Helicity Maximization of Structured Light to Empower Nanoscale Chiral Matter Interaction

Structured light enables the characterization of chirality of optically small nanoparticles by taking advantage of the helicity maximization concept recently introduced in[1]. By referring to fields with nonzero helicity density as chiral fields, we first investigate the properties of two chiral optical beams in obtaining helicity density localization and maximization requirements. The investigated beams include circularly polarized Gaussian beams and also an optical beam properly composed by a combination of a radially and an azi-muthally polarized beam. To acquire further enhancement and localization of helicity density beyond the diffraction limit, we also study chiral fields at the vicinity of a spherical dielectric nanoantenna and demon-strate that the helicity density around such a nanoantenna is a superposition of helicity density of the illu-minating field, scattered field, and an interference helicity term. Moreover, we illustrate when the nanoan-tenna is illuminated by a proper combination of azimuthal and radially polarized beams, the scattered nearfields satisfy the helicity maximization conditions beyond the diffraction limit. The application of the concept of helicity maximization to nanoantennas and generating optimally chiral nearfield result in helici-ty enhancement which is of great advantage in areas like detection of nanoscale chiral samples, microsco-py, and optical manipulation of chiral nanoparticles

Electromagnetic Characterization of Metasurfaces

Electromagnetic characterization of metasurfaces (MSs), electrically/optically thin sheet metamaterials (MMs), is the subject of the current study. Briefly, a MM is a composite material with unusual electromagnetic properties offered by specific response of its constituents and their arrangement. The main goal in this work is to attribute some macroscopic characteristic parameters to MSs.  We first discuss the definitions and present a brief review of the electromagnetic characterization of MMs and MSs. We explain the failures of the traditional characterization approach when applied to MSs. We discuss two known approaches especially suggested for the characterization of MSs in 1990s-2000s.  We continue to introduce a heuristic homogenization model of MSs located on a dielectric interface. Indeed, we derive the general boundary conditions invariant on the polarization of the excitation field. Then, we present the most general algorithm to retrieve the characteristic macroscopic parameters through two-dimensional reflection and transmission dyadics.  We next present two explicit examples of MSs in order to prove the applicability of our theory. The first one is a periodic array of plasmonic nano-spheres while the second one is an array of coupled plasmonic nano-patches positioned in a disordered fashion on a flat surface. We show that our approach works for for both random and periodic MSs. Indeed, the restriction of our theory is a sufficiently small electrical/optical size of a unit cell (area per one particle).  We finally present the main results of the thesis through functional MSs. We theoretically reveal and discuss novel physical effects and various functionalities. We present some discussions on the intrinsically bianisotropic and intrinsically magnetic MSs operating in the visible range. We also discuss the microscopic effect of substrate-induced bianisotropy for a substrated array of plasmonic nano-spheres. Moreover, we reveal the magnetic response within the framework of our homogenization model; i.e., retrieving some magnetic parameters. Furthermore, we obtain the perfect absorbance conditions for different topologies and discuss them in this chapter. Finally, we present a model which explains the different behavior of electric and magnetic resonant modes of MSs in transition from periodic to amorphous arrangements

Effective electric and magnetic properties of metasurfaces in transition from crystalline to amorphous state

In this paper we theoretically study electromagnetic reflection, transmission, and scattering properties of periodic and random arrays of particles which exhibit both electric-mode and magnetic-mode resonances. We compare the properties of regular and random grids and explain recently observed dramatic differences in resonance broadening in the electric and magnetic modes of random arrays. We show that randomness in the particle positioning influences equally on the scattering loss from both electric and magnetic dipoles, however, the observed resonance broadening can be very different depending on the absorption level in different modes as well as on the average electrical distance between the particles. The theory is illustrated by an example of a planar metasurface composed of cut-wire pairs. We show that in this particular case at the magnetic resonance the array response is almost not affected by positioning randomness due to lower frequency and higher absorption losses in that mode. The developed model allows predictions of behavior of random grids based on the knowledge of polarizabilities of single inclusions.Comment: 13 pages, 5 figures, and submitted to PR

Illusion Mechanisms with Cylindrical Metasurfaces: A General Synthesis Approach

We explore the use of cylindrical metasurfaces in providing several illusion mechanisms including scattering cancellation and creating fictitious line sources. We present the general synthesis approach that leads to such phenomena by modeling the metasurface with effective polarizability tensors and by applying boundary conditions to connect the tangential components of the desired fields to the required surface polarization current densities that generate such fields. We then use these required surface polarizations to obtain the effective polarizabilities for the synthesis of the metasurface. We demonstrate the use of this general method for the synthesis of metasurfaces that lead to scattering cancellation and illusion effects, and discuss practical scenarios by using loaded dipole antennas to realize the discretized polarization current densities. This study is the first fundamental step that may lead to interesting electromagnetic applications, like stealth technology, antenna synthesis, wireless power transfer, sensors, cylindrical absorbers, etc.Comment: 12 pages, 9 figure

Shadow-free multimers as extreme-performance meta-atoms

We generalize the concept of parity-time symmetric structures with the goal to create meta-atoms exhibiting extraordinary abilities to overcome the presumed limitations in the scattering of overall lossless particles, such as non-zero forward scattering and the equality of scattering and extinction powers for all lossless particles. Although the forward scattering amplitude and the extinction cross section of our proposed meta-atoms vanish, they scatter incident energy into other directions, with controllable directionality. These meta-atoms possess extreme electromagnetic properties not achievable for passive scatterers. As an example, we study meta-atoms consisting of two or three small dipole scatters. We consider possible microwave realizations in the form of short dipole antennas loaded by lumped elements. The proposed meta-atom empowers extraordinary response of a shadow-free scatterer and theoretically enables most unusual material properties when used as a building block of an artificial medium.Comment: 14 pages, 9 Figure

Functional metasurfaces: Do we need normal polarizations?

We consider reciprocal metasurfaces with engineered reflection and transmission coefficients and study the role of normal (with respect to the metasurface plane) electric and magnetic polarizations on the possibilities to shape the reflection and transmission responses. We demonstrate in general and on a representative example that the presence of normal components of the polarization vectors does not add extra degrees of freedom in engineering the reflection and transmission characteristics of metasurfaces. Furthermore, we discuss advantages and disadvantages of equivalent volumetric and fully planar realizations of the same properties of functional metasurfaces.Comment: 3 pages, 2 figures, XXXIInd International Union of Radio Science General Assembly and Scientific Symposium, paper B7-1, pp. 1-3, Montreal, Canada, 19-26 August 2017 (invited