Samad Jafar-Zanjani

All-dielectric ultrathin conformal metasurfaces: lensing and cloaking applications at 532 nm wavelength

Metasurfaces are ideal candidates for conformal wave manipulation on curved objects due to their low profiles and rich functionalities. Here we design and analyze conformal metasurfaces for practical optical applications at 532 nm visible band for the first time. The inclusions are silicon disk nanoantennas embedded in a flexible supporting layer of polydimethylsiloxane (PDMS). They behave as local phase controllers in subwavelength dimensions for successful modification of electromagnetic responses point by point, with merits of high efficiency, at visible regime, ultrathin films, good tolerance to the incidence angle and the grid stretching due to the curvy substrate. An efficient modeling technique based on field equivalence principle is systematically proposed for characterizing metasurfaces with huge arrays of nanoantennas oriented in a conformal manner. Utilizing the robust nanoantenna inclusions and benefiting from the powerful analyzing tool, we successfully demonstrate the superior performances of the conformal metasurfaces in two specific areas, with one for lensing and compensation of spherical aberration, and the other carpet cloak, both at 532 nm visible spectrum.

Real-time two-dimensional beam steering with gate-tunable materials: a theoretical investigation.

A leaky-wave antenna is proposed that furnishes two-dimensional (2-D) beam scanning in both elevation and azimuth planes via electrical control in real time, and at a single frequency. The structure consists of a graphene sheet on a metal-backed substrate. The 2-D beam-scanning performance is achieved through the proper biasing configuration of graphene. Traditional pixel-by-pixel electrical control makes the biasing network a huge challenge for chip-scale designs in the terahertz regime and beyond. The method presented here enables dynamic control by applying two groups of one-dimensional biasing on the sides of the sheet. They are orthogonal and decoupled, with one group offering monotonic impedance variation along one direction, and the other sinusoidal impedance modulation along the other direction. The conductivity profile of the graphene sheet for a certain radiation angle, realized by applying proper voltage to each pad underneath the sheet, is determined by a holographic technique and can be reconfigured electronically and desirably. Such innovative biasing design makes real-time control of the beam direction and beamwidth simple and highly integrated. The concept is not limited to graphene-based structures, and can be generalized to any available gate-tunable material system.

Electromagnetic Scattering from BI-Periodic Fabric Structures

We develop an efficient semi-analytical technique to calculate the electromagnetic scattering from fabric structures modeled as crossed gratings of circular coated fibers of any material composition, arranged arbitrarily in yarns. The method relies on a matrix formulation based on multipole expansion for modeling conical scattering from uniaxial gratings of fibers, and employs a scattering matrix approach to obtain coand cross-polarized transmission and reflection coefficients. The lattice sums are evaluated using an efficient adaptive algorithm based on Shank’s transformation. The method can be employed for analyzing the scattering characteristics of fabric structures embedded in any arbitrary layered media. The validity of the method is verified through comparison with full-wave finite-difference time-domain simulations. A substantial performance gain is obtained, which makes the proposed method applicable to solve large-scale fabric structures.

Light manipulation with flat and conformal inhomogeneous dispersive impedance sheets: an efficient FDTD modeling.

An efficient auxiliary differential equation method for incorporating 2D inhomogeneous dispersive impedance sheets in the finite-difference time-domain solver is presented. This unique proposed method can successfully solve optical problems of current interest involving 2D sheets. It eliminates the need for ultrafine meshing in the thickness direction, resulting in a significant reduction of computation time and memory requirements. We apply the method to characterize a novel broad-beam leaky-wave antenna created by cascading three sinusoidally modulated reactance surfaces and also to study the effect of curvature on the radiation characteristic of a conformal impedance sheet holographic antenna. Considerable improvement in the simulation time based on our technique in comparison with the traditional volumetric model is reported. Both applications are of great interest in the field of antennas and 2D sheets.

Large enhancement of third-order nonlinear effects with a resonant all-dielectric metasurface

A novel low-profile nonlinear metasurface, consisting of a single-layer of all-dielectric material, is proposed and numerically investigated by a nonlinear full-wave finite-difference time-domain (FDTD) method. The proposed metasurface is transparent for low, and opaque for high values of incident light intensity. The metasurface design is broadly applicable to enhancement of intrinsic nonlinearities of any material with a sufficiently high refractive index contrast. We illustrate the ability of this design to enhance intrinsic nonlinear absorption of a transition metal oxide, vanadium pentoxide (V2O5), with resonant metasurface elements. The complex third-order nonlinear susceptibility (χ(3)) for V2O5, representing both nonlinear refraction and absorption is considered in FDTD simulations. Our design achieves high initial transparency (>90%) for low incident light intensity. An order of magnitude decrease in the required input light intensity threshold for nonlinear response of the metasurface is observe...

Adaptive Genetic Algorithm for Optical Metasurfaces Design

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.