A Light-Field Metasurface for High-Resolution Single-Particle Tracking

Three-dimensional (3D) single-particle tracking (SPT) is a key tool for studying dynamic processes in the life sciences. However, conventional optical elements utilizing light fields impose an inherent trade-off between lateral and axial resolution, preventing SPT with high spatiotemporal resolution across an extended volume. We overcome the typical loss in spatial resolution that accompanies light-field-based approaches to obtain 3D information by placing a standard microscope coverslip patterned with a multifunctional, light-field metasurface on a specimen. This approach enables an otherwise unmodified microscope to gather 3D information at an enhanced spatial resolution. We demonstrate simultaneous tracking of multiple fluorescent particles within a large 0.5 × 0.5 × 0.3 mm³ volume using a standard epi-fluorescent microscope with submicron lateral and micron-level axial resolution.

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.

Dynamic thermal emission control with InAs-based plasmonic metasurfaces

Thermal emission from objects tends to be spectrally broadband, unpolarized, and temporally invariant. These common notions are now challenged with the emergence of new nanophotonic structures and concepts that afford on-demand, active manipulation of the thermal emission process. This opens a myriad of new applications in chemistry, health care, thermal management, imaging, sensing, and spectroscopy. Here, we theoretically propose and experimentally demonstrate a new approach to actively tailor thermal emission with a reflective, plasmonic metasurface in which the active material and reflector element are epitaxially grown, high-carrier-mobility InAs layers. Electrical gating induces changes in the charge carrier density of the active InAs layer that are translated into large changes in the optical absorption and thermal emission from metasurface. We demonstrate polarization-dependent and electrically controlled emissivity changes of 3.6%P (6.5% in relative scale) in the mid-infrared spectral range.

DNA-Assembled Plasmonic Waveguides for Nanoscale Light Propagation to a Fluorescent Nanodiamond

Plasmonic waveguides consisting of metal nanoparticle chains can localize and guide light well below the diffraction limit, but high propagation losses due to lithography-limited large interparticle spacing have impeded practical applications. Here, we demonstrate that DNA-origami-based self-assembly of monocrystalline gold nanoparticles allows the interparticle spacing to be decreased to ∼2 nm, thus reducing propagation losses to 0.8 dB per 50 nm at a deep subwavelength confinement of 62 nm (∼λ/10). We characterize the individual waveguides with nanometer-scale resolution by electron energy-loss spectroscopy. Light propagation toward a fluorescent nanodiamond is directly visualized by cathodoluminescence imaging spectroscopy on a single-device level, thereby realizing nanoscale light manipulation and energy conversion. Simulations suggest that longitudinal plasmon modes arising from the narrow gaps are responsible for the efficient waveguiding. With this scalable DNA origami approach, micrometer-long propagation lengths could be achieved, enabling applications in information technology, sensing, and quantum optics.

Spectrally interleaved topologies using geometric phase metasurfaces

Metasurfaces facilitate the interleaving of multiple topologies in an ultra-thin photonic system. Here, we report on the spectral interleaving of topological states of light using a geometric phase metasurface. We realize that a dielectric spectrally interleaved metasurface generates multiple interleaved vortex beams at different wavelengths. By harnessing the space-variant polarization manipulations that are enabled by the geometric phase mechanism, a vectorial vortex array is implemented. The presented interleaved topologies concept can greatly enhance the functionality of advanced microscopy and communication systems.

Polarization-independent metasurface lens employing the Pancharatnam-Berry phase

Metasurface optical elements, optical phased arrays constructed from a dense arrangement of nanoscale antennas, are promising candidates for the next generation of flat optical components. Metasurfaces that rely on the Pancharatnam-Berry phase facilitate complete and efficient wavefront control. However, their operation typically requires control over the polarization state of the incident light to achieve a desired optical function. Here, we circumvent this inherent sensitivity to the incident polarization by multiplexing two metasurfaces that were designed to achieve the same optical function with incident light of opposite helicity. We analyze the optical performance of different multiplexing approaches, and demonstrate a subwavelength random interleaved polarization-independent metasurface lens operating in the visible spectrum, providing a diffraction-limited spot size for the shared-aperture.

