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.

Efficient radiational outcoupling of electromagnetic energy from hyperbolic metamaterial resonators.

Hyperbolic metamaterials were initially proposed in optics to boost radiation efficiencies of quantum emitters. Adopting this concept for antenna design allows approaching long-standing contests in radio physics. For example, broadband impedance matching, accompanied with moderately high antenna gain, is among the existent challenges. Here we propose employing hyperbolic metamaterials for a broadband impedance matching, while a structured layer on top of a metamaterials slab ensures an efficient and directive energy outcoupling to a free space. In particular, a subwavelength loop antenna, placed underneath the matching layer, efficiently excites bulk metamaterial modes, which have well-resolved spatial-temporal separation owing to the hypebolicity of effective permeability tensor. Interplaying chromatic and modal dispersions enable to map different frequencies into non overlapping spatial locations within a compact subwavelength hyperbolic slab. The outcoupling of energy to the free space is obtained by patterning the slab with additional resonant elements, e.g. high index dielectric spheres. As the result, two-order of magnitude improvement in linear gain of the device is predicted. The proposed new architecture can find a use in applications, where multiband or broadband compact devices are required.

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.

Single-transverse-mode broadband InAs quantum dot superluminescent light emitting diodes by parity-time symmetry.

Parity-time (PT) symmetry breaking in counterintuitive gain/loss coupled waveguide designs is numerically and theoretically investigated. The PT symmetry mode selection conditions are determined theoretically. Single-transverse-mode broadband InAs quantum dot (QD) superluminescent light emitting diodes (SLEDs) are fabricated and characterized; the PT symmetric broad-area SLEDs contain laterally coupled gain and loss PT- symmetric waveguides. Single-transverse-mode operation is achieved by parity-time symmetry breaking. The broadband SLEDs exhibit a uniform Gaussian-like emission spectrum with the 3-dB bandwidth of 110 nm. Far-field characteristics of the coupled waveguide SLEDs exhibit a single-lobe far-field pattern when the gain and loss waveguides are biased at the injection current of 600 mA and 60 mA, respectively.

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.

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.

Enhanced Optical Transmission through MacEtch-Fabricated Buried Metal Gratings.

Metallic films with subwavelength apertures, integrated into a semiconductor by metal-assisted chemical etch (MacEtch), demonstrate enhanced transmission when compared to bare semiconductor surfaces. The resulting "buried" metallic structures are characterized spectroscopically and modeled using rigorous coupled wave analysis. These composite materials offer potential integration with optoelectronic devices, for simultaneous near-uniform electrical contact and strong optical coupling to free space.

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.

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.

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.

Focus issue: hyperbolic metamaterials.

This special issue presents a cross-section of recent progress in the rapidly developing area of optics of hyperbolic metamaterials.

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.