Eigendecomposition-free inverse design of meta-optics devices.

The inverse design of meta-optics has received much attention in recent years. In this paper, we propose a GPU-friendly inverse design framework based on improved eigendecomposition-free rigorous diffraction interface theory, which offers up to 16.2 × speedup over the traditional inverse design based on rigorous coupled-wave analysis. We further improve the framework's flexibility by introducing a hybrid parameterization combining neural-implicit and traditional shape optimization. We demonstrate the effectiveness of our framework through intricate tasks, including the inverse design of reconfigurable free-form meta-atoms.

Extending plasmonic response to the mid-wave infrared with all-epitaxial composites.

Highly doped semiconductor "designer metals" have been shown to serve as high-quality plasmonic materials across much of the long-wavelength portion of the mid-infrared. These plasmonic materials benefit from a technologically mature semiconductor fabrication infrastructure and the potential for monolithic integration with electronic and photonic devices. However, accessing the short-wavelength side of the mid-infrared is a challenge for these designer metals. In this work we study the perspectives for extending the plasmonic response of doped semiconductors to shorter wavelengths by leveraging charge confinement, in addition to doping. We demonstrate, theoretically and experimentally, negative permittivity across the technologically vital mid-wave infrared (3-5 µm) frequency range. The semiconductor composites presented in our work offer an ideal material platform for monolithic integration with a variety of semiconductor optoelectronic devices operating in the mid-wave infrared.

Angle-insensitive plasmonic nanorod metamaterial-based band-pass optical filters.

We demonstrate, experimentally and theoretically, a new class of angle-insensitive band-pass optical filters that utilize anisotropy of plasmonic nanorod metamaterials, in both ε ≃ -1 and epsilon-near-infinity regimes, to minimize dependence of optical path on the incident angle. The operating wavelength and bandwidth of the filter can be engineered by controlling the geometry of the metamaterial. Experimental results are in agreement with full wave numerical and analytical solutions of the Maxwell's equations. Theoretical simulations show that performance of the systems can be further improved by replacing metallic mirrors with dielectric stacks.

Engineering the Berreman mode in mid-infrared polar materials.

We demonstrate coupling to and control over the broadening and dispersion of a mid-infrared leaky mode, known as the Berreman mode, in samples with different dielectric environments. We fabricate subwavelength films of AlN, a mid-infrared epsilon-near-zero material that supports the Berreman mode, on materials with a weakly negative permittivity, strongly negative permittivity, and positive permittivity. Additionally, we incorporate ultra-thin AlN layers into a GaN/AlN heterostructure, engineering the dielectric environment above and below the AlN. In each of the samples, coupling to the Berreman mode is observed in angle-dependent reflection measurements at wavelengths near the longitudinal optical phonon energy. The measured dispersion of the Berreman mode agrees well with numerical modes. Differences in the dispersion and broadening for the different materials is quantified, including a 13 cm-1 red-shift in the energy of the Berreman mode for the heterostructure sample.

Low-frequency nonlocal and hyperbolic modes in corrugated wire metamaterials.

Metamaterials based on arrays of aligned plasmonic nanowires have recently attracted significant attention due to their unique optical properties that combine tunable strong anisotropy and nonlocality. These optical responses provide a platform for implementation of novel sensing, imaging, and quantum optics applications. Basic building blocks, used for construction of those peculiar composites, are plasmonic metals, such as gold and silver, which have moderate negative values of permittivities at the optical spectral range. Scaling the plasmonic behavior to lower frequencies remains a longstanding challenge also owing to the emergence of strong spatial dispersion in homogenized artificial composites. At lower THz and GHz frequencies, the electromagnetic response of noble metals approaches that of perfect electric conductors, preventing straightforward scaling of visible-frequency plasmonics to the frequency domains that are important for a vast range of applications, including wireless communications, microwave technologies and many others. Here we demonstrate that both extreme anisotropy (so-called hyperbolicity) and nonlocality of artificial composites can be achieved and designed in arrays of corrugated perfectly conducting wires at relatively low GHz frequencies. The key concept is based on hybridization of spoof plasmon polariton modes that in turn emulate surface polariton waves in systems with corrugated interfaces. The method makes it possible to map the recent developments in the field of plasmonics and metamaterials to the domain of THz and RF photonics.