Optical emission near a high-impedance mirror

Solid state light emitters rely on metallic contacts with a high sheet-conductivity for effective charge injection. Unfortunately, such contacts also support surface plasmon polariton and lossy wave excitations that dissipate optical energy into the metal and limit the external quantum efficiency. Here, inspired by the concept of radio-frequency high-impedance surfaces and their use in conformal antennas we illustrate how electrodes can be nanopatterned to simultaneously provide a high DC electrical conductivity and high-impedance at optical frequencies. Such electrodes do not support SPPs across the visible spectrum and greatly suppress dissipative losses while facilitating a desirable Lambertian emission profile. We verify this concept by studying the emission enhancement and photoluminescence lifetime for a dye emitter layer deposited on the electrodes.

Light emission of molecules can be largely impacted (enhanced or quenched) by nearby surfaces. Here, Esfandyarpour et al. engineer a high-impedance mirror that increases light emission of adjacent molecules by enhancing the coupling between the molecule and free space.

Metasurface Mirrors for External Control of Mie Resonances

The ability to control and structurally tune the optical resonances of semiconductor nanostructures has far-reaching implications for a wide range of optical applications, including photodetectors, (bio)sensors, and photovoltaics. Such control is commonly obtained by tailoring the nanostructure's geometry, material, or dielectric environment. Here, we combine insights from the field of coherent optics and metasurface mirrors to effectively turn Mie resonances on and off with high spatial control and in a polarization-dependent fashion. We illustrate this in an integrated device by manipulating the photocurrent spectra of a single-nanowire photodetector placed on a metasurface mirror. This approach can be generalized to control spectral, angle-dependent, absorption, and scattering properties of semiconductor nanostructures with an engineered metasurface and without a need to alter their geometric or materials properties.

Spatially controlled doping of two-dimensional SnS2 through intercalation for electronics

Doped semiconductors are the most important building elements for modern electronic devices ¹ . In silicon-based integrated circuits, facile and controllable fabrication and integration of these materials can be realized without introducing a high-resistance interface^2,3. Besides, the emergence of two-dimensional (2D) materials enables the realization of atomically thin integrated circuits^4-9. However, the 2D nature of these materials precludes the use of traditional ion implantation techniques for carrier doping and further hinders device development ¹⁰ . Here, we demonstrate a solvent-based intercalation method to achieve p-type, n-type and degenerately doped semiconductors in the same parent material at the atomically thin limit. In contrast to naturally grown n-type S-vacancy SnS₂, Cu intercalated bilayer SnS₂ obtained by this technique displays a hole field-effect mobility of ~40 cm² V^-1s^-1, and the obtained Co-SnS₂ exhibits a metal-like behaviour with sheet resistance comparable to that of few-layer graphene ⁵ . Combining this intercalation technique with lithography, an atomically seamless p-n-metal junction could be further realized with precise size and spatial control, which makes in-plane heterostructures practically applicable for integrated devices and other 2D materials. Therefore, the presented intercalation method can open a new avenue connecting the previously disparate worlds of integrated circuits and atomically thin materials.

Thermoplasmonic Ignition of Metal Nanoparticles

Explosives, propellants, and pyrotechnics are energetic materials that can store and quickly release tremendous amounts of chemical energy. Aluminum (Al) is a particularly important fuel in many applications because of its high energy density, which can be released in a highly exothermic oxidation process. The diffusive oxidation mechanism (DOM) and melt-dispersion mechanism (MDM) explain the ways powders of Al nanoparticles (NPs) can burn, but little is known about the possible use of plasmonic resonances in NPs to manipulate photoignition. This is complicated by the inhomogeneous nature of powders and very fast heating and burning rates. Here, we generate Al NPs with well-defined sizes, shapes, and spacings by electron beam lithography and demonstrate that their plasmonic resonances can be exploited to heat and ignite them with a laser. By combining simulations with thermal-emission, electron-, and optical-microscopy studies, we reveal how an improved control over NP ignition can be attained.

