OOPS

Recent advances in ultrafast electron and X-ray diffraction have pushed imaging of structural dynamics into the femtosecond time domain, that is, the fundamental time scale of atomic motion. New physics can be reached beyond the scope of traditional diffraction or reciprocal space imaging. By exploiting the high time resolution, it has been possible to directly observe the collapse of nearly innumerable possible nuclear motions to a few key reaction modes that direct chemistry. It is this reduction in dimensionality in the transition state region that makes chemistry a transferable concept, with the same class of reactions being applicable to synthetic strategies to nearly arbitrary levels of complexity. The ability to image the underlying key reaction modes has been achieved with resolution to relative changes in atomic positions to better than 0.01 Å, that is, comparable to thermal motions. We have effectively reached the fundamental space-time limit with respect to the reaction energetics and imaging the acting forces. In the process of ensemble measured structural changes, we have missed the quantum aspects of chemistry. This perspective reviews the current state of the art in imaging chemistry in action and poses the challenge to access quantum information on the dynamics. There is the possibility with the present ultrabright electron and X-ray sources, at least in principle, to do tomographic reconstruction of quantum states in the form of a Wigner function and density matrix for the vibrational, rotational, and electronic degrees of freedom. Accessing this quantum information constitutes the ultimate demand on the spatial and temporal resolution of reciprocal space imaging of chemistry. Given the much shorter wavelength and corresponding intrinsically higher spatial resolution of current electron sources over X-rays, this Perspective will focus on electrons to provide an overview of the challenge on both the theory and the experimental fronts to extract the quantum aspects of molecular dynamics.

Purcell effect enhancement of spontaneous emission rates is demonstrated for isoelectronic trap single-photon emitters. Two-dimensional photonic crystal slabs with L3 defects were fabricated in nitrogen delta-doped GaAs. Photoluminescence spectra of each photonic crystal cavity had a series of sharp and bright lines arising from individual nitrogen luminescence centers, which was confirmed by Hanbury-Brown and Twiss measurements. The emission rates of these lines depended on cavity detuning, indicating a resonant character of the enhancement. The observed emission lifetime in the cavity was 400 ps, which would be the shortest lifetime reported so far for luminescence centers in GaAs.

Lead halide perovskites exhibit good performance in room-temperature exciton–polariton lasers and efficient flow of polariton condensates. Shaping and directing polariton condensates by confining the potential is essential for polariton-based optoelectronic devices, which have seldom been explored based on perovskite materials. Here, we investigate the trapping of polaritons in micron-sized CsPbBr3 flakes embedded in a microcavity by varying the negative detuning energy (from −36 to −172 meV) at room temperature. The confinement by the crystal edge results in quantized polariton states both below and above the condensed threshold. As the cavity is more negatively detuned (Δ ≤ −118 meV), the condensed polaritons undergo a transition from the ground state to metastable states with a finite group velocity (∼50 μm/ps at Δ = −118 meV). The metastable polariton condensates can be optically and stably driven between different polariton states by simply changing the pump fluence. The manipulations of the polariton states reveal the effective control of polariton relaxation in quantized polariton states by the underlying exciton–polariton and polariton–polariton scattering. Our findings pave the way for novel polaritonic light sources and integrated polariton devices through the trap engineering of perovskite microcavities.

There are significant recent interests in using nanophotonic structures to perform differentiation operation on images for edge detection purposes. All previous works using nanophotonic structures, however, can only operate with coherent light. Here we introduce a hybrid optoelectronic approach that enables one to use nanophotonic structures to perform differentiation operation with incoherent light. As a demonstration, we consider a photonic crystal slab structure and show that differentiation operation with incoherent light can be achieved by subtracting the optical transfer function of the structure at two different frequencies. Our method is robust to noise and directly integrable into existing imaging systems and, thus, points to a new avenue for improving image sensors using nanophotonic structures.

