Directional emission from a single plasmonic scatterer

Magnetic transverse unidirectional scattering and longitudinal displacement sensing in silicon nanodimer

Unidirectional scattering, crucial for manipulating light at the nanoscale, has wide-ranging applications from optical manipulation to sensing. While traditionally achieved through interactions between electric multipoles or between electric and magnetic multipoles, reports on unidirectional scattering driven purely by magnetic multipoles are limited. In this study, we undertake a theoretical exploration of transverse unidirectional scattering induced by magnetic multipoles, employing tightly focused azimuthally polarized beams (APBs) in interaction with a silicon nanodimer comprising two non-concentric nanorings. Through numerical simulations and theoretical analysis, we validate the transverse unidirectional scattering, predominantly governed by magnetic dipolar and quadrupolar resonances. Moreover, the directionality of this unidirectional scattering shows a strong correlation with the longitudinal displacement of the nanodimer within a specific range, showcasing its potential for longitudinal displacement sensing. Our study advances optical scattering control in nanostructures and guides the design of on-chip longitudinal displacement sensors.

Cathodoluminescence Nanoscopy: State of the Art and Beyond

Cathodoluminescence (CL) nanoscopy is proven to be a powerful tool to explore nanoscale optical properties, whereby free electron beams achieve a spatial resolution far beyond the diffraction limit of light. With developed methods for the control of electron beams and the collection of light, the dimension of information that CL can access has been expanded to include polarization, momentum, and time, holding promise to provide invaluable insights into the study of materials and optical near-field dynamics. With a focus on the burgeoning field of CL nanoscopy, this perspective outlines the recent advancements and applications of this technique, as illustrated by the salient experimental works. In addition, as an outlook for future research, several appealing directions that may bring about developments and discoveries are highlighted.

Electron-Induced Chirality-Selective Routing of Valley Photons via Metallic Nanostructure

Valleytronics in 2D transition metal dichalcogenides has raised a great impact in nanophotonic information processing and transport as it provides the pseudospin degree of freedom for carrier control. The imbalance of carrier occupation in inequivalent valleys can be achieved by external stimulations such as helical light and electric field. With metasurfaces, it is feasible to separate the valley exciton in real space and momentum space, which is significant for logical nanophotonic circuits. However, the control of valley-separated far-field emission by a single nanostructure is rarely reported, despite the fact that it is crucial for subwavelength research of valley-dependent directional emission. Here, it is demonstrated that the electron beam permits the chirality-selective routing of valley photons in a monolayer WS2 with Au nanostructures. The electron beam can locally excite valley excitons and regulate the coupling between excitons and nanostructures, hence controlling the interference effect of multipolar electric modes in nanostructures. Therefore, the separation degree can be modified by steering the electron beam, exhibiting the capability of subwavelength control of valley separation. This work provides a novel method to create and resolve the variation of valley emission distribution in momentum space, paving the way for the design of future nanophotonic integrated devices.

Multimode hybrid gold-silicon nanoantennas for tailored nanoscale optical confinement

High-index dielectric nanoantennas, which provide an interplay between electric and magnetic modes, have been widely used as building blocks for a variety of devices and metasurfaces, both in linear and nonlinear regimes. Here, we investigate hybrid metal-semiconductor nanoantennas, consisting of a multimode silicon nanopillar core coated with a gold layer, that offer an enhanced degree of control over the mode selection and confinement, and emission of light on the nanoscale exploiting high-order electric and magnetic resonances. Cathodoluminescence spectra revealed a multitude of resonant modes supported by the nanoantennas due to hybridization of the Mie resonances of the core and the plasmonic resonances of the shell. Eigenmode analysis revealed the modes that exhibit enhanced field localization at the gold interface, together with high confinement within the nanopillar volume. Consequently, this architecture provides a flexible means of engineering nanoscale components with tailored optical modes and field confinement for a plethora of applications, including sensing, hot-electron photodetection and nanophotonics with cylindrical vector beams.

Broadband transverse unidirectional scattering and large range nanoscale displacement measuring based on the interaction between a tightly focused azimuthally polarized beam and a silicon hollow nanostructure

We theoretically propose a broadband transverse unidirectional scattering scheme based on the interaction between a tightly focused azimuthally polarized beam (APB) and a silicon hollow nanostructure. When the nanostructure is located at a specific position in the focal plane of the APB, the transverse scattering fields can be decomposed into contributions from transverse components of the electric dipoles, longitudinal components of magnetic dipoles and magnetic quadrupole components. In order to satisfy the transverse Kerker conditions for these multipoles within a wide infrared spectrum, we design a novel nanostructure with hollow parallelepiped shape. Through numerical simulations and theoretical calculations, this scheme exhibits efficient transverse unidirectional scattering effects in the wavelength range of 1440 nm to 1820 nm (380 nm). In addition, by adjusting the position of the nanostructure on the x-axis, efficient nanoscale displacement sensing with large measuring ranges can be achieved. After analyses, the results prove that our research may have potential applications in the field of high-precision on-chip displacement sensors.

Tunable directional emission from electrically driven nano-strip metal-insulator-metal tunnel junctions

Electrically driven nanoantennas for on-chip generation and manipulation of light have attracted significant attention in recent times. Metal-insulator-metal (MIM) tunnel junctions have been extensively used to electrically excite surface plasmons and photons via inelastic electron tunneling. However, the dynamic switching of light from MIM junctions into spatially separate channels has not been shown. Here, we numerically demonstrate switchable, highly directional light emission from electrically driven nano-strip Ag-SiO₂-Ag tunnel junctions. The top electrode of our Ag-SiO₂-Ag stack is divided into 16 nano-strips, with two of the tunnel junctions at the centre (S _L and S _R) acting as sources. Using full-wave electromagnetic simulations, we show that when S _L is excited, the emission is highly directional with an angle of emission of -30° and an angular spread of ∼11°. When the excitation is switched to S _R, the emission is redirected to an angle of 30° with an identical angular spread. A directivity of 29.4 is achieved in the forward direction, with a forward-to-backward ratio of 12. We also demonstrate wavelength-selective directional switching by changing the width, and thereby the resonance wavelength, of the sources. The emission can be tuned by varying the periodicity of the structure, paving the way for electrically driven, reconfigurable light sources.