Purcell effect for active tuning of light scattering from semiconductor optical antennas

Subwavelength, high-refractive index semiconductor nanostructures support optical resonances that endow them with valuable antenna functions. Control over the intrinsic properties, including their complex refractive index, size, and geometry, has been used to manipulate fundamental light absorption, scattering, and emission processes in nanostructured optoelectronic devices. In this study, we harness the electric and magnetic resonances of such antennas to achieve a very strong dependence of the optical properties on the external environment. Specifically, we illustrate how the resonant scattering wavelength of single silicon nanowires is tunable across the entire visible spectrum by simply moving the height of the nanowires above a metallic mirror. We apply this concept by using a nanoelectromechanical platform to demonstrate active tuning.

Electrical tuning of a quantum plasmonic resonance

Surface plasmon (SP) excitations in metals facilitate confinement of light into deep-subwavelength volumes and can induce strong light-matter interaction. Generally, the SP resonances supported by noble metal nanostructures are explained well by classical models, at least until the nanostructure size is decreased to a few nanometres, approaching the Fermi wavelength λ_F of the electrons. Although there is a long history of reports on quantum size effects in the plasmonic response of nanometre-sized metal particles, systematic experimental studies have been hindered by inhomogeneous broadening in ensemble measurements, as well as imperfect control over size, shape, faceting, surface reconstructions, contamination, charging effects and surface roughness in single-particle measurements. In particular, observation of the quantum size effect in metallic films and its tuning with thickness has been challenging as they only confine carriers in one direction. Here, we show active tuning of quantum size effects in SP resonances supported by a 20-nm-thick metallic film of indium tin oxide (ITO), a plasmonic material serving as a low-carrier-density Drude metal. An ionic liquid (IL) is used to electrically gate and partially deplete the ITO layer. The experiment shows a controllable and reversible blue-shift in the SP resonance above a critical voltage. A quantum-mechanical model including the quantum size effect reproduces the experimental results, whereas a classical model only predicts a red shift.

Multifunctional interleaved geometric-phase dielectric metasurfaces

Shared-aperture technology for multifunctional planar systems, performing several simultaneous tasks, was first introduced in the field of radar antennas. In photonics, effective control of the electromagnetic response can be achieved by a geometric-phase mechanism implemented within a metasurface, enabling spin-controlled phase modulation. The synthesis of the shared-aperture and geometric-phase concepts facilitates the generation of multifunctional metasurfaces. Here shared-aperture geometric-phase metasurfaces were realized via the interleaving of sparse antenna sub-arrays, forming Si-based devices consisting of multiplexed geometric-phase profiles. We study the performance limitations of interleaved nanoantenna arrays by means of a Wigner phase-space distribution to establish the ultimate information capacity of a metasurface-based photonic system. Within these limitations, we present multifunctional spin-dependent dielectric metasurfaces, and demonstrate multiple-beam technology for optical rotation sensing. We also demonstrate the possibility of achieving complete real-time control and measurement of the fundamental, intrinsic properties of light, including frequency, polarization and orbital angular momentum.

Optically resonant dielectric nanostructures

Rapid progress in nanophotonics is driven by the ability of optically resonant nanostructures to enhance near-field effects controlling far-field scattering through intermodal interference. A majority of such effects are usually associated with plasmonic nanostructures. Recently, a new branch of nanophotonics has emerged that seeks to manipulate the strong, optically induced electric and magnetic Mie resonances in dielectric nanoparticles with high refractive index. In the design of optical nanoantennas and metasurfaces, dielectric nanoparticles offer the opportunity for reducing dissipative losses and achieving large resonant enhancement of both electric and magnetic fields. We review this rapidly developing field and demonstrate that the magnetic response of dielectric nanostructures can lead to novel physical effects and applications.

