arXiv subjects

Viktar Asadchy

Publications and source records attributed to Viktar Asadchy.

32 records · Page 2Linked to original sources

arXiv2021

Non-trivial point-gap topology and non-Hermitian skin effect in photonic crystals

We show that two-dimensional non-Hermitian photonic crystals made of lossy material can exhibit non-trivial point gap topology in terms of topological winding in its complex frequency band structure. Such crystals can be either made of lossy isotropic media which is reciprocal, or lossy magneto-optical media which is non-reciprocal. We discuss the effects of reciprocity on the properties of the point gap topology. We also show that such a non-trivial point gap topology leads to non-Hermitian skin effect when the photonic crystal is truncated. In contrast to most previous studies on point gap topology which used tight-binding models, our work indicates that such non-Hermitian topology can studied in photonic crystals, which represent a more realistic system that has technological significance.

cond-mat.mes-hall↗

arXiv2020

Reciprocal Metasurfaces for On-axis Reflective Optical Computing

Analog computing has emerged as a promising candidate for real-time and parallel continuous data processing. This paper presents a reciprocal way for realizing asymmetric optical transfer functions (OTFs) in the reflection side of the on-axis processing channels. It is rigorously demonstrated that the presence of Cross-polarization Exciting Normal Polarizabilities (CPENP) of a reciprocal metasurface circumvents the famous challenge of Green's function approach in implementation of on-axis reflective optical signal processing while providing dual computing channels under orthogonal polarizations. Following a comprehensive theoretical discussion and as a proof of concept, an all-dielectric optical metasurface is elaborately designed to exhibit the desired surface polarizabilities, thereby reflecting the first derivative and extracting the edges of images impinging from normal direction. The proposed study offers a flexible design method for on-axis metasurface-based optical signal processing and also, dramatically facilitates the experimental setup required for ultrafast analog computation and image processing.

physics.optics↗

arXiv2020

Three-dimensional random dielectric colloid metamaterial with giant isotropic optical activity

Motivated by the theoretical observation that isotropic chirality can exist even in completely random systems, we design a dielectric metamaterial consisting of a random colloid of meta-atoms, which exhibits unprecedentedly high isotropic optical activity. Each meta-atom is composed of a helically arranged cluster of silicon nanospheres. Such clusters can be fabricated by large-scale DNA self-assembly techniques. It is demonstrated that the use of a high concentration of the meta-atoms in the colloid provides significant suppressions of incoherent scattering losses. As a result, the proposed system shows three orders of magnitude improvement of isotropic optical activity as compared with the previous metamaterial designs. This work highlights the significant potential of completely random system, which are commonly produced in colloidal sciences, for applications as metamaterials towards novel photonic effects and devices.

physics.optics↗

arXiv2020

Tutorial on Electromagnetic Nonreciprocity and Its Origins

This tutorial provides an intuitive and concrete description of the phenomena of electromagnetic nonreciprocity that will be useful for readers with engineering or physics backgrounds. The notion of time reversal and its different definitions are discussed with special emphasis to its relationship with the reciprocity concept. Starting from the Onsager reciprocal relations generally applicable to many physical processes, we present the derivation of the Lorentz theorem and discuss other implications of reciprocity for electromagnetic systems. Next, we identify all possible routes towards engineering nonreciprocal devices and analyze in detail three of them: Based on external bias, based on nonlinear and time-variant systems. The principles of the operation of different nonreciprocal devices are explained. We address the similarity and fundamental difference between nonreciprocal effects and asymmetric transmission in reciprocal systems. In addition to the tutorial description of the topic, the manuscript also contains original findings. In particular, general classification of reciprocal and nonreciprocal phenomena in linear bianisotropic media based on the space- and time-reversal symmetries is presented. This classification serves as a powerful tool for drawing analogies between seemingly distinct effects having the same physical origin and can be used for predicting novel electromagnetic phenomena. Furthermore, electromagnetic reciprocity theorem for time-varying systems is derived and its applicability is discussed.

