Optical properties of single plasmonic holes probed with local electron beam excitation

Controllable Chiral Light Generation and Vortex Field Investigation Using Plasmonic Holes Revealed by Cathodoluminescence

Control of the angular momentum of light is a key technology for next-generation nano-optical devices and optical communications, including quantum communication and encoding. We propose an approach to controllably generate circularly polarized light from a circular hole in a metal film using an electron beam by coherently exciting transition radiation and light scattering from the hole through surface plasmon polaritons. The circularly polarized light generation is confirmed by fully polarimetric four-dimensional cathodoluminescence mapping, where angle-resolved spectra are simultaneously obtained. The obtained intensity and Stokes maps show clear interference fringes as well as almost fully circularly polarized light generation with controllable parities by the electron beam position. By applying this approach to a three-hole system, a vortex field with a phase singularity is visualized in the middle of three holes.

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.

Simultaneous Nanoscale Excitation and Emission Mapping by Cathodoluminescence

Free-electron-based spectroscopies can reveal the nanoscale optical properties of semiconductor materials and nanophotonic devices with a spatial resolution far beyond the diffraction limit of light. However, the retrieved spatial information is constrained to the excitation space defined by the electron beam position, while information on the delocalization associated with the spatial extension of the probed optical modes in the specimen has so far been missing, despite its relevance in ruling the optical properties of nanostructures. In this study, we demonstrate a cathodoluminescence method that can access both excitation and emission spaces at the nanoscale, illustrating the power of such a simultaneous excitation and emission mapping technique by revealing a subwavelength emission position modulation as well as by visualizing electromagnetic energy transport in nanoplasmonic systems. Besides the fundamental interest of these results, our technique grants us access into previously inaccessible nanoscale optical properties.

Origin of Zenneck-like waves excited by optical nanoantennas in non-plasmonic transition metals

The scattering properties of metallic optical antennas are typically examined through the lens of their plasmonic resonances. However, non-plasmonic transition metals also sustain surface waves in the visible. We experimentally investigate in this work the far-field diffraction properties of apertured optical antennas milled on non-plasmonic W films and compare the results with plasmonic references in Ag and Au. The polarization-dependent diffraction patterns and the leakage signal emerging from apertured antennas in both kinds of metals are recorded and analyzed. This thorough comparison with surface plasmon waves reveals that surface waves are launched on W and that they have the common abilities to confine the visible light at metal-dielectric interfaces offering the possibility to tailor the far-field emission. The results have been analyzed through theoretical models accounting for the propagation of a long range surface mode launched by subwavelength apertures, that is scattered in free space by the antenna. This surface mode on W can be qualitatively described as an analogy in the visible of the Zenneck wave in the radio regime. The nature of the new surface waves have been elucidated from a careful analysis of the asymptotic expansion of the electromagnetic propagators, which provides a convenient representation for explaining the Zenneck-like character of the excited waves and opens new ways to fundamental studies of surface waves at the nanoscale beyond plasmonics.

Nanoscale mapping of optically inaccessible bound-states-in-the-continuum

Bound-states-in-the-continuum (BIC) is an emerging concept in nanophotonics with potential impact in applications, such as hyperspectral imaging, mirror-less lasing, and nonlinear harmonic generation. As true BIC modes are non-radiative, they cannot be excited by using propagating light to investigate their optical characteristics. In this paper, for the 1st time, we map out the strong near-field localization of the true BIC resonance on arrays of silicon nanoantennas, via electron energy loss spectroscopy with a sub-1-nm electron beam. By systematically breaking the designed antenna symmetry, emissive quasi-BIC resonances become visible. This gives a unique experimental tool to determine the coherent interaction length, which we show to require at least six neighboring antenna elements. More importantly, we demonstrate that quasi-BIC resonances are able to enhance localized light emission via the Purcell effect by at least 60 times, as compared to unpatterned silicon. This work is expected to enable practical applications of designed, ultra-compact BIC antennas such as for the controlled, localized excitation of quantum emitters.

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.

Electrons Generate Self-Complementary Broadband Vortex Light Beams Using Chiral Photon Sieves

Planar electron-driven photon sources have been recently proposed as miniaturized light sources, with prospects for ultrafast conjugate electron-photon microscopy and spectral interferometry. Such sources usually follow the symmetry of the electron-induced polarization: transition-radiation-based sources, for example, only generate p-polarized light. Here we demonstrate that the polarization, the bandwidth, and the directionality of photons can be tailored by utilizing photon-sieve-based structures. We design, fabricate, and characterize self-complementary chiral structures made of holes in an Au film and generate light vortex beams with specified angular momentum orders. The incoming electron interacting with the structure generates chiral surface plasmon polaritons on the structured Au surface that scatter into the far field. The outcoupled radiation interferes with transition radiation creating TE- and TM-polarized Laguerre-Gauss light beams with a chiral intensity distribution. The generated vortex light and its unique spatiotemporal features can form the basis for the generation of structured-light electron-driven photon sources.