Surface-enhanced coherent anti-Stokes Raman scattering based on coupled nanohole-slit arrays

Metal nanohole arrays show excellent performance when applied for sensing, optical fibers, and surface-enhanced spectroscopy, but they are not ideal candidates for surface-enhanced coherent anti-Stokes Raman scattering (SECARS) because of their low enhancement factor (EF). Here, the finite element method was used to study the dependence of the period, width, and thickness of nanoslits on the EF of SECARS and optical transmission in Au nanohole-slit arrays. Nanoslits across the nanoholes significantly modulated the SECARS signal, and we observed an ∼10⁶ improvement in the EF of SECARS compared with the nanohole-only structure. Uniform and stable 2D hotspots at the open surface of plasmonic nanohole-slit structures provided a huge SECARS EF as high as 18 orders of magnitude. Directional SECARS emission revealed strong forward and backscattering with high directionality, showing a smaller divergence angle of 14° on the reflective side of the nanohole-slit array. These results provide a fundamental understanding of SECARS in coupled nanohole-slit arrays and are useful for designing a SECARS platform with high sensitivity.

Directional quantum dot emission by soft-stamping on silicon Mie resonators

We present a soft-stamping method to selectively print a homogenous layer of CdSeTe/ZnS core-shell quantum dots (QDs) on top of an array of Si nanocylinders with Mie-type resonant modes. Using this new method, we gain accurate control of the quantum dot's angular emission through engineered coupling of the QDs to these resonant modes. Using numerical simulations we show that the emission into or away from the Si substrate can be precisely controlled by the QD position on the nanocylinder. QDs centered on a 400 nm diameter nanocylinder surface show 98% emission directionality into the Si substrate. Alternatively, for homogenous ensembles placed over the nanocylinder top-surface, the upward emission is enhanced 10-fold for 150 nm diameter cylinders. Experimental PL intensity measurements corroborate the simulated trends with cylinder diameter. PL lifetime measurements reflect well the variations of the local density of states at the QD position due to coupling to the resonant cylinders. These results demonstrate that the soft imprint technique provides a unique manner to directly integrate optical emitters with a wide range of nanophotonic geometries, with potential applications in LEDs, luminescent solar concentrators, and up- and down-conversion schemes for improved photovoltaics.

Study on Localized Surface Plasmon Coupling with Many Radiators

Localized surface plasmon (LSP) coupling with many radiators are investigated. The LSP is generated by excitation of laser or electron beam on the random Ag nano particles (NPs) and arrayed ones embedded in the p-GaN of green LEDs. They couple with the excitons or radiative recombination in the quantum well (QW) and electron beam, which enhance or suppress the luminescence of the radiators. The photoluminescence (PL) intensity of periodic Ag NPs can get as much as 4.5 times higher than that of bare LED. In addition to the periodic structure, the morphology of Ag NPs also affects the localized SP (LSP) resonance intensity and light scattering efficiency. In the finite difference time domain (FDTD) simulation, five x-polarized dipoles are approximated to five quantum wells. Considering the interaction between the five dipoles and their feedback effect on LSP, the enhancement effect of SP dipole coupling with Ag NPs is amplified and the energy dissipation is reduced. The enhancement of cathodoluminescence (CL) was also found in green LEDs with Ag NPs. The three-body model composed of two orthogonal dipoles and an Ag NP is used for 3D FDTD simulation. The LSP-QWs coupling effect is separated from the electron beam (e-beam)-LSP-QW system by linear approximation. Under the excitation of electron beam, the introduction of z-dipole greatly reduces the energy dissipation. In the cross-sectional sample, z-polarized dipoles in QWs show more coupling strength to the dipole and quadrupole modes of LSP. The perturbation theory is used to separate the LSP coupling effects to x-dipole and z-dipole. At last, the resonator and the antenna effects are discussed for LSP coupling at different positions to the Ag NP.

Nanometric displacement sensor with a switchable measuring range using a cylindrical vector beam excited silicon nanoantenna

We demonstrate a nanometric displacement sensor with a switchable measuring range by using a single silicon nanoantenna. It is revealed that the interference between the longitudinal and transverse dipolar scattering can be well tuned by moving the nanoantenna in the focal field of the cylindrical vector beam. As a result, a position related scattering directivity is found and is used as a displacement sensor with a 4.5 nm lateral resolution. Interestingly, the measuring range of this displacement sensor can be extended by twice through simply changing the excitation from the azimuthally polarized beam to the radially polarized beam. Our results provide a facile way to tune the measuring range of the nanometric displacement sensor and may open up an avenue to super-resolution microscopy and optical nanometrology.

Visualization of Directional Beaming of Weakly Localized Raman from a Random Network of Silicon Nanowires

Disordered optical media are an emerging class of materials that can strongly scatter light. These materials are useful to investigate light transport phenomena and for applications in imaging, sensing and energy storage. While coherent light can be generated using such materials, its directional emission is typically hampered by their strong scattering nature. Here, the authors directly image Rayleigh scattering, photoluminescence and weakly localized Raman light from a random network of silicon nanowires via real-space microscopy and Fourier imaging. Direct imaging enables us to gain insight on the light transport mechanisms in the random material, to visualize its weak localization length and to demonstrate out-of-plane beaming of the scattered coherent Raman light. The direct visualization of coherent light beaming in such random networks of silicon nanowires offers novel opportunities for fundamental studies of light propagation in disordered media. It also opens venues for the development of next generation optical devices based on disordered structures, such as sensors, light sources, and optical switches.