physics.app-ph↗

arXiv2020

Sub-Wavelength Passive Optical Isolators Using Photonic Structures Based on Weyl Semimetals

We design sub-wavelength high-performing non-reciprocal optical devices using recently discovered magnetic Weyl semimetals. These passive bulk topological materials exhibit anomalous Hall effect which results in magnetooptical effects that are orders of magnitude higher than those in conventional materials, without the need of any external magnetic bias. We design two optical isolators of both Faraday and Voigt geometries. These isolators have dimensions that are reduced by three orders of magnitude compared to conventional magneto-optical configurations. Our results indicate that the magnetic Weyl semimetals may open up new avenues in photonics for the design of various nonreciprocal components.

physics.optics↗

arXiv2019

Omega-bianisotropic metasurface for converting a propagating wave into a surface wave

Although a rigorous theoretical ground on metasurfaces has been established in the recent years on the basis of the equivalence principle, the majority of metasurfaces for converting a propagating wave into a surface wave are developed in accordance with the so-called generalized Snell's law being a simple heuristic rule for performing wave transformations. Recently, for the first time, Tcvetkova et al. [Phys. Rev. B 97, 115447 (2018)] have rigorously studied this problem by means of a reflecting anisotropic metasurface, which is, unfortunately, difficult to realize, and no experimental results are available. In this paper, we propose an alternative practical design of a metasurface-based converter by separating the incident plane wave and the surface wave in different half-spaces. It allows one to preserve the polarization of the incident wave and substitute the anisotropic metasurface by an omega-bianisotropic one. The problem is approached from two sides: By directly solving the corresponding boundary problem and by considering the ``time-reversed'' scenario when a surface wave is converted into a nonuniform plane wave. We develop a practical three-layer metasurface based on a conventional printed circuit board technology to mimic the omega-bianisotropic response. The metasurface incorporates metallic walls to avoid coupling between adjacent unit cells and accelerate the design procedure. The design is validated with full-wave three-dimensional numerical simulations and demonstrates high conversion efficiency.

physics.app-ph↗

arXiv2019

Materiatronics: Modular analysis of arbitrary meta-atoms

Within the paradigm of metamaterials and metasurfaces, electromagnetic properties of composite materials can be engineered by shaping or modulating their constituents, so-called meta-atoms. Synthesis and analysis of complex-shape meta-atoms with general polarization properties is a challenging task. In this paper, we demonstrate that the most general response can be conceptually decomposed into a set of basic, fundamental polarization phenomena, which enables immediate all-direction characterization of electromagnetic properties of arbitrary linear metamaterials and metasurfaces. The proposed platform of modular characterization (called "materiatronics") is tested on several examples of bianisotropic and nonreciprocal meta-atoms. As a demonstration of the potential of the modular analysis, we use it to design a single-layer metasurface of vanishing thickness with unitary circular dichroism. The analysis approach developed in this paper is supported by a ready-to-use computational code and can be further extended to meta-atoms engineered for other types of wave interactions, such as acoustics and mechanics.

physics.optics↗

arXiv2018

Applications and Potentials of Reciprocal Bianisotropic Metasurfaces

We discuss different applications and potentials of reciprocal bianisotropic metasurfaces including uniform and gradient metasurfaces, in particular, with anisotropic, chiral, and omega properties. Based on an analytic analysis, we discuss the necessary conditions to observe asymmetric co- and cross- polarization reflection and/or transmission, polarization conversion and rotation, asymmetric absorption, etc. We consider two kinds of incident wave scenarios based on linear and circular polarization.