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.

Hybridized plasmonic modes and Fabry-Perot effect in nanoscale bowtie aperture waveguide

Based on Bethe's theory, light is hard to transmit through sub-wavelength apertures. However, a special designed sub-wavelength bowtie aperture is found to be able to transmit light with high efficiency. In this letter, modal analysis is used to study the hybridized plasmonic modes and Fabry-Perot effect of the nanoscale bowtie aperture waveguide. High frequency structure simulator (HFSS) simulations in perfect electrically conductor (PEC) and real metals are performed to calculate the fundamental mode, higher order mode, as well as their own cutoff wavelength. Mode analysis can give a better understanding of the intrinsic link between the plasmonic effects and Fabry-Perot effect. The TE₁₀ and TE₃₀ modes hybridize with channel plasmon polaritons (CPPs) modes and surface plasmon polaritons (SPPs) modes respectively. Experiments are carried out to verify the numerical results. These results are of great significance for understanding the internal mechanism of the bowtie aperture for coupling light to a sub-diffraction limited spot with high transmission efficiency.

Limits of Babinet's principle for solid and hollow plasmonic antennas

We present an experimental and theoretical study of Babinet’s principle of complementarity in plasmonics. We have used spatially-resolved electron energy loss spectroscopy and cathodoluminescence to investigate electromagnetic response of elementary plasmonic antenna: gold discs and complementary disc-shaped apertures in a gold layer. We have also calculated their response to the plane wave illumination. While the qualitative validity of Babinet’s principle has been confirmed, quantitative differences have been found related to the energy and quality factor of the resonances and the magnitude of related near fields. In particular, apertures were found to exhibit stronger interaction with the electron beam than solid antennas, which makes them a remarkable alternative of the usual plasmonic-antennas design. We also examine the possibility of magnetic near field imaging based on the Babinet’s principle.

Two-Dimensional Drexhage Experiment for Electric- and Magnetic-Dipole Sources on Plasmonic Interfaces

Fifty years ago, Drexhage et al. showed how photon emission from an electric dipole can be modified by a nearby mirror. Here, we study the two-dimensional analog for surface plasmon polaritons (SPPs). We print Eu^{3+}-doped nanoparticles, which act as both electric- and magnetic-dipole sources of SPPs, near plasmonic reflectors on flat Ag films. We measure modified SPP radiation patterns and emission rates as a function of reflector distance and source symmetry. The results, which agree with an analytical self-interference model, provide simple strategies to control SPP radiation in plasmonic devices.

Second harmonic generation hotspot on a centrosymmetric smooth silver surface

Second harmonic generation (SHG) is forbidden for materials with inversion symmetry, such as bulk metals. Symmetry can be broken by morphological or dielectric discontinuities, yet SHG from a smooth continuous metallic surface is negligible. Using non-linear microscopy, we experimentally demonstrate enhanced SHG within an area of smooth silver film surrounded by nanocavities. Nanocavity-assisted SHG is locally enhanced by more than one order of magnitude compared to a neighboring silver surface area. Linear optical measurements and cathodoluminescence (CL) imaging substantiate these observations. We suggest that plasmonic modes launched from the edges of the nanocavities propagate onto the smooth silver film and annihilate, locally generating SHG. In addition, we show that these hotspots can be dynamically controlled in intensity and location by altering the polarization of the incoming field. Our results show that switchable nonlinear hotspots can be generated on smooth metallic films, with important applications in photocatalysis, single-molecule spectroscopy and non-linear surface imaging.

Cathodoluminescence as a probe of the optical properties of resonant apertures in a metallic film

Here we present the results of an investigation of resonances of azimuthal trimer arrangements of rectangular slots in a gold film on a glass substrate using cathodoluminescence (CL) as a probe. The variation in the CL signal collected from specific locations on the sample as a function of wavelength and the spatial dependence of emission into different wavelength bands provides considerable insight into the resonant modes, particularly sub-radiant modes, of these apertures. By comparing our experimental results with electromagnetic simulations we are able to identify a Fabry-Pérot mode of these cavities as well as resonances associated with the excitation of surface plasmon polaritons on the air-gold boundary. We obtain evidence for the excitation of dark (also known as sub-radiant) modes of apertures and aperture ensembles.

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.