Confining light to sharp metal tips has become a versatile technique to study optical and electronic properties far below the diffraction limit. Particularly near-field microscopy in the mid-infrared spectral range has found a variety of applications in probing nanostructures and their dynamics. Yet, the ongoing quest for ultimately high spatial resolution down to the single-nanometer regime and quantitative three-dimensional nano-tomography depends vitally on a precise knowledge of the spatial distribution of the near fields emerging from the probe. Here, we perform finite element simulations of a tip with realistic geometry oscillating above a dielectric sample. By introducing a novel Fourier demodulation analysis of the electric field at each point in space, we reliably quantify the distribution of the near fields above and within the sample. Besides inferring the lateral field extension, which can be smaller than the tip radius of curvature, we also quantify the probing volume within the sample. Finally, we visualize the scattering process into the far field at a given demodulation order, for the first time, and shed light onto the nanoscale distribution of the near fields, and its evolution as the tip-sample distance is varied. Our work represents a crucial step in understanding and tailoring the spatial distribution of evanescent fields in optical nanoscopy.

Accurate, real-time, and noninvasive monitoring of health signals, such as heart rate, is indispensable for mobile health care. A highly sensitive and flexible photoelectric detector is the key component for collection of biological signals. Herein, a Sb2Se3-based photodiode is fabricated on a flexible polyimide substrate. The device exhibited impressive detection performance with 95 dB linear dynamic range, 83% external quantum efficiency, 0.42 A W–1 photoresponsivity, less than 10–14 A Hz–1/2 noise current, and 500 kHz −3 dB bandwidth. Moreover, thanks to the 1D crystal structure of Sb2Se3, our devices demonstrate ultrahigh flexibility with negligible performance degradation after bending at 69° for a 1000× cycle. Owing to these decent features, the flexible Sb2Se3 photodiodes are employed for additional light-source-free heart rate detection. This work opens up an avenue toward wearable Sb2Se3-based photoelectric devices, such as flexible solar cells.

Directional coupling of light in nanophotonic circuits has recently attracted increasing interest, with numerous experimental realizations based on broken rotational or mirror symmetries of the light–matter system. The most prominent underlying effect is the spin–orbit interaction of light in subwavelength structures. Unfortunately, coupling of light to such structures is, in general, very inefficient. In this work, we experimentally demonstrate an order of magnitude enhancement of the directional coupling between two nanowaveguides by means of a whispering gallery microcavity. We also show that both transverse magnetic and transverse electric modes can be used for the enhancement.

Zero reflection and complete light absorption are required in a wide range of applications ranging from sensing devices to solar heaters and photoelectrodes. However, simultaneously satisfying the requirements of the broadband spectrum, omnidirectionality, polarization insensitivity, and scalability is very challenging. Combining the light-trapping characteristics of microscale copper nanowires (Cu NWs) with the unique optical properties of carbon nanotubes (CNTs), we experimentally demonstrate a novel perfect absorber that has an average total reflectance of 0.75% over the broad 400–1000 nm wavelength range and an average specular reflectance as low as 0.1%. Importantly, our cactus-like, hierarchical structure retains a similar performance independently of light polarization and for a broad range of incident angles. We furthermore developed a model that elucidates how the Cu NW and CNT components synergistically contribute to the suppression of both specular and diffuse reflections while maximizing light absorption. Thanks to the scalability of the fabrication process, on the basis of the thermal oxidation and chemical vapor deposition methods, our broadband and omnidirectional perfect absorber exhibits a large potential for boosting the performance of many light-harnessing devices.