Observing Plasmon Damping Due to Adhesion Layers in Gold Nanostructures Using Electron Energy Loss Spectroscopy

Gold plasmonic nanostructures with several different adhesion layers have been studied with monochromated electron energy loss spectroscopy in the scanning transmission electron microscope (STEM-EELS) and with surface enhanced Raman spectroscopy (SERS). Compared to samples with no adhesion layer, those with 2nm of Cr or Ti show broadened, lower intensity plasmon peaks as measured with EELS. This broadening is observed in both optically active ("bright") and inactive ("dark") plasmon modes. When the former are probed with SERS, the signal enhancement factor is lower for samples with Cr or Ti, another indication of reduced plasmon resonance. This work illustrates the capability of STEM-EELS to provide direct near-field measurement of changes in plasmon excitation probability with nano-scale spatial resolution. Additionally, it demonstrates that applications which require high SERS enhancement, such as biomarker detection and cancer diagnostics, can be improved by avoiding the use of a metallic adhesion layer.

Dynamic Reflection Phase and Polarization Control in Metasurfaces

Optical metasurfaces are two-dimensional optical elements composed of dense arrays of subwavelength optical antennas and afford on-demand manipulation of the basic properties of light waves. Following the pioneering works on active metasurfaces capable of modulating wave amplitude, there is now a growing interest to dynamically control other fundamental properties of light. Here, we present metasurfaces that facilitate electrical tuning of the reflection phase and polarization properties. To realize these devices, we leverage the properties of actively controlled plasmonic antennas and fundamental insights provided by coupled mode theory. Indium-tin-oxide is embedded into gap-plasmon resonator-antennas as it offers electrically tunable optical properties. By judiciously controlling the resonant properties of the antennas from under- to overcoupling regimes, we experimentally demonstrate tuning of the reflection phase over 180°. This work opens up new design strategies for active metasurfaces for displacement measurements and tunable waveplates.

Active flat optics using a guided mode resonance

Dynamically-controlled flat optics relies on achieving active and effective control over light-matter interaction in ultrathin layers. A variety of metasurface designs have achieved efficient amplitude and phase modulation. Particularly, noteworthy progress has been made with the incorporation of newly emerging electro-optical materials into such metasurfaces, including graphene, phase change materials, and transparent conductive oxides. In this Letter, we demonstrate dynamic light-matter interaction in a silicon-based subwavelength grating that supports a guided mode resonance. By overcoating the grating with indium tin oxide as an electrically tunable material, its reflectance can be tuned from 4% to 86%. Guided mode resonances naturally afford higher optical quality factors than the optical antennas used in the construction of metasurfaces. As such, they facilitate more effective control over the flow of light within the same layer thickness.

Fabry-Perot description for Mie resonances of rectangular dielectric nanowire optical resonators

We show that a dielectric nanowire (NW) with a rectangular cross section can effectively be modeled as a Fabry-Perot cavity formed by truncating a dielectric slab waveguide. By calculating the mode indices of the supported waveguide modes and the reflection phase pickup of the guided waves from the end facets, we can numerically predict the spectral locations of optical, Mie-like resonances for such NWs. This type of analysis must be performed twice in order to account for all resonances of these structures, corresponding to light propagating in the vertical or horizontal directions. The model shows excellent agreement with full-field simulations. We show how the refractive index of both the NW itself and neighboring materials and substrates impact the resonant properties. Our results can aid the development of NW-based optoelectronic devices, for which rectangular cross sections are much simpler to fabricate using top-down fabrication procedures.

Electron energy-loss spectroscopy of branched gap plasmon resonators

The miniaturization of integrated optical circuits below the diffraction limit for high-speed manipulation of information is one of the cornerstones in plasmonics research. By coupling to surface plasmons supported on nanostructured metallic surfaces, light can be confined to the nanoscale, enabling the potential interface to electronic circuits. In particular, gap surface plasmons propagating in an air gap sandwiched between metal layers have shown extraordinary mode confinement with significant propagation length. In this work, we unveil the optical properties of gap surface plasmons in silver nanoslot structures with widths of only 25 nm. We fabricate linear, branched and cross-shaped nanoslot waveguide components, which all support resonances due to interference of counter-propagating gap plasmons. By exploiting the superior spatial resolution of a scanning transmission electron microscope combined with electron energy-loss spectroscopy, we experimentally show the propagation, bending and splitting of slot gap plasmons.