Spontaneous and stimulated electron-photon interactions in nanoscale plasmonic near fields

The interplay between free electrons, light, and matter offers unique prospects for space, time, and energy resolved optical material characterization, structured light generation, and quantum information processing. Here, we study the nanoscale features of spontaneous and stimulated electron-photon interactions mediated by localized surface plasmon resonances at the tips of a gold nanostar using electron energy-loss spectroscopy (EELS), cathodoluminescence spectroscopy (CL), and photon-induced near-field electron microscopy (PINEM). Supported by numerical electromagnetic boundary-element method (BEM) calculations, we show that the different coupling mechanisms probed by EELS, CL, and PINEM feature the same spatial dependence on the electric field distribution of the tip modes. However, the electron-photon interaction strength is found to vary with the incident electron velocity, as determined by the spatial Fourier transform of the electric near-field component parallel to the electron trajectory. For the tightly confined plasmonic tip resonances, our calculations suggest an optimum coupling velocity at electron energies as low as a few keV. Our results are discussed in the context of more complex geometries supporting multiple modes with spatial and spectral overlap. We provide fundamental insights into spontaneous and stimulated electron-light-matter interactions with key implications for research on (quantum) coherent optical phenomena at the nanoscale.

Selectively steering photon spin angular momentum via electron-induced optical spin Hall effect

The development of the optical spin Hall effect (OSHE) realizes the splitting of different spin components, contributing to the manipulation of photon spin angular momentum that acts as the information carrier for quantum technology. However, OSHE with optical excitation lacks active control of photon angular momentum at deep subwavelength scale because of the optical diffraction limit. Here, we experimentally demonstrate a selective manipulation of photon spin angular momentum at a deep subwavelength scale via electron-induced OSHE in Au nanoantennas. The inversion of the OSHE radiation pattern is observed by angle-resolved cathodoluminescence polarimetry with the electron impact position shifting within 80 nm in a single antenna unit. By this selective steering of photon spin, we propose an information encoding with robustness, privacy, and high level of integration at a deep subwavelength scale for the future quantum applications.

Fundamental Limit of Plasmonic Cathodoluminescence

We use cathodoluminescence (CL) spectroscopy in a transmission electron microscope to probe the radial breathing mode of plasmonic silver nanodisks. A two-mirror detection system sandwiching the sample collects the CL emission in both directions, that is, backward and forward with respect to the electron beam trajectory. We unambiguously identify a spectral shift of about 8 nm in the CL spectra acquired from both sides and show that this asymmetry is induced by the electron beam itself. By numerical simulations, we confirm the observations and identify the underlying physical effect due to the interference of the CL emission patterns of an electron-beam-induced dipole and the breathing mode. This effect can ultimately limit the achievable fidelity in CL measurements on any system involving multiple excitations and should therefore be considered with care in high-precision experiments.

Deep subwavelength control of valley polarized cathodoluminescence in h-BN/WSe2/h-BN heterostructure

Valley pseudospin in transition metal dichalcogenides monolayers intrinsically provides additional possibility to control valley carriers, raising a great impact on valleytronics in following years. The spin-valley locking directly contributes to optical selection rules which allow for valley-dependent addressability of excitons by helical optical pumping. As a binary photonic addressable route, manipulation of valley polarization states is indispensable while effective control methods at deep-subwavelength scale are still limited. Here, we report the excitation and control of valley polarization in h-BN/WSe₂/h-BN and Au nanoantenna hybrid structure by electron beam. Near-field circularly polarized dipole modes can be excited via precise stimulation and generate the valley polarized cathodoluminescence via near-field interaction. Effective manipulation of valley polarization degree can be realized by variation of excitation position. This report provides a near-field excitation methodology of valley polarization, which offers exciting opportunities for deep-subwavelength valleytronics investigation, optoelectronic circuits integration and future quantum information technologies.

Here, the authors generate near-field circularly polarized dipole modes in a hBN/WSe₂/hBN–Au nanoantenna hybrid structure by electron beam excitation, and show nanoscale control of the valley polarization through spatial variation of the electron beam excitation position.

Plasmonic nanoarcs: a versatile platform with tunable localized surface plasmon resonances in octave intervals

The tunability of the longitudinal localized surface plasmon resonances (LSPRs) of metallic nanoarcs is demonstrated with key relationships identified between geometric parameters of the arcs and their resonances in the infrared. The wavelength of the LSPRs is tuned by the mid-arc length of the nanoarc. The ratio between the attenuation of the fundamental and second order LSPRs is governed by the nanoarc central angle. Beneficial for plasmonic enhancement of harmonic generation, these two resonances can be tuned independently to obtain octave intervals through the design of a non-uniform arc-width profile. Because the character of the fundamental LSPR mode in nanoarcs combines an electric and a magnetic dipole, plasmonic nanoarcs with tunable resonances can serve as versatile building blocks for chiroptical and nonlinear optical devices.

Phase-Resolved Surface Plasmon Scattering Probed by Cathodoluminescence Holography

High-energy (1-100 keV) electrons can coherently couple to plasmonic and dielectric nanostructures, creating cathodoluminescence (CL) of which the spectral features reveal details of the material's resonant modes at a deep-subwavelength spatial resolution. While CL provides fundamental insight in optical modes, detecting its phase has remained elusive. Here, we use Fourier-transform CL holography to determine the far-field phase distribution of fields scattered from plasmonic nanoholes, nanocubes, and helical nanoapertures and reconstruct the angle-resolved phase distributions. From the derived fields, we derive the relative strength and phase of induced scattering dipoles. Fourier-transform CL holography opens up a new world of coherent light scattering and surface wave studies with nanoscale spatial resolution.