Many previously observed strong chiroptical effects are limited to chiral metamaterials with complex three-dimensional building blocks. Recently, chiral metamirrors consisting of planar elements are proposed to selectively reflect one particular circularly polarized light (CPL) while absorbing the other, with the reflected one either preserving or altering its spin state. However, they are limited by complicated subwavelength chiral profiles of their unit cells, which are challenging to fabricate in the visible-near-infrared (NIR) region. We present an extrinsically chiral metamirror that reflects one CPL preserving its handedness while strongly absorbing the other, reaching a circular conversion dichroism (CCD) in reflectance as large as 0.43 in the NIR region. Our polarization-conserving mirror possesses an Au pillar array and a continuous Al film separated by a SiO2 dielectric layer, where the Au pillars are periodically arranged by a rectangular lattice. The rather simple geometry of mirror significantly simplifies its fabrication in the high frequency region. The pronounced CCD originates from a chiral triad of the achiral resonator array and light’s propagation direction. Such a chiral mirror provides an efficient methodology toward handedness-selective modulation of the CPL and finds applications in planar photonic devices such as molecular spectroscopy, quantum information, and polarimetric imaging.

Strong light–matter coupling of a photon mode to tightly bound Frenkel excitons in organic materials has emerged as a versatile, room-temperature platform to study nonlinear many-particle physics and bosonic condensation. However, various aspects of the optical response of Frenkel excitons in this regime remained largely unexplored. Here, a hemispheric optical cavity filled with the fluorescent protein mCherry is utilized to address two important questions. First, combining the high quality factor of the microcavity with a well-defined mode structure allows to address whether temporal coherence in such systems can be competitive with their low-temperature counterparts. To this end, a coherence time greater than 150 ps is evidenced via interferometry, which exceeds the polariton lifetime by 2 orders of magnitude. Second, the narrow line width of the device allows to reliably trace the emission energy of the condensate with increasing particle density and thus to establish a fundamental picture that quantitatively explains the core nonlinear processes. It is found that the blue-shift of the Frenkel exciton–polaritons is largely dominated by the reduction of the Rabi splitting due to phase space filling effects, which is influenced by the redistribution of polaritons in the system. The highly coherent emission at ambient conditions establishes organic materials as a promising active medium in room-temperature polariton lasers, and the detailed insights on the nonlinearity are of great benefit toward implementing nonlinear polaritonic devices, optical switches, and lattices based on exciton–polaritons at room temperature.

Single-molecule localization microscopy is a powerful technique with vast potential to study light–matter interactions at the nanoscale. Nanostructured environments can modify the fluorescence emission of single molecules, and the induced decay-rate modification can be retrieved to map the local density of optical states (LDOS). However, the modification of the emitter’s point spread function (PSF) can lead to its mislocalization, setting a major limitation to the reliability of this approach. In this paper, we address this by simultaneously mapping the position and decay rate of single molecules and by sorting events by their decay rate and PSF size. With the help of numerical simulations, we are able to infer the dipole orientation and to retrieve the real position of mislocalized emitters. We have applied our approach of single-molecule fluorescence lifetime imaging microscopy (smFLIM) to study the LDOS modification of a silver nanowire over a field of view of ∼10 μm2 with a single-molecule localization precision of ∼15 nm. This is possible thanks to the combined use of an EMCCD camera and an array of single-photon avalanche diodes, enabling multiplexed and super-resolved fluorescence lifetime imaging.

In the strong coupling regime, photons and excitons exchange energy very rapidly such that they blend together, forming hybrid plexcitonic states. Recently, strong coupling of plasmons with a single fluorescent molecule was realized for the first time at room temperature using plasmonic nanocavities. Here, we show that electron beam excitation of a plasmonic nanocavity allows us to dynamically control the light–matter interaction forming the plexciton without changing the plasmonic nanocavity. Unlike dark-field microscopy, an electron beam has the ability to excite dark plasmons, which, due to their nonradiative nature, allow for more energy cycles between the plasmon and the emitter to survive in a very lossy system. Directly comparing effective two-dimensional (nanowire) and three-dimensional (nanosphere) plasmonic nanocavities we demonstrate that the dark modes inherently present at lower frequencies in the nanowire-nanocavity can be used as a means to extend the quantum coherence between the plasmon and emitter, opening the way toward dynamic tuning of the strong-coupling dynamics via the electron velocity. This opens new routes for dynamically addressing and controlling quantum emitters in the strong coupling regime via (the velocity of) the electrons.

