Younes Ra'di

Total Absorption of Electromagnetic Waves in Ultimately Thin Layers

We consider single-layer arrays of electrically small lossy particles that completely absorb electromagnetic waves at normal incidence. Required conditions for electromagnetic properties of bi-anisotropic particles have been identified in the most general case of uniaxial reciprocal and nonreciprocal particles. We consider the design possibilities offered by the particles of all four fundamental classes of bi-anisotropic inclusions: reciprocal chiral and omega particles and nonreciprocal Tellegen and moving particles. We also study the reflection/transmission properties of asymmetric structures with different properties when illuminated from the opposite sides of the sheet. It has been found that it is possible to realize single-layer grids which exhibit the total absorption property when illuminated from one side but are totally transparent when illuminated from the other side (an ultimately thin isolator). Other possible properties are co-polarized or twist polarized reflection from the side opposite to the absorbing one. Finally, we discuss possible approaches to practical realization of particles with the properties required for single-layer perfect absorbers and other proposed devices.

Functional metamirrors using bianisotropic elements.

Conventional mirrors obey the simple reflection law that a plane wave is reflected as a plane wave, at the same angle. To engineer spatial distributions of fields reflected from a mirror, one can either shape the reflector or position some phase-correcting elements on top of a mirror surface. Here we show, both theoretically and experimentally, that full-power reflection with general control over the reflected wave phase is possible with a single-layer array of deeply subwavelength inclusions. These proposed artificial surfaces, metamirrors, provide various functions of shaped or nonuniform reflectors without utilizing any mirror. This can be achieved only if the forward and backward scattering of the inclusions in the array can be engineered independently, and we prove that it is possible using electrically and magnetically polarizable inclusions. The proposed subwavelength inclusions possess desired reflecting properties at the operational frequency band, while at other frequencies the array is practically transparent. The metamirror concept leads to a variety of applications over the entire electromagnetic spectrum, such as optically transparent focusing antennas for satellites, multifrequency reflector antennas for radio astronomy, low-profile conformal antennas for telecommunications, and nanoreflectarray antennas for integrated optics.

Perfect control of reflection and refraction using spatially dispersive metasurfaces

Nonuniform metasurfaces (electrically thin composite layers) can be used for shaping refracted and reflected electromagnetic waves. However, known design approaches based on the generalized refraction and reflection laws do not allow realization of perfectly performing devices: there are always some parasitic reflections into undesired directions. In this paper we introduce and discuss a general approach to the synthesis of metasurfaces for full control of transmitted and reflected plane waves and show that perfect performance can be realized. The method is based on the use of an equivalent impedance matrix model which connects the tangential field components at the two sides on the metasurface. With this approach we are able to understand what physical properties of the metasurface are needed in order to perfectly realize the desired response. Furthermore, we determine the required polarizabilities of the metasurface unit cells and discuss suitable cell structures. It appears that only spatially dispersive metasurfaces allow realization of perfect refraction and reflection of incident plane waves into arbitrary directions. In particular, ideal refraction is possible only if the metasurface is bianisotropic (weak spatial dispersion), and ideal reflection without polarization transformation requires spatial dispersion with a specific, strongly nonlocal response to the fields.

Broadband Reflectionless Metasheets: Frequency-Selective Transmission and Perfect Absorption

Energy of propagating electromagnetic waves can be fully absorbed in a thin lossy layer, but only in a narrow frequency band, as follows from the causality principle. On the other hand, it appears that there are no fundamental limitations on broadband matching of thin absorbing layers. However, known thin absorbers produce significant reflections outside of the resonant absorption band. In this paper we explore possibilities to realize a thin absorbing layer which produces no reflected waves in a very wide frequency range, while the transmission coefficient has a narrow peak of full absorption. Here we show, both theoretically and experimentally, that a wide-band-matched thin resonant absorber, invisible in reflection, can be realized if one and the same resonant mode of the absorbing array unit cells is utilized to create both electric and magnetic responses. We test this concept using chiral particles in each unit cells, arranged in a periodic planar racemic array, utilizing chirality coupling in each unit cell but compensating the field coupling at the macroscopic level. We prove that the concept and the proposed realization approach also can be used to create non-reflecting layers for full control of transmitted fields. Our results can have a broad range of potential applications over the entire electromagnetic spectrum including, for example, perfect ultra-compact wave filters and selective multi-frequency sensors.