Color Routing via Cross-Polarized Detuned Plasmonic Nanoantennas in Large-Area Metasurfaces

Bidirectional nanoantennas are of key relevance for advanced functionalities to be implemented at the nanoscale and, in particular, for color routing in an ultracompact flat-optics configuration. Here we demonstrate a novel approach avoiding complex collective geometries and/or restrictive morphological parameters based on cross-polarized detuned plasmonic nanoantennas in a uniaxial (quasi-1D) bimetallic configuration. The nanofabrication of such a flat-optics system is controlled over a large area (cm²) by a novel self-organized technique exploiting ion-induced nanoscale wrinkling instability on glass templates to engineer tilted bimetallic nanostrip dimers. These nanoantennas feature broadband color routing with superior light scattering directivity figures, which are well described by numerical simulations and turn out to be competitive with the response of lithographic nanoantennas. These results demonstrate that our large-area self-organized metasurfaces can be implemented in real-world applications of flat-optics color routing from telecom photonics to optical nanosensing.

Constructing a library of metal and metal-oxide nanoparticle heterodimers through colloidal assembly

Nanoparticle dimers composed of different metals or metal oxides, as well as different shapes and sizes, are of wide interest for applications ranging from nanoplasmonic sensing to nanooptics to biomedical engineering. Shaped nanoparticles, like triangles and nanorods, can be particularly useful in applications due to the strong localized plasmonic hot-spot that forms at the tips or corners. By placing catalytic, but traditionally weakly- or non-plasmonic nanoparticles, such as metal oxides and metals like palladium, in these hot-spots, an enhanced function for sensing, photocatalysis or optical use is predicted. Here, we present an electrostatic colloidal assembly strategy for nanoparticles, incorporating different sizes, shapes and metal or metal oxide compositions into heterodimers with smaller gaps than are achievable using nanofabrication techniques. This versatile method is demonstrated on 14 combinations, including a variety of shaped gold nanoparticles as well as palladium, iron oxide, and titanium oxide nanoparticles. These colloidal nanoparticles are stabilized with traditional surfactants, such as citrate, CTAB, PVP and oleic acid/oleylamines, indicating the wide applicability of our approach. Heterodimers of gold and palladium are further analyzed using cathodoluminescence to demonstrate the tunability of these "plasmonic molecules". Since systematically altering the absorption and emission of the plasmonic nanoparticles dimers is crucial to extending their functionality, and small gap sizes produce the strongest hot-spots, this method indicates that the electrostatic approach to heterodimer assembly can be useful in creating new nanoparticle dimers for many applications.

Tunable plasmonic HfN nanoparticles and arrays

We present the fabrication of tunable plasmonic hafnium nitride (HfN) nanoparticles. HfN is a metallic refractory material with the potential of supporting plasmon resonances in the visible range, similar to silver and gold, but with the additional benefits of high melting point, chemical stability, and mechanical hardness. However, the preparation of HfN nanoparticles and the experimental demonstration of their plasmonic potential are still in their infancy. Here, high quality HfN thin films were fabricated, for which ellipsometry shows their plasmonic potential. From these thin films, nanorods and nanotriangles were milled using a focused ion beam and the plasmon resonances were identified using cathodoluminescence mapping. As an alternative fabrication strategy, an optimized electron-beam lithography procedure was used to prepare arrays of HfN nanoparticles, which also exhibited clear surface plasmon resonances. These results pave the way to further explore HfN nanoparticles in plasmonically-powered applications where materials robustness is essential.

Electron-beam spectroscopy for nanophotonics

Progress in electron-beam spectroscopies has recently enabled the study of optical excitations with combined space, energy and time resolution in the nanometre, millielectronvolt and femtosecond domain, thus providing unique access into nanophotonic structures and their detailed optical responses. These techniques rely on ~1-300 keV electron beams focused at the sample down to sub-nanometre spots, temporally compressed in wavepackets a few femtoseconds long, and in some cases controlled by ultrafast light pulses. The electrons undergo energy losses and gains (also giving rise to cathodoluminescence light emission), which are recorded to reveal the optical landscape along the beam path. This Review portraits these advances, with a focus on coherent excitations, emphasizing the increasing level of control over the electron wavefunctions and ensuing applications in the study and technological use of optically resonant modes and polaritons in nanoparticles, 2D materials and engineered nanostructures.

Dual-band directional scattering with all-dielectric trimer in the near-infrared region

A silicon trimer is explored to tailor unidirectional forward scattering at multiple wavelengths in the near-infrared region with low loss using theoretical calculations and numerical simulations, which leads to the dramatic enhancement in unidirectional forward scattering and suppression of backward scattering. The higher moments in the trimer can be properly excited and balanced by breaking the symmetry of the trimer. The generalized Kerker conditions at two different wavelengths can be achieved in the trimer to further improve the scattering directivity. Our results provide insights into future development of all-dielectric low-loss nanoantennas in the near-infrared region.

Nanostructured metals for light-based technologies

The basic theoretical understanding of light interacting with nanostructured metals that has existed since the early 1900s has become more relevant in the last two decades, largely because of new approaches to structure metals down to the nanometer scale or smaller. Here, a broad overview of the concepts and applications of nanostructuring metals for light-based technologies is given. The theory of the response of metals to an applied oscillating field is given, including a discussion of nonlocal, nonlinear and quantum effects. Using this metal response, the guiding of electromagnetic (light) waves using metals is given, with a particular emphasis on the impact of nanostructured metals for tighter confinement and slower propagation. Similarly, the influence of metal nanostructures on light scattering by isolated metal structures, like nanoparticles and nanoantennas, is described, with basic results presented including plasmonic/circuit resonances, the single channel limit, directivity enhancement, the maximum power transfer theorem, limits on the magnetic response from kinetic inductance and the scaling of gap plasmons to the nanometer scale and smaller. A brief overview of nanofabrication approaches to creating metal nanostructures is given. Finally, existing and emerging light-based applications are presented, including those for sensing, spectroscopy (including local refractive index, Raman, IR absorption), detection (including Schottky detectors), switching (including terahertz photoconductive antennas), modulation, energy harvesting and photocatalysis, light emission (including lasers and tunneling based light emission), optical tweezing, nonlinear optics, subwavelength imaging and lithography and high density data storage.