A direct epitaxial approach to achieving ultrasmall and ultrabright InGaN micro light-emitting diodes (μLEDs) has been developed, leading to the demonstration of ultrasmall, ultraefficient, and ultracompact green μLEDs with a dimension of 3.6 μm and an interpitch of 2 μm. The approach does not involve any dry-etching processes which are exclusively used by any current μLED fabrication approaches. As a result, our approach has entirely eliminated any damage induced during the dry-etching processes. Our green μLED array chips exhibit a record external quantum efficiency (EQE) of 6% at ∼515 nm in the green spectral region, although our measurements have been performed on bare chips which do not have any coating, passivation, epoxy, or reflector, which are generally used for standard LED packaging in order to enhance extraction efficiency. A high luminance of >107 cd/m2 has been obtained on the μLED array bare chips. Temperature-dependent measurements show that our μLED array structure exhibits an internal quantum efficiency (IQE) of 28%. It is worth highlighting that our epitaxial approach is fully compatible with any existing microdisplay fabrication techniques.

Aluminum, with its distinctively favorable dielectric characteristics down to deep ultraviolet (UV) regime, has recently emerged as a broad-band and low-cost alternative to noble metals. However, low Q-factor resonances (Q ∼ 2–4), offered by Al nanostructures, pose a fundamental bottleneck for many practical applications. Here, we show that it is possible to realize Al-nanoantenna with remarkably large extinction cross sections and strong resonance characteristics surpassing those of their noble metal counterparts. By quenching radiation damping through far-field coherent dipolar interactions, we experimentally demonstrate exceptionally narrow line width (∼15 nm) and high Q-factor (∼27) dipolar plasmonic resonances in the blue-violet region of the optical spectrum (∼3 eV) beyond the practical operational limits of traditional plasmonic metals. To realize high Q-factor Al resonators, we introduce a novel space mapping algorithm enabling inverse design of Al nanoantenna arrays at arbitrary sub/superstrate material interfaces with diminished radiative losses. We show that radiatively coupled Al nanoantenna arrays offer remarkably high-Q factor (27 ≤ Q ≤ 53) resonances over the entire visible spectrum and readily outperform similarly optimized silver (Ag) nanoantenna arrays in green-blue-violet wavelengths (≤550 nm) and near UV regime. This report shows that it is possible to realize high Q-factor aluminum resonators by suppressing radiative losses and that Al-based plasmonics holds enormous potential as a viable and low-cost alternative to noble metals. Our inverse-design technique, on the other hand, provides a general and efficient approach in engineering of high Q-factor resonator arrays, independently from the metals and sub/superstrates used.

Carefully designed nanostructures can inspire a new type of optomechanical interactions and allow surpassing limitations set by classical diffractive optical elements. Apart from strong near-field localization, a nanostructured environment allows controlling scattering channels and might tailor many-body interactions. Here we investigate an effect of optical binding, where several particles demonstrate a collective mechanical behavior of bunching together in a light field. In contrast to classical binding, where separation distances between particles are diffraction limited, an auxiliary hyperbolic metasurface is shown here to break this barrier by introducing several controllable near-field interaction channels. Strong material dispersion of the hyperbolic metamaterial along with high spatial confinement of optical modes, which it supports, allows achieving superior tuning capabilities and efficient control over binding distances on the nanoscale. In addition, a careful choice of the metamaterial slab’s thickness enables decreasing optical binding distances by orders of magnitude compared to free space scenarios due to the multiple reflections of volumetric modes from the substrate. Auxiliary tunable metamaterials, which allow controlling collective optomechanical interactions on the nanoscale, open a venue for new investigations including collective nanofluidic interactions, triggered biochemical reactions, and many others.

