arXiv subjects

Michael B. Sinclair

Publications and source records attributed to Michael B. Sinclair.

17 recordsLinked to original sources

arXiv2023

Near-field Imaging of Optical Resonances in Silicon Metasurfaces Using Photoelectron Microscopy

Precise control of light-matter interactions at the nanoscale lies at the heart of nanophotonics. Experimental examination at this length scale is challenging, however, since the corresponding electromagnetic near-field is often confined within volumes below the resolution of conventional optical microscopy. In semiconductor nanophotonics electromagnetic fields are further restricted within the confines of individual subwavelength resonators, limiting access to critical light-matter interactions in these structures. In this work, we demonstrate that photoelectron emission microscopy (PEEM) can be used for polarization resolved near-field spectroscopy and imaging of electromagnetic resonances supported by broken-symmetry silicon metasurfaces. We find that the photoemission results, enabled through an in-situ potassium surface layer, are consistent with full-wave simulations and far-field reflectance measurements across visible and near-infrared wavelengths. In addition, we uncover a polarization dependent evolution of collective resonances near the metasurface array edge taking advantage of the far-field excitation and full-field imaging of PEEM. Here, we deduce that coupling between eight resonators or more establishes the collective excitations of this metasurface. All told, we demonstrate that the high-spatial resolution hyperspectral imaging and far-field illumination of PEEM can be leveraged for the metrology of collective, non-local, optical resonances in semiconductor nanophotonic structures.

physics.optics↗

arXiv2022

Sub picosecond steering of ultrafast incoherent emission from semiconductor metasurfaces

The ability to dynamically steer fs pulses from a monolithically integrated source is a critical milestone for the fields of nanophotonics and ultrafast optics. The nascent field of reconfigurable metasurfaces -- made of optically resonant meta-atoms -- has shown great promise in manipulating light-matter interactions through subwavelength control of the phase, amplitude, and polarization of light. These active metasurfaces arbitrarily transform an incident wavefront using a reconfigurable spatial phase profile and thus have been limited to manipulating coherent external sources. Light emitting metasurfaces obtained through integration of incoherent emitters with meta-atoms, have been used to statically increase the quantum efficiency of the emission through Purcell factor enhancement and control the far-field emission properties of the light to collimate and focus spontaneous emission. Active manipulation of such incoherent light sources, however, remains a challenge as current phase-sensitive approaches used for coherent sources cannot be directly applied. Spatiotemporal control at ultrafast timescales of incoherent light emission could lead to a transformative technological leap allowing low-power light emitting diodes (LEDs) to replace high-power coherent laser sources, enabling holographic LED displays and other key optical transceiver applications including remote-sensing, perception, and high-speed optical communication systems. In this work, we theoretically predict and experimentally demonstrate for the first time, sub-picosecond steering over a 70° range of ultrafast incoherent emission from a light emitting metasurface.

physics.optics↗

arXiv2021

Cascaded optical nonlinearities in dielectric metasurfaces

Since the discovery of the laser, optical parametric nonlinearities have been at the core of efficient light conversion sources. Typically, thick transparent crystals or quasi-phase matched waveguides, are utilized in conjunction with phase-matching techniques to select a single parametric process. In recent years, due to the rapid developments in artificially structured materials, optical frequency mixing has been achieved at the nanoscale in subwavelength resonators arrayed as metasurfaces. Phase matching becomes relaxed for these wavelength-scale structures, and all allowed nonlinear processes can, in principle, occur on an equal footing. This could promote harmonic generation via a cascaded (consisting of several frequency mixing steps) process. However, so far, all reported work on dielectric metasurfaces have assumed frequency mixing from a direct (single step) nonlinear process. In this work, we prove the existence of cascaded second-order optical nonlinearities by analyzing the second and third wave mixing from a highly nonlinear metasurface in conjunction with polarization selection rules and crystal symmetries. We find that the third wave mixing signal from a cascaded process can be of comparable strength to that from conventional third harmonic generation, and that surface nonlinearities are the dominant mechanism that contributes to cascaded second order nonlinearities

physics.optics↗

arXiv2019

High-harmonic generation from an epsilon-near-zero material

High-harmonic generation (HHG) from a compact, solid-state medium is highly desirable for applications such as coherent attosecond pulse generation and extreme ultra-violet (EUV) spectroscopy, yet the typically weak conversion of pump light to HHG can largely hinder its applications. Here, we use a material operating in its epsilon-near-zero (ENZ) region, where the real part of its permittivity vanishes, to greatly boost the efficiency of the HHG process at the microscopic level. In experiments, we report high-harmonic emission up to the 9th order directly from a low-loss, solid-state ENZ medium: indium-doped cadmium oxide, with an excitation intensity at the GW cm-2 level. Furthermore, the observed HHG signal exhibits a pronounced spectral red-shift as well as linewidth broadening, resulting from the photo-induced electron heating and the consequent time-dependent resonant frequency of the ENZ film. Our results provide a novel nanophotonic platform for strong field physics, reveal new degrees of freedom for spectral and temporal control of HHG, and open up possibilities of compact solid-state attosecond light sources.