Control of the emission from electric and magnetic dipoles by gold nanocup antennas

The control of the emission from electric and magnetic dipoles is highly desired for the development of optic chips. Although the emission of electric dipole has been successfully controlled by plasmonic nanoantenna, the control of magnetic dipole emission is relatively difficult. Here, we systematically study the effect of electric and magnetic modes of Au nanocups on the emission of electric and magnetic dipoles. The emission of electric dipole can be enhanced by both the electric and magnetic mode of the Au nanocup, while the emission of the magnetic dipole is only increased by the magnetic mode. The enhancement exhibits wavelength dependence. The wavelength of the largest enhancement is determined by the resonance wavelength of electric and magnetic modes. The enhancement values for electric and magnetic dipoles are determined by the near-field electric and magnetic field enhancements, respectively. More importantly, the emission pattern of magnetic dipole is greatly modified by the magnetic mode of Au nanocup. The directional emission of magnetic dipole is first time realized by use of the magnetic mode of the Au nanocup. Our findings deepen the understanding of the plasmon-controlled emission of electric and magnetic dipoles and will be very helpful to the development of the nanophotonic chips.

Spatial Resolution of Coherent Cathodoluminescence Super-Resolution Microscopy

We investigate the nanoscale excitation of Ag nanocubes with coherent cathodoluminescence imaging spectroscopy (CL) to resolve the factors that determine the spatial resolution of CL as a deep-subwavelength imaging technique. The 10-30 keV electron beam coherently excites localized plasmons in 70 nm Ag cubes at 2.4 and 3.1 eV. The radiation from these plasmon modes is collected in the far-field together with the secondary electron intensity. CL line scans across the nanocubes show exponentially decaying tails away from the cube that reveal the evanescent coupling of the electron field to the resonant plasmon modes. The measured CL decay lengths range from 8 nm (10 keV) to 12 nm (30 keV) and differ from the calculated ones by only 1-3 nm. A statistical model of electron scattering inside the Ag nanocubes is developed to analyze the secondary electron images and compare them with the CL data. The Ag nanocube edges are derived from the CL line scans with a systematic error less than 3 nm. The data demonstrate that CL probes the electron-induced plasmon fields with nanometer accuracy.

Probing the Band Structure of Topological Silicon Photonic Lattices in the Visible Spectrum

We study two-dimensional hexagonal photonic lattices of silicon Mie resonators with a topological optical band structure in the visible spectral range. We use 30 keV electrons focused to nanoscale spots to map the local optical density of states in topological photonic lattices with deeply subwavelength resolution. By slightly shrinking or expanding the unit cell, we form hexagonal superstructures and observe the opening of a band gap and a splitting of the double-degenerate Dirac cones, which correspond to topologically trivial and nontrivial phases. Optical transmission spectroscopy shows evidence of topological edge states at the domain walls between topological and trivial lattices.

Characterization of Overlapped Plasmon Modes in a Gold Hexagonal Plate Revealed by Three-Dimensional Near-Field Optical Microscopy

A detailed characterization of plasmon modes is important not only for a deeper understanding of plasmons but also for their practical applications. In this study, we investigated the three-dimensional near-field characteristics of high-order plasmon modes excited in a gold hexagonal nanoplate. From the near-field spectroscopic images, we found that both in-plane and out-of-plane plasmon modes observed near 900 nm were spectrally and spatially overlapped. We performed three-dimensional near-field measurement to reveal the optical characteristics of the overlapped modes in detail. We found that the steric near-field distribution near the nanoplate strongly depended on the plasmon mode, and the out-of-plane mode confines electromagnetic fields more tightly than the in-plane mode. We also found that the in-plane mode was dominantly visualized as the probe tip-sample distance increased. These findings demonstrate that the three-dimensional near-field technique enables selective visualization of a single plasmon mode even if multiple modes are spatially and spectrally overlapped.

Experimental Observation of Superscattering

Superscattering, induced by degenerate resonances, breaks the fundamental single-channel limit of the scattering cross section of subwavelength structures; in principle, an arbitrarily large total cross section can be achieved via superscattering. It thus provides a unique way to strengthen the light-matter interaction at the subwavelength scale, and has many potential applications in sensing, energy harvesting, bioimaging (such as magnetic resonance imaging), communication, and optoelectronics. However, the experimental demonstration of superscattering remains an open challenge due to its vulnerability to structural imperfections and intrinsic material losses. Here we report the first experimental evidence for superscattering by demonstrating the superscattering simultaneously in two different frequency regimes through both the far-field and near-field measurements. The underlying mechanism for the observed superscattering is the degenerate resonances of confined surface waves, by utilizing a subwavelength metasurface-based multilayer structure. Our work paves the way towards practical applications based on superscattering.

In-plane coherent control of plasmon resonances for plasmonic switching and encoding

Considerable attention has been paid recently to coherent control of plasmon resonances in metadevices for potential applications in all-optical light-with-light signal modulation and image processing. Previous reports based on out-of-plane coherent control of plasmon resonances were established by modulating the position of a metadevice in standing waves. Here we show that destructive and constructive absorption can be realized in metallic nano-antennas through in-plane coherent control of plasmon resonances, which is determined by the distribution rule of electrical-field components of nano-antennas. We provide proof-of-principle demonstrations of plasmonic switching effects in a gold nanodisk monomer and dimer, and propose a plasmonic encoding strategy in a gold nanodisk chain. In-plane coherent control of plasmon resonances may open a new avenue toward promising applications in optical spectral enhancement, imaging, nanolasing, and optical communication in nanocircuits.