Signal intensity fluctuations are a ubiquitous characteristic of single-molecule surface-enhanced Raman scattering (SERS). In this work, we observed SERS intensity fluctuations (SIFs) from single nanoparticles fully coated with an adsorbate layer. Fluctuations from dry, fully coated nanoparticles are assigned to a dynamic molecule/metal environment wherein atomic-scale reconstructions support SERS. Using super resolution imaging techniques, we were able to pinpoint the positions of the fluctuations with subparticle precision. We observed that the fluctuation events were separated spatially, temporally, and were unique to different laser excitation wavelengths and polarizations. Dual-wavelength super-resolution SERS imaging with green and red lasers reveal various classes of SIFs that occur either simultaneously or nonsimultaneously and from either the same or different location on a single nanoparticle. Similar results were seen when the particle is excited with different polarizations. This suggests that single molecule responses from several different hotspots in the same nanoparticle were readily probed. Furthermore, each nanoparticle contains multiple unique hotspots of different strengths, and resonance conditions, which are accessible by the different illumination conditions. The plasmon resonances localized by the roughness features at the nanoparticle’s surface play a significant role in the fluctuation events. Our experiments show that SERS hotspots that support single molecules are not a static feature of the nanoparticle. This information should be useful to guide future single-molecule SERS experiments.

Dynamic tuning of color filters finds numerous applications including displays or image sensors. Plasmonic resonators are subwavelength nanostructures which can tailor the phase, polarization, and amplitude of the optical field, but they are limited in color vibrancy when used as filters. In this work, birefringence-induced colors of plasmonic resonators and a fast switching thin liquid crystal cell are combined in a multicolored electrically tunable filter. With this mechanism, the color gamut of the plasmonic surface and the liquid crystal cell is mutually enhanced in order to generate all primary additive and subtractive colors with high saturation as well as different tones of white. A single filter is able to cover more than 70% of the color gamut of standard RGB filters by applying a voltage ranging between 2 and 6.5 V. This spectral selectivity is added in transmission without any loss in the image resolution. The presented approach is foreseen to be implemented in a variety of devices including miniature sensors or smart-phone cameras to enhance the color information, ultraflat multispectral imagers, wearable or head-worn displays, as well as high resolution display panels.

All-inorganic lead halide perovskites are ideal platforms to investigate the fundamental physics of the light–matter interactions, due to their strong oscillator strength at room temperature and various microstructures. In this paper, we investigated strong exciton-photon coupling and coherent photonic lasing in a same high-quality self-assembled CsPbBr3 perovskite microcuboid grown by a chemical vapor deposition method. The vacuum Rabi splitting of polariton up to 309 meV, and the exciton-like and photon-like components in low polariton states at different cavity-exciton detuning, were revealed by angle-resolved photoluminescence spectra at room temperature. Moreover, we realized a coherent photonic lasing with a high quality factor (4153) and narrow line width (0.13 nm) in the microcuboid above threshold (16 μJ/cm2), originated from population inversion. Significantly, the interference pattern of the coherent lasing through the Young’s double-slit interference method based on far-field Fourier optical system, directly indicate the parity (odd) of the lasing mode and the asymmetric electric-field distribution in the CsPbBr3 microstructure. Our work demonstrates for the first time a transition from the strong coupling regime (vertical Fabry–Pérot oscillation) to weak coupling regime (lateral Fabry–Pérot oscillation) in such self-assembled microcuboid under the competition between gain and internal loss. Based on this mechanism, a considerable promise is expected to enrich the functions of the micronanostructure photoelectric devices by precisely controlling the quality factor and gain of such microstructures.

In the past decade, advances in nanotechnology have led to the development of plasmonic nanocavities that facilitate light–matter strong coupling in ambient conditions. The most robust example is the nanoparticle-on-mirror (NPoM) structure whose geometry is controlled with subnanometer precision. The excited plasmons in such nanocavities are extremely sensitive to the exact morphology of the nanocavity, giving rise to unexpected optical behaviors. So far, most theoretical and experimental studies on such nanocavities have been based solely on their scattering and absorption properties. However, these methods do not provide a complete optical description of the nanocavities. Here, the NPoM is treated as an open nonconservative system supporting a set of photonic quasinormal modes (QNMs). By investigating the morphology-dependent optical properties of nanocavities, we propose a simple yet comprehensive nomenclature based on spherical harmonics and report spectrally overlapping bright and dark nanogap eigenmodes. The near-field and far-field optical properties of NPoMs are explored and reveal intricate multimodal interactions.