physics.app-ph↗

arXiv2017

An optical metamixer

A frequency mixer is a nonlinear device that combines electromagnetic waves to create waves at new frequencies. Mixers are ubiquitous components in modern radio-frequency technology and are widely used in microwave signal processing. The development of versatile frequency mixers for optical frequencies remains challenging: such devices generally rely on weak nonlinear optical processes and, thus, must satisfy phase matching conditions. In this work, we utilize a GaAs-based dielectric metasurface to demonstrate an optical frequency mixer that concurrently generates eleven new frequencies spanning the ultraviolet to near-infrared (NIR) spectral range. Our approach combines strong intrinsic material nonlinearities, enhanced electromagnetic fields, and relaxed phase-matching requirements, to allow seven different nonlinear optical processes to occur simultaneously. Specifically, when pumped by two femtosecond NIR pulses, we observe second-, third- and fourth-harmonic generation, sum-frequency generation, two-photon absorption induced photoluminescence, four-wave mixing, and six-wave mixing. Such ultracompact optical mixers may enable a plethora of applications in biology, chemistry, sensing, communications and quantum optics.

physics.optics↗

arXiv2017

Active tuning of high-Q dielectric metasurfaces

We demonstrate the active tuning of all-dielectric metasurfaces exhibiting high-quality factor (high-Q) resonances. The active control is provided by embedding the asymmetric silicon meta-atoms with liquid crystals, which allows the relative index of refraction to be controlled through heating. It is found that high quality factor resonances ($Q=270\pm30$) can be tuned over more than three resonance widths. Our results demonstrate the feasibility of using all-dielectric metasurfaces to construct tunable narrow-band filters.

physics.optics↗

arXiv2016

Resonantly enhanced second-harmonic generation using III-V semiconductor all-dielectric metasurfaces

Nonlinear optical phenomena in nanostructured materials have been challenging our perceptions of nonlinear optical processes that have been explored since the invention of lasers. For example, the ability to control optical field confinement, enhancement, and scattering almost independently, allows nonlinear frequency conversion efficiencies to be enhanced by many orders of magnitude compared to bulk materials. Also, the subwavelength length scale renders phase matching issues irrelevant. Compared with plasmonic nanostructures, dielectric resonator metamaterials show great promise for enhanced nonlinear optical processes due to their larger mode volumes. Here, we present, for the first time, resonantly enhanced second-harmonic generation (SHG) using Gallium Arsenide (GaAs) based dielectric metasurfaces. Using arrays of cylindrical resonators we observe SHG enhancement factors as large as 104 relative to unpatterned GaAs. At the magnetic dipole resonance we measure an absolute nonlinear conversion efficiency of ~2X10^(-5) with ~3.4 GW/cm2 pump intensity. The polarization properties of the SHG reveal that both bulk and surface nonlinearities play important roles in the observed nonlinear process.

physics.optics↗

arXiv2016

Broken symmetry dielectric resonators for high quality-factor Fano metasurfaces

We present a new approach to dielectric metasurface design that relies on a single resonator per unit cell and produces robust, high quality-factor Fano resonances. Our approach utilizes symmetry breaking of highly symmetric resonator geoemetries, such as cubes, to induce couplings between the otherwise orthogonal resonator modes. In particular, we design perturbations that couple "bright" dipole modes to "dark" dipole modes whose radiative decay is suppressed by local field effects in the array. Our approach is widely scalable from the near-infrared to radio frequencies. We first unravel the Fano resonance behavior through numerical simulations of a germanium resonator-based metasurface that achieved a quality-factor of ~1300 at ~10.8 um. Then, we present two experimental demonstrations operating in the near-infrared (~1 um): a silicon-based implementation that achieved a quality-factor of ~350; and a gallium arsenide-based structure that achieves a quality-factor of ~600 - the highest near-infrared quality-factor experimentally demonstrated to date with this kind of metasurfaces. Importantly, large electromagnetic field enhancements appear within the resonators at the Fano resonant frequencies. We envision that combining high-quality factor, high field enhancement resonances with nonlinear and active/gain materials such as gallium arsenide will lead to new classes of active optical devices.