Multipole Radiations from Large Gold Nanospheres Excited by Evanescent Wave

We proposed the use of the evanescent wave generated in a total internal reflection configuration to excite large gold nanospheres and investigated the radiations of the high-order plasmon modes supported in gold nanospheres. It was revealed that the evanescent wave excitation is equivalent to the excitation by using both the incident and reflected light, offering us the opportunity to control the orientation of the electric field used to excite nanoparticles. In addition, it was found that the scattering light intensity is greatly enhanced and the background noise is considerably suppressed, making it possible to detect the radiations from high-order plasmon modes. Moreover, the influence of the mirror images on the scattering induced by a metal substrate is eliminated as compared with the surface plasmon polariton excitation. By exciting a gold nanosphere with s-polarized light and detecting the scattering light with a p-polarized analyzer, we were able to reveal the radiation from the electric quadrupole mode of the gold nanosphere in both the spatial and the frequency domains. Our findings are important for characterizing the radiations from the high-order modes of large nanoparticles and useful for designing nanoscale photonic devices.

Tunable polarization-controlled perfect switching of transverse scattering

We demonstrate the polarization-controlled perfect switching effect in near-infrared frequency ranges using silicon particles embedded in a liquid crystal cell. The effect has electric field tunable properties in a wide frequency band. The phenomenon originates from the interaction between electric multipole modes and magnetic multipole modes, which can also be generalized to the visible and other frequency ranges. It is expected that such an electric field tunable switching device can play important role in the optical information process.

Experimental demonstration of linear and spinning Janus dipoles for polarisation- and wavelength-selective near-field coupling

The electromagnetic field scattered by nano-objects contains a broad range of wavevectors and can be efficiently coupled to waveguided modes. The dominant contribution to scattering from subwavelength dielectric and plasmonic nanoparticles is determined by electric and magnetic dipolar responses. Here, we experimentally demonstrate spectral and phase selective excitation of Janus dipoles, sources with electric and magnetic dipoles oscillating out of phase, in order to control near-field interference and directional coupling to waveguides. We show that by controlling the polarisation state of the dipolar excitations and the excitation wavelength to adjust their relative contributions, directionality and coupling strength can be fully tuned. Furthermore, we introduce a novel spinning Janus dipole featuring cylindrical symmetry in the near and far field, which results in either omnidirectional coupling or noncoupling. Controlling the propagation of guided light waves via fast and robust near-field interference between polarisation components of a source is required in many applications in nanophotonics and quantum optics.

Plasmonic Dispersion Relations and Intensity Enhancement of Metal-Insulator-Metal Nanodisks

We show that the plasmon modes of vertically stacked Ag-SiO₂-Ag nanodisks can be understood and classified as hybridized surface and edge modes. We describe their universal dispersion relations and demonstrate that coupling-induced spectral shifts are significantly stronger for surface modes than for edge modes. The experimental data correspond well to numerical simulations. In addition, we estimate optical intensity enhancements of the stacked nanodisks in the range of 1000.

Light Extraction Enhancement in Flexible Organic Light-Emitting Diodes by a Light-Scattering Layer of Dewetted Ag Nanoparticles at Low Temperatures

We demonstrated light extraction improvement by applying a scattering layer of Ag nanoparticles physically synthesized through a low-temperature annealing process to flexible organic light-emitting diodes (OLEDs). In general, increasing the size of Ag nanoparticles is preferred to increase light scattering, but a high-temperature annealing process (∼400 °C) is required to produce them. However, flexible substrates generally cannot withstand high-temperature processes. In this study, we formed Ag nanoparticles at a low temperature of ∼200 °C by inserting a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate buffer layer, thus promoting Ag dewetting. As a result, the scattering layer of enlarged Ag nanoparticles formed at low temperatures increased the external quantum efficiency by 24% in a flexible OLED compared to a reference device.

Laser-Splashed Three-Dimensional Plasmonic Nanovolcanoes for Steganography in Angular Anisotropy

Planar optics constructed from subwavelength artificial atoms have been suggested as a route to the physical realization of steganography with controlled intrinsic redundancy at single-pixel levels. Unfortunately, two-dimensional geometries with uniform flat profiles offer limited structural redundancy and make it difficult to create advanced crypto-information in multiplexed physical divisions. Here, we reveal that splashing three-dimensional (3D) plasmonic nanovolcanoes could allow for a steganographic strategy in angular anisotropy, with high resolution, full coloration, and transient control of structural profiles. Highly reproducible 3D morphologies of volcanic nanosplashes are demonstrated by creating a standardized recipe of laser parameters. Such single nanovolcanoes can be well controlled individually at different splashing stages and thus provide a lithography-free fashion to access various spectral responses of angularly coordinated transverse and vertical modes, leading to the full-range coloration. This chip-scale demonstration of steganographic color images in angular anisotropy unfolds a long-ignored scheme for structured metasurfaces and thereby provides a paradigm for information security and anticounterfeiting.

Quantifying single plasmonic nanostructure far-fields with interferometric and polarimetric k-space microscopy

Optically resonant nanoantennae are key building blocks for metasurfaces, nanosensors, and nanophotonic light sources due to their ability to control the amplitude, phase, directivity, and polarization of scattered light. Here, we report an experimental technique for the full recovery of all degrees of freedom encoded in the far-field radiated by a single nanostructure using a high-NA Fourier microscope equipped with digital off-axis holography. This method enables full decomposition of antenna-physics in its multipole contributions and gives full access to the orbital and spin angular momentum properties of light scattered by single nano-objects. Our results demonstrate these capabilities through a quantitative assessment of the purity of the "selection rules" for orbital angular momentum transfer by plasmonic spiral nanostructures.