Diffractive Interface Theory: nonlocal polarizability approach to the optics of metasurfaces: erratum.

A typo in the software implementation of Diffractive Interface Theory [Opt. Express23, 2764 (2015)10.1364/OE.23.002764] was found during subsequent research. The typo was corrected, yielding better-than-originally-reported agreement between Diffractive Interface Theory and full-wave numerical solutions of Maxwell equations.

Interscale mixing microscopy: numerically stable imaging of wavelength- scale objects with sub-wavelength resolution and far field measurements.

We present an imaging technique that allows the recovery of the profile of wavelength-scale objects with deep subwavelength resolution based on far-field intensity measurements. The approach, interscale mixing microscopy (IMM), relies on diffractive elements positioned in the near-field proximity of an object in order to scatter information carried by evanescent waves into propagating part of the spectrum. A combination of numerical solutions of Maxwell equations and nonlinear fitting is then used to recover the information about the object based on far-field intensity measurements. It is demonstrated that IMM has the potential to recover wavelength/20 features of wavelength-scale objects in the presence of 10% noise.

Diffractive interface theory: nonlocal susceptibility approach to the optics of metasurfaces.

We present a formalism for understanding the electromagnetism of metasurfaces, optically thin composite films with engineered diffraction. The technique, diffractive interface theory (DIT), takes explicit advantage of the small optical thickness of a metasurface, eliminating the need for solving for light propagation inside the film and providing a direct link between the spatial profile of a metasurface and its diffractive properties. Predictions of DIT are compared with full-wave numerical solutions of Maxwell's equations, demonstrating DIT's validity and computational advantages for optically thin structures. Applications of the DIT range from understanding of fundamentals of light-matter interaction in metasurfaces to efficient analysis of generalized refraction to metasurface optimization.

Looking into meta-atoms of plasmonic nanowire metamaterial.

Nanowire-based plasmonic metamaterials exhibit many intriguing properties related to the hyperbolic dispersion, negative refraction, epsilon-near-zero behavior, strong Purcell effect, and nonlinearities. We have experimentally and numerically studied the electromagnetic modes of individual nanowires (meta-atoms) forming the metamaterial. High-resolution, scattering-type near-field optical microscopy has been used to visualize the intensity and phase of the modes. Numerical and analytical modeling of the mode structure is in agreement with the experimental observations and indicates the presence of the nonlocal response associated with cylindrical surface plasmons of nanowires.

Hyperbolic metamaterials: new physics behind a classical problem.

Hyperbolic materials enable numerous surprising applications that include far-field subwavelength imaging, nanolithography, and emission engineering. The wavevector of a plane wave in these media follows the surface of a hyperboloid in contrast to an ellipsoid for conventional anisotropic dielectric. The consequences of hyperbolic dispersion were first studied in the 50's pertaining to the problems of electromagnetic wave propagation in the Earth's ionosphere and in the stratified artificial materials of transmission lines. Recent years have brought explosive growth in optics and photonics of hyperbolic media based on metamaterials across the optical spectrum. Here we summarize earlier theories in the Clemmow's prescription for transformation of the electromagnetic field in hyperbolic media and provide a review of recent developments in this active research area.

Low-diffraction beaming in plasmonic crystals.

We analyze the propagation of electromagnetic modes guided by periodic plasmonic structures. We use full-wave solutions of Maxwell equations to calculate dispersion of these modes and derive analytical description of their optical properties. Finally, we demonstrate that, at a certain frequency range that can be controlled by the geometry, diffraction of these guided states is strongly suppressed, leading to formation of low-diffraction beams. A beaming phenomenon, consistent with earlier experiments, can be used as the foundation for on-chip communication or microscopy.

Collective phenomena in photonic, plasmonic and hybrid structures.

Preface to a focus issue of invited articles that review recent progress in studying the fundamental physics of collective phenomena associated with coupling of confined photonic, plasmonic, electronic and phononic states and in exploiting these phenomena to engineer novel devices for light generation, optical sensing, and information processing.

Enhanced bandwidth and reduced dispersion through stacking multiple optical metamaterials.