physics.optics↗

arXiv2016

III-V semiconductor nano-resonators-a new strategy for passive, active, and nonlinear all-dielectric metamaterials

Metamaterials comprising assemblies of dielectric resonators have attracted much attention due to their low intrinsic loss and isotropic optical response. In particular, metasurfaces made from silicon dielectric resonators have shown desirable behaviors such as efficient nonlinear optical conversion, spectral filtering and advanced wave-front engineering. To further explore the potential of dielectric metamaterials, we present all-dielectric metamaterials fabricated from epitaxially grown III-V semiconductors that can exploit the high second-order optical susceptibilities of III-V semiconductors, as well as the ease of monolithically integrating active/gain media. Specifically, we create GaAs nano-resonators using a selective wet oxidation process that forms a low refractive index AlGaO (n~1.6) under layer similar to silicon dielectric resonators formed using silicon-on-insulator wafers. We further use the same fabrication processes to demonstrate multilayer III-V dielectric resonator arrays that provide us with new degrees of freedom in device engineering. For these arrays, we experimentally measure ~100% reflectivity over a broad spectral range. We envision that all-dielectric III-V semiconductor metamaterials will open up new avenues for passive, active and nonlinear all dielectric metamaterials

physics.optics↗

arXiv2015

Realizing high-quality, ultra-large momentum states using semiconductor hyperbolic metamaterials

We employ both the effective medium approximation (EMA) and Bloch theory to compare the dispersion properties of semiconductor hyperbolic metamaterials (SHMs) at mid-infrared frequencies and metallic hyperbolic metamaterials (MHMs) at visible frequencies. This analysis reveals the conditions under which the EMA can be safely applied for both MHMs and SHMs. We find that the combination of precise nanoscale layering and the longer infrared operating wavelengths puts the SHMs well within the effective medium limit and, in contrast to MHMs, allows the attainment of very high photon momentum states. In addition, SHMs allow for new phenomena such as ultrafast creation of the hyperbolic manifold through optical pumping. In particular, we examine the possibility of achieving ultrafast topological transitions through optical pumping which can photo-dope appropriately designed quantum wells on the femtosecond time scale.

physics.optics↗

arXiv2015

Enhanced third harmonic generation from the epsilon-near-zero modes of ultrathin films

We experimentally demonstrate efficient third harmonic generation from an indium tin oxide (ITO) nanofilm (lambda/42 thick) on a glass substrate for a pump wavelength of 1.4 um. A conversion efficiency of 3.3x10^-6 is achieved by exploiting the field enhancement properties of the epsilon-near-zero (ENZ) mode with an enhancement factor of 200. This nanoscale frequency conversion method is applicable to other plasmonic materials and reststrahlen materials in proximity of the longitudinal optical phonon frequencies.

physics.optics↗

arXiv2014

Tailoring dielectric resonator geometries for directional scattering and Huygens' metasurfaces

In this paper we describe a methodology for tailoring the design of metamaterial dielectric resonators, which represent a promising path toward low-loss metamaterials at optical frequencies. We first describe a procedure to decompose the far field scattered by subwavelength resonators in terms of multipolar field components, providing explicit expressions for the multipolar far fields. We apply this formulation to confirm that an isolated high-permittivity cube resonator possesses frequency separated electric and magnetic dipole resonances, as well as a magnetic quadrupole resonance in close proximity to the electric dipole resonance. We then introduce multiple dielectric gaps to the resonator geometry in a manner suggested by perturbation theory, and demonstrate the ability to overlap the electric and magnetic dipole resonances, thereby enabling directional scattering by satisfying the first Kerker condition. We further demonstrate the ability to push the quadrupole resonance away from the degenerate dipole resonances to achieve local behavior. These properties are confirmed through the multipolar expansion and show that the use of geometries suggested by perturbation theory is a viable route to achieve purely dipole resonances for metamaterial applications such as wave-front manipulation with Huygens' metasurfaces. Our results are fully scalable across any frequency bands where high-permittivity dielectric materials are available, including microwave, THz, and infrared frequencies.