Topologically Enclosed Aluminum Voids as Plasmonic Nanostructures

Recent advances in the ability to synthesize metallic nanoparticles with tailored geometries have led to a revolution in the field of plasmonics. However, studies of the important complementary system, an inverted nanostructure, have so far been limited to two-dimensional sphere-segment voids or holes. Here we reveal the localized surface plasmon resonances (LSPRs) of nanovoids that are topologically enclosed in three-dimensions: an "anti-nanoparticle". We combine this topology with the favorable plasmonic properties of aluminum to observe strongly localized field enhancements with LSPR energies in the extreme UV range, well beyond those accessible with noble metals or yet achieved with aluminum. We demonstrate the resonance tunability by tailoring the shape and size of the nanovoids, which are truncated octahedra in the 10-20 nm range. This system is pristine: the nanovoid cavity is free from any oxide or supporting substrate that would affect the LSPRs. We exploit this to infer LSPRs of pure, sub-20-nm Al nanoparticles, which have yet to be synthesized. Access to this extreme UV range will allow applications in LSPR-enhanced UV photoemission spectroscopy and photoionization.

Secondary Electron Cloaking with Broadband Plasmonic Resonant Absorbers

Scanning electron microscopy (SEM) is one of the most powerful tools for nanoscale inspection and imaging. It is broadly used for biomedicine, materials science, and nanotechnology, enabling spatial resolution beyond the optical diffraction limit. In SEM, a high-energy electron beam illuminates a specimen, and the emitted secondary electrons are routed to a positively biased, synchronized detector for image creation. Here, for the first time, we experimentally demonstrate a cloaking of metallic objects from a secondary electron image. We make a metallic disc with a diameter of 300 nm almost invisible to a secondary electron detector with <5 nm spatial resolution. The secondary electron cloaking is based on broadband optical radiation absorption in the near field. Our secondary electron images are in good agreement with full-wave numerical solution of Maxwell's equations at optical frequencies, confirming the concept of secondary electron cloaking based on broadband optical radiation absorption.

Nanostructure Formation by controlled dewetting on patterned substrates: A combined theoretical, modeling and experimental study

We perform systematic two-dimensional energetic analysis to study the stability of various nanostructures formed by dewetting solid films deposited on patterned substrates. Our analytical results show that by controlling system parameters such as the substrate surface pattern, film thickness and wetting angle, a variety of equilibrium nanostructures can be obtained. Phase diagrams are presented to show the complex relations between these system parameters and various nanostructure morphologies. We further carry out both phase field simulations and dewetting experiments to validate the analytically derived phase diagrams. Good agreements between the results from our energetic analyses and those from our phase field simulations and experiments verify our analysis. Hence, the phase diagrams presented here provide guidelines for using solid-state dewetting as a tool to achieve various nanostructures.

Coupling of plasmonic nanopore pairs: facing dipoles attract each other

Control of the optical properties of nano-plasmonic structures is essential for next-generation optical circuits and high-throughput biosensing platforms. Realization of such nano-optical devices requires optical couplings of various nanostructured elements and field confinement at the nanoscale. In particular, symmetric coupling modes, also referred to as dark modes, have recently received considerable attention because these modes can confine light energy to small spaces. Although the coupling behavior of plasmonic nanoparticles has been relatively well studied, couplings of inverse structures, that is, holes and pores, remain partially unexplored. Even for the most fundamental coupling system of two dipolar holes, comparison of the symmetric and anti-symmetric coupling modes has not been performed. Here we present, for the first time, a systematic study of the symmetric and anti-symmetric coupling of nanopore pairs using cathodoluminescence by scanning transmission electron microscopy and electromagnetic simulation. The symmetric coupling mode, approximated as a pair of facing dipoles, is observed at a lower energy than that of the anti-symmetric coupling mode, indicating that the facing dipoles attract each other. The anti-symmetric coupling mode splits into the inner- and outer-edge localized modes as the coupling distance decreases. These coupling behaviors cannot be fully explained as inverses of coupled disks. Symmetric and anti-symmetric coupling modes are also observed in a short-range ordered pore array, where one pore supports multiple local resonance modes, depending on the distance to the neighboring pore. Accessibility to the observed symmetric modes by far field is also discussed, which is important for nanophotonic device applications.

