multiple scattering from metal pillars embedded in a quantum cascade (QC) gain medium. Compared with the dielectric pillars or air hole approaches used in previous random lasers, metal pillars provide high scattering efficiency over a broader range of frequencies and with low ohmic losses. Complex emission spectra are observed with over 25 emission peaks across a 0.4 THz frequency range, limited primarily by the gain bandwidth of the QC wafer employed. The experimental results are corroborated by numerical simulations that show the lasing modes are strongly localized.

Tailoring Multipolar Mie Scattering with Helicity and Angular Momentum
Xavier Zambrana-Puyalto *- ,
Xavier Vidal - ,
Paweł Woźniak - ,
Peter Banzer - , and
Gabriel Molina-Terriza *
Linear scattering processes are usually described as a function of the parameters of the incident beam. The wavelength, the intensity distribution, the polarization or the phase are among them. Here, we discuss and experimentally demonstrate how the angular momentum and the helicity of light influence the light scattering of spherical particles. We measure the backscattering of a 4 μm diameter TiO2 single particle deposited on a glass substrate. The particle is probed at different wavelengths by different beams with total angular momenta ranging from −8 to +8 units. It is observed that the spectral behavior of the particle is highly dependent on the angular momentum and helicity of the incoming beam. While some of the properties of the scattered field can be described with a simple resonator model, the scattering of high angular momentum beams requires a deeper understanding of the multipolar modes induced in the sphere. We observe that tailoring these induced multipolar modes can cause a shift and a spectral narrowing of the peaks of the scattering spectrum. Furthermore, specific combinations of helicity and angular momentum for the excitation lead to differences in the conservation of helicity by the system, which has clear consequences on the scattering pattern.

Plasmonic Manipulation of Targeted Metallic Particles by Polarization-Sensitive Metalens
Xianyou Wang - ,
Yanmeng Dai - ,
Yuquan Zhang *- ,
Changjun Min - , and
Xiaocong Yuan *
As a tool in the manipulation of micro- and nano-objects, optical tweezers are found in applications in many areas. However, selective trapping still poses challenges. Recently, a meta-surface technique offers an approach to improve optical trapping and manipulation capabilities. Here, we demonstrate the selective trapping of metallic nanoparticles with tailored plasmonic fields using a polarization sensitive metalens. We show, both by theory and experiments, modulated trapping and antitrapping forces when beam polarizations are tuned. Combining the effects of two orthogonal circular polarizations, single target particles were stably trapped in the center, while all other particles were repelled. This particle isolation points toward targeted manipulations that may find applications in single-particle assistant molecular Raman detection and assembly of plasmonic structures.

Picosecond Random Lasing Based on Three-Photon Absorption in Organometallic Halide CH3NH3PbBr3 Perovskite Thin Films
Guoen Weng - ,
Juanjuan Xue - ,
Jiao Tian - ,
Xiaobo Hu - ,
Xumin Bao - ,
Hechun Lin - ,
Shaoqiang Chen *- ,
Ziqiang Zhu - , and
Junhao Chu
Organometallic halide perovskites have been demonstrated to be very promising for nonlinear optics and practical frequency upconversion devices in integrated photonics. In this work, high quality organometallic halide CH3NH3PbBr3 perovskite thin films were synthesized through a solution-based one-step spin-coating method. With femtosecond optical pumping at 1300 nm, frequency-upconverted random lasing (RL) from the bromide perovskite films were achieved via three-photon (3P) absorption processes. The RL spectra show no spikes due to the large scattering mean-free path in the perovskite crystals, meaning the incoherent RL emission with incoherent feedback. In comparison with the one-photon pumped situation, it is found that both the two-photon and 3P excitations are more effective in reducing the RL threshold, despite the low conversion efficiency of their nonlinear multiphoton schemes. Moreover, the time- and spectral-resolved lasing characteristics of the laser pulses were systematically explored by time-resolved photoluminescence based on an optical Kerr-gate method. The measured ultrashort 3.1 ps output pulse is the shortest one that has been observed so far in bromide perovskite random lasers, without any postprocessing. In addition, wavelength dependence of the pulse width and delay time of the RL pulses were clearly demonstrated, and could be unravelled by intraband carrier relaxation dynamics, which is an important physical mechanism in ultrafast lasers. Our results demonstrate that organometallic halide perovskites are excellent gain medium for high-performance frequency upconversion random lasers and have great potential for use in gain-switched semiconductor lasers with ultrashort output pulses and tunable emission wavelengths across the entire visible spectrum.

Plasmonic Enhancement of Two-Photon-Excited Luminescence of Single Quantum Dots by Individual Gold Nanorods
Weichun Zhang - ,
Martín Caldarola - ,
Xuxing Lu - , and
Michel Orrit *
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Plasmonic enhancement of two-photon-excited fluorescence is not only of fundamental interest but also appealing for many bioimaging and photonic applications. The high peak intensity required for two-photon excitation may cause shape changes in plasmonic nanostructures, as well as transient plasmon broadening. Yet, in this work, we report on strong enhancement of the two-photon-excited photoluminescence of single colloidal quantum dots close to isolated chemically synthesized gold nanorods. Upon resonant excitation of the localized surface plasmon resonance, a gold nanorod can enhance the photoluminescence of a single quantum dot more than 10 000-fold. This strong enhancement arises from the combined effect of local field amplification and the competition between radiative and nonradiative decay rate enhancements, as is confirmed by time-resolved fluorescence measurements and numerical simulations.
Additions and Corrections
Correction to “Free Carrier Front Induced Indirect Photonic Transitions: A New Paradigm for Frequency Manipulation on Chip
Mahmoud A. Gaafar *- ,
Alexander Yu. Petrov - , and
Manfred Eich
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