As optical metasurfaces become progressively ubiquitous, the expectations from them are becoming increasingly complex. The limited number of structural parameters in the conventional metasurface building blocks, and existing phase engineering rules do not completely support the growth rate of metasurface applications. In this paper, we present digitized-binary elements, as alternative high-dimensional building blocks, to accommodate the needs of complex-tailorable-multifunctional applications. To design these complicated platforms, we demonstrate adaptive genetic algorithm (AGA), as a powerful evolutionary optimizer, capable of handling such demanding design expectations. We solve four complex problems of high current interest to the optics community, namely, a binary-pattern plasmonic reflectarray with high tolerance to fabrication imperfections and high reflection efficiency for beam-steering purposes, a dual-beam aperiodic leaky-wave antenna, which diffracts TE and TM excitation waveguides modes to arbitrarily chosen directions, a compact birefringent all-dielectric metasurface with finer pixel resolution compared to canonical nano-antennas, and a visible-transparent infrared emitting/absorbing metasurface that shows high promise for solar-cell cooling applications, to showcase the advantages of the combination of binary-pattern metasurfaces and the AGA technique. Each of these novel applications encounters computational and fabrication challenges under conventional design methods, and is chosen carefully to highlight one of the unique advantages of the AGA technique. Finally, we show that large surplus datasets produced as by-products of the evolutionary optimizers can be employed as ingredients of the new-age computational algorithms, such as, machine learning and deep leaning. In doing so, we open a new gateway of predicting the solution to a problem in the fastest possible way based on statistical analysis of the datasets rather than researching the whole solution space.

Metagratings are flat and thin surfaces that rely on unique, periodically repeating (non-gradient), arbitrary shaped light scattering units for wave manipulation. However, the absence of an empirical relationship between the structural and diffraction properties of the units enforces utilization of brute force numerical optimization techniques to determine the unit shape for a desired application. Here, we present an artificial neural network based methodology to develop a fast-paced numerical relationship between the two. We demonstrate the training and the performance of a numerical function, utilizing simulated diffraction efficiencies of a large set of units, that can instantaneously mimic the optical response of any other arbitrary shaped unit of the same class. We validate the performance of the trained neural network on a previously unseen set of test samples and discuss the statistical significance. We then utilize the virtually instantaneous network operations to inverse design the metagrating unit shapes for a desired diffraction efficiency distribution. The proposed inter-disciplinary combination of advanced information processing techniques with Maxwell's equation solvers opens a pathway for the fast-paced prediction of metagrating designs rather than full wave computation.

We present a comprehensive study of enhanced light funneling through a subwavelength aperture with realistic (lossy) epsilon-near-zero (ENZ) materials. We realize experimentally an inclusion-free ENZ material layer operating at optical frequencies and characterize its performance. An analytical expression describing light funneling through several structures involving ENZ coupling layers is developed, validated with numerical solutions of Maxwell equations, and utilized to relate the performance of the ENZ coupling systems to their main limiting factor, material losses.

distribution and adaptive beamforming. Toward the realization of these active engineered structures various mechanisms have been utilized including mechanical reconfiguration, [4,5] photoswitching of dye molecules, [6] nonlinear optical effects, [7] thermal phase transitions, [8,9] and electrooptical field-effect modulation. [10][11][12][13][14][15][16][17][18] Among the aforementioned techniques, electro-optical controllable devices offer continuous tunability, shorter response time, and relatively wide tuning range comparing to thermal and mechanical tunable stimulations. Furthermore, the possibility of direct and independent electrical biasing of each inclusion within an optical platform makes this approach more favorable than all-optical tunability approaches. [19,20] Wide range of electrooptical materials have recently emerged like graphene, liquid crystals, [11] doped semiconductors (InSb and GaAs), [12,13] and transparent conducting oxides materials (TCOs) [14,15] including indium tin oxide (ITO), doped zinc oxide (ZnO), and aluminum-, gallium-, and indium-doped zinc-oxide (AZO, GZO, and IZO). In the mid-infrared and far-infrared spectra, the surface conductivity of graphene is widely tunable by the change of its electrochemical potential via applying an external gate voltage. Due to the extreme thinness, conformable to diverse patterning schemes, and broadband operation, it has been exploited in reconfigurable metadevices for manipulation of the surface plasmons and dynamical tuning of the geometrical resonances. [16][17][18] In the near-infrared (NIR) regime, TCOs are of particular interest due to the short response time (≈ns), fabrication feasibility, the controllable optical and electrical properties through the pre-and post-depositional processes, and large variation of complex refractive index (unity-order index change) in the charge accumulation/depletion regime. Also, the epsilon-near-zero (ENZ) property has been observed at the NIR regime and telecommunication frequency range when the carrier concentration of TCO is in the range of 10 20 -10 21 cm −3 . [21][22][23][24] ITO as the most well-known TCO has been exploited in the design of various optoelectronic devices. [25][26][27][28][29][30][31][32][33][34][35] Due to the fact that the ultrathin active layer of ITO has limited interaction length with the normal impinging wave, two approaches have been proposed to overcome the weak interaction between light and ITO. First, integration of ITO into a subwavelength grating A. Forouzmand, M. M. Salary, Dr.

Synthesization of multiple functionalities over a flat metasurface platform offers a promising approach to achieving integrated photonic devices with minimized footprint. Metasurfaces capable of diverse wavefront shaping according to wavelengths and polarizations have been demonstrated. Here we propose a class of angle-selective metasurfaces, over which beams are reflected following different and independent phase gradients in the light of the beam direction. Such powerful feature is achieved by leveraging the local phase modulation and the non-local lattice diffraction via inverse scattered field and geometry optimization in a monolayer dielectric grating, whereas most of the previous designs utilize the local phase modulation only and operate optimally for a specific angle. Beam combiner/splitter and independent multibeam deflections with up to 4 incident angles are numerically demonstrated respectively at the wavelength of 700 nm. The deflection efficiency is around 45% due to the material loss and the compromise of multi-angle responses. Flexibility of the approach is further validated by additional designs of angle-switchable metagratings as splitter/reflector and transparent/opaque mirror. The proposed designs hold great potential for increasing information density of compact optical components from the degree of freedom of angle.

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.

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