Directional light beams by design from electrically driven elliptical slit antennas

We report on the low-energy, electrical generation of light beams in specific directions from planar elliptical microstructures. The emission direction of the beam is determined by the microstructure eccentricity. A very simple, broadband, optical antenna design is used, which consists of a single elliptical slit etched into a gold film. The light beam source is driven by an electrical nanosource of surface plasmon polaritons (SPP) that is located at one focus of the ellipse. In this study, SPPs are generated through inelastic electron tunneling between a gold surface and the tip of a scanning tunneling microscope.

Unveiling and Imaging Degenerate States in Plasmonic Nanoparticles with Nanometer Resolution

Metal nanoparticles host localized plasmon excitations that allow the manipulation of optical fields at the nanoscale. Despite the availability of several techniques for imaging plasmons, direct access into the symmetries of these excitations remains elusive, thus hindering progress in the development of applications. Here, we present a combination of angle-, polarization-, and space-resolved cathodoluminescence spectroscopy methods to selectively access the symmetry and degeneracy of plasmonic states in lithographically fabricated gold nanoprisms. We experimentally reveal and spatially map degenerate states of multipole plasmon modes with nanometer spatial resolution and further provide recipes for resolving optically dark and out-of-plane modes. Full-wave simulations in conjunction with a simple tight-binding model explain the complex plasmon structure in these particles and reveal intriguing mode-symmetry phenomena. Our approach introduces systematics for a comprehensive symmetry characterization of plasmonic states in high-symmetry nanostructures.

On the scattering directionality of a dielectric particle dimer of High Refractive Index

Low-losses and directionality effects exhibited by High Refractive Index Dielectric particles make them attractive for applications where radiation direction control is relevant. For instance, isolated metallo-dielectric core-shell particles or aggregates (dimers) of High Refractive Index Dielectric particles have been proposed for building operational switching devices. Also, the possibility of using isolated High Refractive Index Dielectric particles for optimizing solar cells performance has been explored. Here, we present experimental evidence in the microwave range, that a High Refractive Index Dielectric dimer of spherical particles is more efficient for redirecting the incident radiation in the forward direction than the isolated case. In fact, we report two spectral regions in the dipolar spectral range where the incident intensity is mostly scattered in the forward direction. They correspond to the Zero-Backward condition (also observed for isolated particles) and to a new condition, denoted as "near Zero-Backward" condition, which comes from the interaction effects between the particles. The proposed configuration has implications in solar energy harvesting devices and in radiation guiding.

Generalized Kerker effects in nanophotonics and meta-optics [Invited]

The original Kerker effect was introduced for a hypothetical magnetic sphere, and initially it did not attract much attention due to a lack of magnetic materials required. Rejuvenated by the recent explosive development of the field of metamaterials and especially its core concept of optically-induced artificial magnetism, the Kerker effect has gained an unprecedented impetus and rapidly pervaded different branches of nanophotonics. At the same time, the concept behind the effect itself has also been significantly expanded and generalized. Here we review the physics and various manifestations of the generalized Kerker effects, including the progress in the emerging field of meta-optics that focuses on interferences of electromagnetic multipoles of different orders and origins. We discuss not only the scattering by individual particles and particle clusters, but also the manipulation of reflection, transmission, diffraction, and absorption for metalattices and metasurfaces, revealing how various optical phenomena observed recently are all ubiquitously related to the Kerker's concept.

Ultrasmall Plasmonic Single Nanoparticle Light Source Driven by a Graphene Tunnel Junction

Metal nanoparticles that can couple light into tightly confined surface plasmons bridge the size mismatch between the wavelength of light and nanostructures are one of the smallest building blocks of nano-optics. However, plasmonic nanoparticles have been primarily studied to concentrate or scatter incident light as an ultrasmall antenna, while studies of their intrinsic plasmonic light emission properties have been limited. Although light emission from plasmonic structures can be achieved by inelastic electron tunneling, this strategy cannot easily be applied to isolated single nanoparticles due to the difficulty in making electrical connections without disrupting the particle plasmon mode. Here, we solve this problem by placing gold nanoparticles on a graphene tunnel junction. The monolayer graphene provides a transparent counter electrode for tunneling while preserving the ultrasmall footprint and plasmonic mode of nanoparticle. The tunneling electrons excite the plasmonic mode, followed by radiative decay of the plasmon. We also demonstrate that a dielectric overlayer atop the graphene tunnel junction can be used to tune the light emission. We show the simplicity and scalability of this approach by achieving electroluminescence from single nanoparticles without bulky contacts as well as millimeter-sized arrays of nanoparticles.

How Dark Are Radial Breathing Modes in Plasmonic Nanodisks?

Due to a vanishing dipole moment, radial breathing modes in small flat plasmonic nanoparticles do not couple to light and have to be probed with a near-field source, as in electron energy loss spectroscopy (EELS). With increasing particle size, retardation gives rise to light coupling, enabling probing breathing modes optically or by cathodoluminescence (CL). Here, we investigate single silver nanodisks with diameters of 150-500 nm by EELS and CL in an electron microscope and quantify the EELS/CL ratio, which corresponds to the ratio of full to radiative damping of the breathing mode. For the investigated diameter range, we find the CL signal to increase by about 1 order of magnitude, in agreement with numerical simulations. Due to reciprocity, our findings corroborate former optical experiments and enable a quantitative understanding of the light coupling of dark plasmonic modes.

Janus and Huygens Dipoles: Near-Field Directionality Beyond Spin-Momentum Locking

Unidirectional scattering from circularly polarized dipoles has been demonstrated in near-field optics, where the quantum spin-Hall effect of light translates into spin-momentum locking. By considering the whole electromagnetic field, instead of its spin component alone, near-field directionality can be achieved beyond spin-momentum locking. This unveils the existence of the Janus dipole, with side-dependent topologically protected coupling to waveguides, and reveals the near-field directionality of Huygens dipoles, generalizing Kerker's condition. Circular dipoles, together with Huygens and Janus sources, form the complete set of all possible directional dipolar sources in the far- and near-field. This allows the designing of directional emission, scattering, and waveguiding, fundamental for quantum optical technology, integrated nanophotonics, and new metasurface designs.