Hybridization between nanocavities for a polarimetric color sorter at the sub-micron scale

Metallic hole arrays have been recently used for color generation and filtering due to their reliability and color tunability. However, color generation is still limited to several microns. Understanding the interaction between the individual elements of the whole nanostructure may push the resolution to the sub-micron level. Herein, we study the hybridization between silver nanocavities in order to obtain active color generation at the micron scale. To do so, we use five identical triangular cavities which are separated by hundreds of nanometers from each other. By tuning either the distance between the cavities or the optical polarization state of the incoming field, the transmitted light through the cavities is actively enhanced at specific frequencies. Consequently, a rainbow of colors is observed from a sub-micron scale unit. The reason for this is that the metallic surface plays a vital role in the hybridization between the cavities and contributes to higher frequency modes. Cathodoluminescence measurements have confirmed this assumption and have revealed that these five triangular cavities act as a unified entity surrounded by the propagated surface plasmons. In such plasmonic structures, multi-color tuning can be accomplished and may open the possibility to improve color generation and high-quality pixel fabrication.

Antenna-coupled photon emission from hexagonal boron nitride tunnel junctions

The ultrafast conversion of electrical signals to optical signals at the nanoscale is of fundamental interest for data processing, telecommunication and optical interconnects. However, the modulation bandwidths of semiconductor light-emitting diodes are limited by the spontaneous recombination rate of electron-hole pairs, and the footprint of electrically driven ultrafast lasers is too large for practical on-chip integration. A metal-insulator-metal tunnel junction approaches the ultimate size limit of electronic devices and its operating speed is fundamentally limited only by the tunnelling time. Here, we study the conversion of electrons (localized in vertical gold-hexagonal boron nitride-gold tunnel junctions) to free-space photons, mediated by resonant slot antennas. Optical antennas efficiently bridge the size mismatch between nanoscale volumes and far-field radiation and strongly enhance the electron-photon conversion efficiency. We achieve polarized, directional and resonantly enhanced light emission from inelastic electron tunnelling and establish a novel platform for studying the interaction of electrons with strongly localized electromagnetic fields.

Modal engineering of Surface Plasmons in apertured Au Nanoprisms

Crystalline gold nanoprisms of sub-micrometric size sustain high order plasmon modes in the visible and near infrared range that open a new realm for plasmon modal design, integrated coplanar devices and logic gates. In this article, we explore the tailoring of the surface plasmon local density of states (SP-LDOS) by embedding a single defect, namely a small hole, carved in the platelet by focused ion beam (FIB). The change in the SP-LDOS of the hybrid structure is monitored by two-photon luminescence (TPL) microscopy. The dependency of the two-dimensional optical field intensity maps on the linear polarization of the tightly focused femtosecond laser beam reveals the conditions for which the hole defect significantly affects the initial modes. A detailed numerical analysis of the spectral characteristics of the SP-LDOS based on the Green dyadic method clearly indicates that the hole size and location can be exploited to tune or remove selected SP modes.

Transforming the optical landscape

Electromagnetism provides us with some of the most powerful tools in science, encompassing lasers, optical microscopes, magnetic resonance imaging scanners, radar, and a host of other techniques. To understand and develop the technology requires more than a set of formal equations. Scientists and engineers have to form a vivid picture that fires their imaginations and enables intuition to play a full role in the process of invention. It is to this end that transformation optics has been developed, exploiting Faraday's picture of electric and magnetic fields as lines of force, which can be manipulated by the electrical permittivity and magnetic permeability of surrounding materials. Transformation optics says what has to be done to place the lines of force where we want them to be.

High aspect ratio 10-nm-scale nanoaperture arrays with template-guided metal dewetting

We introduce an approach to fabricate ordered arrays of 10-nm-scale silica-filled apertures in a metal film without etching or liftoff. Using low temperature (<400°C) thermal dewetting of metal films guided by nano-patterned templates, apertures with aspect ratios up to 5:1 are demonstrated. Apertures form spontaneously during the thermal process without need for further processing. Although the phenomenon of dewetting has been well studied, this is the first demonstration of its use in the fabrication of nanoapertures in a spatially controllable manner. In particular, the achievement of 10-nm length-scale patterning at high aspect ratio with thermal dewetting is unprecedented. By varying the nanotemplate design, we show its strong influence over the positions and sizes of the nanoapertures. In addition, we construct a three-dimensional phase field model of metal dewetting on nano-patterned substrates. The simulation data obtained closely corroborates our experimental results and reveals new insights to template dewetting at the nanoscale. Taken together, this fabrication method and simulation model form a complete toolbox for 10-nm-scale patterning using template-guided dewetting that could be extended to a wide range of material systems and geometries.

Magnetic field modification of optical magnetic dipoles

Acting on optical magnetic dipoles opens novel routes to govern light-matter interaction. We demonstrate magnetic field modification of the magnetic dipolar moment characteristic of resonant nanoholes in thin magnetoplasmonic films. This is experimentally shown through the demonstration of the magneto-optical analogue of Babinet's principle, where mirror imaged MO spectral dependencies are obtained for two complementary magnetoplasmonic systems: holes in a perforated metallic layer and a layer of disks on a substrate.