physics.optics↗

arXiv2017

Refracting Metasurfaces without Spurious Diffraction

Refraction represents one of the most fundamental operations that may be performed by a metasurface. However, simple phasegradient metasurface designs suffer from restricted angular deflection due to spurious diffraction orders. It has been recently shown, using a circuit-based approach, that refraction without spurious diffraction, or diffraction-free, can fortunately be achieved by a transverse metasurface exhibiting either loss-gain or bianisotropy. Here, we rederive these conditions using a medium-based - and hence more insightfull - approach based on Generalized Sheet Transition Conditions (GSTCs) and surface susceptibility tensors, and experimentally demonstrate two diffraction-free refractive metasurfaces that are essentially lossless, passive, bianisotropic and reciprocal.

physics.class-ph↗

arXiv2017

Perfectly Refractive Metasurface using Bianisotropy

A passive and reciprocal perfectly refractive metasurface is designed using a general susceptiblity synthesis method. The metasurface uses weak spatial dispersion in the form of bianisotropy to achieve perfect refraction. Its operation is validated using a full-wave simulation of the metasurface implemented with cascaded metallic layers separated by dielectric.

physics.class-ph↗

arXiv2017

From the generalized reflection law to the realization of perfect anomalous reflectors

The use of the generalized Snell's law opens wide possibilities for the manipulation of transmitted and reflected wavefronts. However, known structures designed to shape reflection wave fronts suffer from significant parasitic reflections in undesired directions: In fact, the desired field distributions do not satisfy Maxwell's equations if the boundary conditions are specified in accordance with the generalized Snell's law. In this work, we explore the limitations of the existing solutions for the design of passive planar reflectors and demonstrate that strongly non-local response is required for perfect performance. Ideal reflective surfaces capable of steering the energy into any desired direction have to localize and carry energy along the inhomogeneous reflective surface. A new paradigm for the design of perfect reflectors based on energy surface channeling is introduced. We realize and experimentally verify a theoretically perfect design of an anomalously reflective surface using an array of rectangular metal patches backed by a metallic plate. This conceptually new mechanism for wavefront manipulation allows the design of thin perfect reflectors, offering a versatile design method applicable to other scenarios such as focusing reflectors or surface wave manipulations, extendible to other frequencies.

physics.optics↗

arXiv2014

A Single-Layer Meta-Atom Absorber

We realized and experimentally tested a conceptually new kind of electrically thin absorbers of electromagnetic waves. The idea is to utilize a single layer of precisely designed meta-atoms. This allows one to design an absorber with unprecedentedly small thickness. The absorber implies absence of a ground plane. High efficiency of the realized structure in the S band is demonstrated. The conceptual idea of the proposed absorber can find many applications especially at optical frequencies.

physics.optics↗

arXiv2014

Metamirrors

We introduce the concept of non-uniform metamirrors (full-reflection metasurfaces) providing full control of reflected wave fronts independently from the two sides of the mirror. Metamirror is a single planar array of electrically small bianisotropic inclusions. The electric and magnetic responses of the inclusions enable creating controlled gradient of phase discontinuities over the surface. Furthermore, presence of electromagnetic coupling in the inclusions allows independent control of reflection phase from the opposite sides of the mirror. Based on the proposed concept, we design and simulate metamirrors for highly efficient light bending and near-diffraction-limit focusing with a sub-wavelength focal distance.

physics.optics↗

arXiv2013

One-way transparent sheets

In this paper we introduce the concept of metasurfaces which are fully transparent when looking from one of the two sides of the sheet and have controllable functionalities for waves hitting the opposite side (one-way transparent sheets). We address the question on what functionalities are allowed, considering limitations due to reciprocity and passivity. In particular, we have found that it is possible to realize one-way transparent sheets which have the properties of a twist-polarizer in reflection or transmission when illuminated from the other side. Also one-way transparent sheets with controllable co-polarized reflection and transmission from the opposite side are feasible. We show that particular non-reciprocal magneto-electric coupling inside the sheet is necessary to realize lossless non-active transparent sheets. Furthermore, we derive the required polarizabilities of constituent dipole particles such that the layers composed of them form one-way transparent sheets. We conclude with design and simulations of an example of a nonreciprocal one-way transparent sheet functioning as an isolating twist-polarizer.

physics.optics↗