Photonic Multitasking Interleaved Si Nanoantenna Phased Array

Metasurfaces provide unprecedented control over light propagation by imparting local, space-variant phase changes on an incident electromagnetic wave. They can improve the performance of conventional optical elements and facilitate the creation of optical components with new functionalities and form factors. Here, we build on knowledge from shared aperture phased array antennas and Si-based gradient metasurfaces to realize various multifunctional metasurfaces capable of achieving multiple distinct functions within a single surface region. As a key point, we demonstrate that interleaving multiple optical elements can be accomplished without reducing the aperture of each subelement. Multifunctional optical elements constructed from Si-based gradient metasurface are realized, including axial and lateral multifocus geometric phase metasurface lenses. We further demonstrate multiwavelength color imaging with a high spatial resolution. Finally, optical imaging functionality with simultaneous color separation has been obtained by using multifunctional metasurfaces, which opens up new opportunities for the field of advanced imaging and display.

Porous Silicon Gradient Refractive Index Micro-Optics

The emergence and growth of transformation optics over the past decade has revitalized interest in how a gradient refractive index (GRIN) can be used to control light propagation. Two-dimensional demonstrations with lithographically defined silicon (Si) have displayed the power of GRIN optics and also represent a promising opportunity for integrating compact optical elements within Si photonic integrated circuits. Here, we demonstrate the fabrication of three-dimensional Si-based GRIN micro-optics through the shape-defined formation of porous Si (PSi). Conventional microfabrication creates Si square microcolumns (SMCs) that can be electrochemically etched into PSi elements with nanoscale porosity along the shape-defined etching pathway, which imparts the geometry with structural birefringence. Free-space characterization of the transmitted intensity distribution through a homogeneously etched PSi SMC exhibits polarization splitting behavior resembling that of dielectric metasurfaces that require considerably more laborious fabrication. Coupled birefringence/GRIN effects are studied by way of PSi SMCs etched with a linear (increasing from edge to center) GRIN profile. The transmitted intensity distribution shows polarization-selective focusing behavior with one polarization focused to a diffraction-limited spot and the orthogonal polarization focused into two laterally displaced foci. Optical thickness-based analysis readily predicts the experimentally observed phenomena, which strongly match finite-element electromagnetic simulations.

Superabsorbing, Artificial Metal Films Constructed from Semiconductor Nanoantennas

In 1934, Wilhelm Woltersdorff demonstrated that the absorption of light in an ultrathin, freestanding film is fundamentally limited to 50%. He concluded that reaching this limit would require a film with a real-valued sheet resistance that is exactly equal to R = η/2 ≈ 188.5Ω/□, where [Formula: see text] is the impedance of free space. This condition can be closely approximated over a wide frequency range in metals that feature a large imaginary relative permittivity εr″, that is, a real-valued conductivity σ = ε0εr″ω. A thin, continuous sheet of semiconductor material does not facilitate such strong absorption as its complex-valued permittivity with both large real and imaginary components preclude effective impedance matching. In this work, we show how a semiconductor metafilm constructed from optically resonant semiconductor nanostructures can be created whose optical response mimics that of a metallic sheet. For this reason, the fundamental absorption limit mentioned above can also be reached with semiconductor materials, opening up new opportunities for the design of ultrathin optoelectronic and light harvesting devices.

Nanoscale Spatial Coherent Control over the Modal Excitation of a Coupled Plasmonic Resonator System

We demonstrate coherent control over the optical response of a coupled plasmonic resonator by high-energy electron beam excitation. We spatially control the position of an electron beam on a gold dolmen and record the cathodoluminescence and electron energy loss spectra. By selective coherent excitation of the dolmen elements in the near field, we are able to manipulate modal amplitudes of bonding and antibonding eigenmodes. We employ a combination of CL and EELS to gain detailed insight in the power dissipation of these modes at the nanoscale as CL selectively probes the radiative response and EELS probes the combined effect of Ohmic dissipation and radiation.