In this work, temperature-dependent optical properties of refractory plasmonic transition metal nitrides and dielectric thin films are utilized to design and realize a planar, direct-contact, nanophotonic metacavity for remote, all-optical sensing of a wide range of surface temperatures (from room temperature to above 1000 °C). The proposed hybrid metacavity device integrates the plasmonic cavity with a planar metasurface that utilizes refractory material components, namely, titanium nitride (TiN) and silicon nitride (Si3N4), and operates in a spectral wavelength window of 900–1400 nm. The unique feature of this approach is that metacativy is located directly on the hot surface, while other components are kept remote. The thermally variant optical properties of the constituent materials (TiN, Si3N4) enable metacavity operation with a strong polarization-dependent resonant reflectance response. At the cavity resonance, relative amplitude variations of above 30% are detected in the temperature-dependent reflectance spectra that act as the read-out from the experimentally demonstrated sensor. The proposed high-efficiency, planar optical refractory sensor located directly on hot surfaces also allows for great scalability. The device enables true remote all-optical measurements by keeping other ancillary systems outside of the hot ambient conditions and, therefore, is especially relevant for applications in harsh environments.

We propose and demonstrate a two-dimensional 4-bit fully optical nonvolatile memory using Ge2Sb2Te5 (GST) phase change materials, with encoding via a 1550 nm laser. Using the telecom-band laser, we are able to reach deeper into the material due to the low-loss nature of GST at this wavelength range, hence, increasing the number of optical write/read levels compared to previous demonstrations, while simultaneously staying within acceptable read/write energies (maximum 60 nJ/bit for write, depending on the number of pulses). For our experimental results, 50 ns long pulses with a 25 ns fall time, a peak power of 200 mW, and a 125 kHz repetition rate were used. We verify our design and experimental results via rigorous numerical simulations based on finite element and nucleation theory, and we successfully write and read a string of characters using direct hexadecimal encoding.

We propose an ultrafast on-chip CMOS compatible graphene plasmonic photodetector based on the photothermoelectric effect (PTE) that occurs across an entire homogeneous graphene channel and operating beyond 500 GHz. The proposed photodetector incorporates the long-range dielectric-loaded surface plasmon polariton (LR-DLSPP) waveguide with a metal stripe serving simultaneously as a plasmon supporting metallic material and one of the metal electrodes. The large in-plane component of the transverse magnetic (TM) plasmonic mode can couple efficiently to the graphene causing large electron temperature increases across an entire graphene channel with a maximum located at the metal stripe edge. As a result, the electronic temperatures exceeding 6000 K at input power of only a few tens of μW can be obtained at the telecom wavelength of 1550 nm. Even with limitations such as the melting temperature of graphene (T = 4510 K), a responsivity exceeding at least 200 A/W is achievable at a telecom wavelength of 1550 nm. It is also shown that, under certain operation conditions, the PTE channel photocurrent can be isolated from photovoltaic and p–n junction PTE contributions providing an efficient way for optimizing the overall photodetector performance.

This paper reports a method to enable, for the first time, reconfigurable control of resonance splitting of one or multiple arbitrarily selected azimuthal orders in a microring resonator. This is accomplished by inscribing Bragg gratings in photosensitive Ge23Sb7S70 chalcogenide microring resonators via a novel cavity-enhanced photoinscription process, in which injection of light at the targeted C-band resonance frequency induces a spatially varying refractive index change. The so-formed Bragg grating precisely matches the selected resonance order without introducing optical losses. Long-term room temperature stability of the photoinscribed Bragg gratings has been verified in darkness and during operation with reduced optical power levels. The Bragg gratings can be reconfigured by first erasure with flood illumination of visible light at 561 nm and subsequent reinscription. We also report controlled splitting of multiple resonances by inscribing superimposed Bragg gratings.