All-semiconductor, highly anisotropic metamaterials provide a straightforward path to negative refraction in the mid-infrared. However, their usefulness in applications is restricted by strong frequency dispersion and limited spectral bandwidth. In this work, we show that by stacking multiple metamaterials of varying thickness and doping into one compound metamaterial, bandwidth is increased by 27% over a single-stack metamaterial, and dispersion is reduced.

Sub-diffraction negative and positive index modes in mid-infrared waveguides.

We characterize a strongly anisotropic waveguide consisting of alternating 80 nm layers of n(+)-InGaAs and i-AlInAs on InP substrate. A strong increase in the transverse magnetic (TM) reflection at lambda = 8.4 microm corresponds to a characteristic low-order mode cutoff for the left-handed waveguide. The subsequent decrease of TM reflection at lambda = 11.5 microm represents the onset of right-handed no-cutoff light guiding. Good qualitative agreement is found when the experimental results are compared to finite element and transfer-matrix frequency domain simulations.

Spontaneous emission in non-local materials.

Light-matter interactions can be strongly modified by the surrounding environment. Here, we report on the first experimental observation of molecular spontaneous emission inside a highly non-local metamaterial based on a plasmonic nanorod assembly. We show that the emission process is dominated not only by the topology of its local effective medium dispersion, but also by the non-local response of the composite, so that metamaterials with different geometric parameters but the same local effective medium properties exhibit different Purcell factors. A record-high enhancement of a decay rate is observed, in agreement with the developed quantitative description of the Purcell effect in a non-local medium. An engineered material non-locality introduces an additional degree of freedom into quantum electrodynamics, enabling new applications in quantum information processing, photochemistry, imaging and sensing with macroscopic composites.

Light emission in nonlocal plasmonic metamaterials.

We present analytical and computational studies of light emission in nonlocal metamaterials formed by arrays of aligned plasmonic nanowires. We demonstrate that the emission lifetime in these composites is a complex function of geometrical and material parameters of the system that cannot be reduced to the "trivial" hyperbolic or elliptical dispersion topology of a homogenised metamaterial. In particular, our studies suggest that the Purcell factor can often be maximized when the composite operates in the elliptic regime, with strong radiation coupling to an additional TM-polarized mode supported by the nonlocal composite, in contrast to the accepted "hyperbolicity related" enhancement.

Hypergratings: nanophotonics in planar anisotropic metamaterials.

We present a technique capable of producing subwavelength focal spots in planar nonresonant structures not limited to the near-field of the source. The approach combines the diffraction gratings that generate the high-wave-vector-number modes and planar slabs of homogeneous anisotropic metamaterials that propagate these waves and combine them at the subwavelength focal spots. In a sense, the technique combines the benefits of Fresnel lens, near-field zone plates, hyperlens, and superlens and at the same time resolves their fundamental limitations. Several realizations of the proposed technique for visible, near-IR, and mid-IR frequencies are proposed, and their performance is analyzed theoretically and numerically. Generalizations of the developed approach for subdiffractional imaging and on-chip photonics are suggested.

Optical nonlocalities and additional waves in epsilon-near-zero metamaterials.

We analyze the optical properties of plasmonic nanorod metamaterials in the epsilon-near-zero regime and show, both theoretically and experimentally, that the performance of these composites is strongly affected by nonlocal response of the effective permittivity tensor. We provide the evidence of interference between main and additional waves propagating in the room-temperature nanorod metamaterials and develop an analytical description of this phenomenon. Additional waves are present in the majority of low-loss epsilon-near-zero structures and should be explicitly considered when designing applications of epsilon-near-zero composites, as they represent a separate communication channel.

Scattering-free plasmonic optics with anisotropic metamaterials.

We develop an approach to utilize anisotropic metamaterials to solve one of the fundamental problems of modern plasmonics--parasitic scattering of surface waves into free-space modes, opening the road to truly two-dimensional plasmonic optics. We illustrate the developed formalism on the examples of plasmonic refractor and plasmonic crystal, and discuss limitations of the developed technique and its possible applications for sensing and imaging structures, high-performance mode couplers, optical cloaking structures, and dynamically reconfigurable electroplasmonic circuits.