arXiv subjects

Mojtaba Dehmollaian

Publications and source records attributed to Mojtaba Dehmollaian.

5 recordsLinked to original sources

arXiv2022

Transmittable Nonreciprocal Cloaking

Cloaking is typically reciprocal. We introduce here the concept of \emph{transmittable nonreciprocal cloaking} whereby the cloaking system operates as a standard omnidirectional cloak for external illumination, but can transmit light from its center outwards at will. We demonstrate a specific implementation of such cloaking that consists in a set of concentric bianisotropic metasurfaces whose innermost element is nonreciprocal and designed to simultaneously block inward waves and pass -- either omnidirectionaly or directionally -- outward waves. Such cloaking represents a fundamental diversification of conventional cloaking and may find applications in areas such as stealth, blockage avoidance, illusion and cooling.

physics.app-ph↗

arXiv2020

General Mapping between Complex Spatial and Temporal Frequencies by Analytical Continuation

This paper introduces a general technique for inter-mapping the complex spatial frequency (or propagation constant) $γ=α+jβ$ and the temporal frequency $ω= ω_\text{r}+jω_\text{i}$ of an arbitrary electromagnetic structure. This technique, based on the analytic property of complex functions describing physical phenomena, invokes the analytic continuity theorem to assert the unicity of the mapping function, and find this function from known data within a restricted domain of its analycity by curve-fitting to a generic polynomial expansion. It is not only applicable to canonical problems admitting an analytical solution, but to \emph{any} problems, from eigen-mode or driven-mode full-wave simulation results. The proposed technique is demonstrated for several systems, namely an unbounded lossy medium, a dielectric-filled rectangular waveguide, a periodically-loaded transmission line, a one-dimensional photonic crystal and a series-fed patch (SFP) leaky-wave antenna (LWA), and it is validated either by analytical results or by full-wave simulated results.

physics.app-ph↗

arXiv2020

Metasurface Modeling by a Thin Slab

We investigate the possibility to model a metasurface, defined as a zero-thickness sheet of surface polarization currents, by a thin slab, characterized by a subwavelength thickness and usual voluminal medium parameters. First, we elaborate a general equivalence relation between the metasurface and the slab in terms of average electromagnetic fields. Then, we derive exact relations between the metasurface and slab susceptibilities and validate them by full-wave simulations. Finally, we discuss the simple and insightful Average Field Approximation (AFA) formula, illustrate its inappropriate for strong metasurface field transformations, and establish its range of validity. All of these developments are restricted to the simplest case of a uniform isotropic metasurface under normal plane wave incidence. We conclude from the complexity of the equivalence for this case, that a metasurface is generally best modeled in terms of Generalized Sheet Transition Conditions (GSTCs).

physics.app-ph↗

arXiv2019

Comparison of Tensor Boundary Conditions (TBCs) with Generalized Sheet Transition Conditions (GSTCs)

This paper compares Tensor Boundary Conditions (TBCs), which were introduced to model multilayered dielectric structures, with Generalized Sheet Transition Conditions (GSTCs), which have been recently used to model metasurfaces. It shows that TBCs, with their 3 scalar parameters, are equivalent to the direct-isotropic -- cross-antiisotropic, reciprocal and nongyrotropic subset of GSTCs, whose 16 tangential (particular case of zero normal polarizations) or 36 general susceptibility parameters can handle the most general bianisotropic sheet structures. It further shows that extending that TBCs scalar parameters to tensors and allowing a doubly-occurring parameter to take different values leads to a TBCs formulation that is equivalent to the tangential GSTCs, but without reflecting the polarization physics of sheet media, such as metasurfaces and two-dimensional material allotropes.

physics.app-ph↗

arXiv2018

Wave Scattering by a Cylindrical Metasurface Cavity of Arbitrary Cross-Section: Theory and Applications

This paper presents a technique, combining the integral equations (IE) and the Generalized Sheet Transition Conditions (GSTCs) with bianisotropic susceptibility tensors, to compute electromagnetic wave scattering by cylindrical metasurfaces -- forming two-dimensional porous cavities -- of arbitrary cross sections. Moreover, it applies this technique to two problems -- cloaking with circular and rhombic shapes and illusion optics with an elliptic shape -- that both validate it, from comparison with specifications used in an exact synthesis of the metasurfaces, and reveal interesting capabilities of such metasurface structures. Particularly, active cylindrical metasurfaces can perfectly cloak and hence eliminate the extinction cross section of various cylindrical shapes, and simple purely passive versions of them, practically more accessible, still perform quite good cloaking and provide remakable extinction cross section reduction.

physics.comp-ph↗