Metalens is one kind of two-dimensional ultrathin lenses with subwavelength artificial structures that can focus light in a compact, flexible way. However, most strategies for designing metalenses only work on one specific spin state of light (i.e., either right- or left-circularly polarized light), hindering simultaneous control of both spins. Utilizing both the Pancharatnam-Berry phase and the propagation phase, we can rationally control the phase for each spin state of light. As proof-of-concept demonstrations, here we numerically and experimentally realize the independent focusing and manipulation of both spins of light by V-antenna metasurfaces, which can be regarded as the demonstration of the photonic spin Hall effect. Our multidimensional metalens is able to focus light of different spins at designated positions along both transverse and longitudinal directions. It can be used as a polarization analyzer to distinguish the polarization state of incident light. In addition, our multifunctional metalens can act either as a convex lens or an axicon, depending on the spin of light. The demonstrated multidimensional and multifunctional metalens has versatile potentials in spin-dependent nanophotonics, ranging from optical imaging and micro/nano-object manipulation to optical sensing.

Raman enhancement techniques are essential for fundamental studies in light–matter interactions and find widespread application in microelectronics, biochemical sensing, and clinical diagnosis. Two-dimensional (2D) materials and their van der Waals heterostructures (vdWHs) are emerging rapidly as potential platforms for Raman enhancement. Here, we experimentally demonstrate a new technique of Raman enhancement driven by nonradiative energy transfer (NRET), achieving a 10-fold enhancement in the Raman intensity in a vertical vdWH comprising of a monolayer transition metal dichalcogenide (1L-TMD) placed on a multilayer SnSe2. Consequently, several weak Raman peaks become visible, which are otherwise imperceptible. We also show a strong modulation of the enhancement factor by tuning the spectral overlap between the 1L-TMD and SnSe2 through temperature variation, and the results are in remarkable agreement with a Raman polarizability model capturing the effect of NRET. The observed NRET-driven Raman enhancement is a novel mechanism that has not been experimentally demonstrated thus far and is distinct from conventional surface (SERS), tip (TERS), or interference enhanced Raman scattering (IERS) mechanisms that are driven solely by charge transfer or electric field enhancement. The mechanism can also be used in synergy with plasmonic nanostructures to achieve additional selectivity and sensitivity beyond hot spot engineering for applications like molecular detection using 2D/molecular hybrids. Our results open new avenues for engineering Raman enhancement techniques coupling the advantages of uniform enhancement accessible across a wide junction area in vertical vdWHs.

Avalanche photodiodes (APDs) on Si operating at optical communication wavelength band are crucial for the Si-based transceiver application. In this paper, we report the first O-band InAs quantum dot (QD) waveguide APDs monolithically grown on Si with a low dark current of 0.1 nA at unit gain and a responsivity of 0.234 A/W at 1.310 μm at unit gain (−5 V). In the linear gain mode, the APDs have a maximum gain of 198 and show a clear eye diagram up to 8 Gbit/s. These QD-based APDs enjoy the benefit of sharing the same epitaxial layers and processing flow as QD lasers, which could potentially facilitate the integration with laser sources on a Si platform.

Spin and orbital angular momenta of light have been a subject of fundamental interest since long ago, classically associated with circular polarization and wave vector. In recent years, extraordinary spin angular momenta in structured electromagnetic waves have been investigated, mostly in subwavelength evanescent fields at the nanoscale. Here we present an in-depth theoretical analysis of the transverse spin density and related momentum induced by mode confinement inside waveguides, with alternating spin layers governed by guided mode spatial symmetry, different from and indeed richer than that in the evanescent region outside. Furthermore, hybrid guided modes with intrinsic helicity exhibit in addition longitudinal spin density. Such fundamental features are manifested through fascinating phenomenology relevant to spin–orbit coupling in nanophotonic waveguides. Thus, guided light intrinsically carrying a wealth of spin momenta holds promise of superb devices to control spin–orbit interaction within confined geometries throughout the electromagnetic spectra.