Effect of dipole polarization orientation on surface plasmon coupling with green emitting quantum wells by cathodoluminescence

Ag nanoparticles (NPs) are fabricated on the cross-section of green emitting quantum wells (QWs). The effect of the dipole polarization orientation on the localized surface plasmon (LSP)-QW coupling can be studied by setting the incident direction of the electron beam parallel to the plane of the QWs. Cathodoluminescence (CL) measurements on the QWs show that the intensity with the Ag NPs is enhanced 6.1 times compared with that without the Ag NPs. Total energy loss profiles for an electron beam in the GaN and Ag NP are accurately simulated using a Monte Carlo program (CASINO). The orientations of the in-plane dipoles in the QWs can vary from 0° to 360°. Through a two-step simulation process using the three-dimensional (3D) finite difference time domain (FDTD) method, the weighted average of CL intensities are simulated for QWs with the Ag NPs. The simulation results agree well with the experimental results. Lastly, the dipole orientation dependent LSP-QW coupling process is discussed.

Ag nanoparticles (NPs) are fabricated on the cross-section of green emitting quantum wells (QWs).

Localized excitation of polarized light emission by cathodoluminescence spectroscopy

Surface plasmons (SPs), the resonance of free electrons on the metal-air interface, may strongly interact with light and generate some extraordinary optical effects. Instead of using conventional planar light excitation, here we excite SPs with a focused electron beam on metallic nanostructures with different geometrical symmetries. With the help of a polarizer and filter in the detection system, we obtain cathodoluminescence (CL) images with different polarizations at certain wavelengths. The maxima in the CL images show that the focused electron beam may efficiently excite luminescence with different polarizations at different spots. Comparing the data collected on the structures with specific geometrical symmetry, we demonstrate that the polarization of the emitted light depends on both the structural symmetry and the excitation location. We suggest that this Letter is enlightening to understand the relationship between the SP resonance on the structure and the emission of CL with different polarizations.

Electrically Driven Unidirectional Optical Nanoantennas

Directional antennas revolutionized modern day telecommunication by enabling precise beaming of radio and microwave signals with minimal loss of energy. Similarly, directional optical nanoantennas are expected to pave the way toward on-chip wireless communication and information processing. Currently, on-chip integration of such antennas is hampered by their multielement design or the requirement of complicated excitation schemes. Here, we experimentally demonstrate electrical driving of in-plane tunneling nanoantennas to achieve broadband unidirectional emission of light. Far-field interference, as a result of the spectral overlap between the dipolar emission of the tunnel junction and the fundamental quadrupole-like resonance of the nanoantenna, gives rise to a directional radiation pattern. By tuning this overlap using the applied voltage, we record directivities as high as 5 dB. In addition to electrical tunability, we also demonstrate passive tunability of the directivity using the antenna geometry. These fully configurable electrically driven nanoantennas provide a simple way to direct optical energy on-chip using an extremely small device footprint.

Electromagnetic scattering laws in Weyl systems

Wavelength determines the length scale of the cross section when electromagnetic waves are scattered by an electrically small object. The cross section diverges for resonant scattering, and diminishes for non-resonant scattering, when wavelength approaches infinity. This scattering law explains the colour of the sky as well as the strength of a mobile phone signal. We show that such wavelength scaling comes from the conical dispersion of free space at zero frequency. Emerging Weyl systems, offering similar dispersion at non-zero frequencies, lead to new laws of electromagnetic scattering that allow cross sections to be decoupled from the wavelength limit. Diverging and diminishing cross sections can be realized at any target wavelength in a Weyl system, providing the ability to tailor the strength of wave–matter interactions for radiofrequency and optical applications.

Scattering characteristics are important optical properties but they depend strongly on the relative electromagnetic size and environment of a particle. Here, the authors study the frequency-dependence of the scattering cross section for a scatterer located inside a photonic Weyl system.

Magnetic Dipole Scattering from Metallic Nanowire for Ultrasensitive Deflection Sensing

It is generally believed that when a single metallic nanowire is sufficiently small, it scatters like a point electric dipole. We show theoretically when a metallic nanowire is placed inside specially designed beams, the magnetic dipole contribution along with the electric dipole resonance can lead to unidirectional scattering in the far field, fulfilling Kerker's condition. Remarkably, this far-field unidirectional scattering encodes information that is highly dependent on the nanowire's deflection at a scale much smaller than the wavelength. The special roles of small but essential magnetic response along with the plasmonic resonance are highlighted for this extreme sensitivity as compared with the dielectric counterpart. In addition, the same essential role of the magnetic dipole contribution is also presented for a very small metallic nanosphere.

Monocrystalline Nanopatterns Made by Nanocube Assembly and Epitaxy

Monocrystalline materials are essential for optoelectronic devices such as solar cells, LEDs, lasers, and transistors to reach the highest performance. Advances in synthetic chemistry now allow for high quality monocrystalline nanomaterials to be grown at low temperature in solution for many materials; however, the realization of extended structures with control over the final 3D geometry still remains elusive. Here, a new paradigm is presented, which relies on epitaxy between monocrystalline nanocube building blocks. The nanocubes are assembled in a predefined pattern and then epitaxially connected at the atomic level by chemical growth in solution, to form monocrystalline nanopatterns on arbitrary substrates. As a first demonstration, it is shown that monocrystalline silver structures obtained with such a process have optical properties and conductivity comparable to single-crystalline silver. This flexible multiscale process may ultimately enable the implementation of monocrystalline materials in optoelectronic devices, raising performance to the ultimate limit.