Tailoring Reflections From Thin Composite Metamirrors

We propose an effective route to fully control the phase of plane waves reflected from electrically (optically) thin composite sheets. This becomes possible using engineered artificial full-reflection layers (metamirrors) formed by arrays of electrically small resonant bi-anisotropic particles. In this scenario, fully reflecting mirrors do not contain any continuous ground plane, but only arrays of small particles. Bi-anisotropic omega coupling is required to get asymmetric response in reflection phase for plane waves incident from the opposite sides of the composite mirror. It is shown that with this concept one can independently tailor the phase of electromagnetic waves reflected from both sides of the mirror array.

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.

Eliminating Electromagnetic Scattering From Small Particles

This paper presents and discusses the conditions for zero electromagnetic scattering by electrically small particles. We consider the most general bi-anisotropic particles, characterized by four dyadic polarizabilities and study the case of uniaxially symmetric objects. Conditions for zero backward and forward scattering are found for a general uniaxial bi-anisotropic particle and specialized for all fundamental classes of bi-anisotropic particles: omega, “moving”, chiral, and Tellegen particles. Possibility for zero total scattering is also discussed for aforementioned cases. The scattering pattern and polarization of the scattered wave are also determined for each particle class. In particular, we analyze the interplay between different scattering mechanisms and show that in some cases it is possible to compensate scattering from a polarizable particle by appropriate magneto-electric coupling. Examples of particles providing zero backscattering and zero forward scattering are presented and studied numerically.

Least Upper Bounds of the Powers Extracted and Scattered by Bi-anisotropic Particles

The least upper bounds of the powers extracted and scattered by bi-anisotropic particles are investigated analytically. A rigorous derivation for particles having invertible polarizability tensors is presented, and the particles with singular polarizability tensors that have been reported in the literature are treated explicitly. The analysis concludes that previous upper bounds presented for isotropic particles can be extrapolated to bi-anisotropic particles. In particular, it is shown that neither nonreciprocal nor magnetoelectric coupling phenomena can further increase those upper bounds on the extracted and scattered powers. The outcomes are illustrated further with approximate circuit model examples of two dipole antennas connected via a generic lossless network.

Polarizabilities of Nonreciprocal Bianisotropic Particles

For two electrically small nonreciprocal scatterers an analytical electromagnetic model of polarizabilities is developed. Both particles are bianisotropic: the so-called Tellegen-omega particle and moving-chiral particle. Analytical results are compared to the full-wave numerical simulations. Both models satisfy to main physical restrictions and leave no doubts in the possibility to realize these particles experimentally. This paper is a necessary step towards applications of nonreciprocal bianisotropic particles such as perfect electromagnetic isolators, twist polarizers, thin-sheet phase shifters, and other devices.

Perfect plasmonic absorbers for photovoltaic applications

Thin-film photovoltaic (PV) cells enhanced by lighttrapping structures (LTSs), capable to prevent both reflection from the cell and parasitic transmission through its PV layer [1] have not yet been adapted by the industry in spite of rather long history of corresponding research started by work [2]. Perhaps, this is so due to the lack of efficiency of suggested LTSs and/or impractical design solutions in the cases when these LTSs are efficient enough. Most popular LTSs are those based on plasmonic nanostructures which allow the incident light to concentrate in strongly subwavelength regions inside the PV layers [3]. These structures were, however, heavily criticized for parasitic losses in the metal nanoelements (see e.g. in [4]). Really, high losses in the metal constituents make the light-trapping functionality meaningless. Without an LTS some incident power is absorbed in the bottom electrode, in presence of a plasmonic LTS it can be absorbed inside the latter one, which is not better. Besides of the direct losses of the incident light energy this implies a decrease of the PV conversion efficiency due to the heating of the PV layer [1].A novel regime of perfect absorption in a thin plasmonic layer corresponds to a collective mode of an array of plasmonic nanospheres. In our theoretical study we show that the absorption of the incident light occurs mainly in the semiconductor material hosting plasmonic nanospheres, whereas the absorption in the metal is very small. The regime survives when the uniform host layer is replaced by a practical photovoltaic cell. Trapping the light allows the thickness of the doped semiconductor to be reduced to values for which the degradation under light exposure should be insufficient. The light-trapping regime is compatible with both the metal-backed variant of the photovoltaic cell and its semitransparent variant when both electrodes are preformed of a conductive oxide. Negligible parasitic losses, a variety of design solutions and a reasonable operational band make our perfect plasmonic absorbers promising for photovoltaic applications.