physics.optics↗

arXiv2014

Directional perfect absorption using deep subwavelength low permittivity films

We experimentally demonstrate single beam directional perfect absorption (to within experimental accuracy) of p-polarized light in the near-infrared using unpatterned, deep subwavelength films of indium tin oxide (ITO) on Ag. The experimental perfect absorption occurs slightly above the epsilon-near-zero (ENZ) frequency of ITO where the permittivity is less than one. Remarkably, we obtain perfect absorption for films whose thickness is as low as ~1/50th of the operating free-space wavelength and whose single pass attenuation is only ~ 5%. We further derive simple analytical conditions for perfect absorption in the subwavelength-film regime that reveal the constraints that the ITO permittivity must satisfy if perfect absorption is to be achieved. Then, to get a physical insight on the perfect absorption properties, we analyze the eigenmodes of the layered structure by computing both the real-frequency/complex-wavenumber and the complex-frequency/real-wavenumber modal dispersion diagrams. These analyses allow us to attribute the experimental perfect absorption behavior to the crossover between bound and leaky behavior of one eigenmode of the layered structure. Both modal methods show that perfect absorption occurs at a frequency slightly larger than the ENZ frequency, in agreement with experimental results, and both methods predict a second perfect absorption condition at higher frequencies attributed to another crossover between bound and leaky behavior of the same eigenmode. Our results greatly expand the list of materials that can be considered for use as ultrathin perfect absorbers and also provide a methodology for the design of absorber systems at any desired frequency.

physics.optics↗

arXiv2014

Electrodynamic modeling of strong coupling between a metasurface and intersubband transitions in quantum wells

Strong light-matter coupling has recently been demonstrated in sub-wavelength volumes by coupling engineered optical transitions in semiconductor heterostructures (e.g., quantum wells) to metasurface resonances via near fields. It has also been shown that different resonator shapes may lead to different Rabi splittings, though this has not yet been well explained. In this paper, our aim is to understand the correlation between resonator shape and Rabi splitting, and in particular determine and quantify the physical parameters that affect strong coupling by developing an equivalent circuit network model whose elements describe energy and dissipation. Because of the subwavelength dimension of each metasurface element, we resort to the quasi-static (electrostatic) description of the near-field and hence define an equivalent capacitance associated to each dipolar element of a flat metasurface, and we show that this is also able to accurately model the phenomenology involved in strong coupling between the metasurface and the intersubband transitions in quantum wells. We show that the spectral properties and stored energy of a metasurface/quantum-well system obtained using our model are in good agreement with both full-wave simulation and experimental results. We then analyze metasurfaces made of three different resonator geometries and observe that the magnitude of the Rabi splitting increases with the resonator capacitance in agreement with our theory, providing a phenomenological explanation for the resonator shape dependence of the strong coupling process.

physics.optics↗

arXiv2014

Optical Magnetic Mirrors without Metals

The reflection of an optical wave from a metal, arising from strong interactions between the optical electric field and the free carriers of the metal, is accompanied by a phase reversal of the reflected electric field. A far less common route to achieve high reflectivity exploits strong interactions between the material and the optical magnetic field to produce a magnetic mirror which does not reverse the phase of the reflected electric field. At optical frequencies, the magnetic properties required for strong interaction can only be achieved through the use of artificially tailored materials. Here we experimentally demonstrate, for the first time, the magnetic mirror behavior of a low-loss, all-dielectric metasurface at infrared optical frequencies through direct measurements of the phase and amplitude of the reflected optical wave. The enhanced absorption and emission of transverse electric dipoles placed very close to magnetic mirrors can lead to exciting new advances in sensors, photodetectors, and light sources.

physics.optics↗

arXiv2011

Infrared Dielectric Resonator Metamaterial

We demonstrate, for the first time, an all-dielectric metamaterial resonator in the mid-wave infrared based on high-index tellurium cubic inclusions. Dielectric resonators are desirable compared to conventional metallo-dielectric metamaterials at optical frequencies as they are largely angular invariant, free of ohmic loss, and easily integrated into three-dimensional volumes. With these low-loss, isotropic elements, disruptive optical metamaterial designs, such as wide-angle lenses and cloaks, can be more easily realized.

physics.optics↗

arXiv2011

Interaction between Metamaterial Resonators and Inter-subband transitions in Semiconductor Quantum Wells

We report on the coupling and interaction between the fundamental resonances of planar metamaterials (split ring resonators) and inter-subband transitions in GaAs/AlGaAs quantum wells structures in the mid-infrared. An incident field polarized parallel to the sample surface is converted by the metamaterial resonators into a field with a finite component polarized normal to the surface, and interacts strongly with the large dipole moment associated with quantum well inter-subband transitions.

physics.optics↗