Optimizing an electromagnetic wave absorber for bi-anisotropic metasurfaces based on toroidal modes

The design and optimization of an electromagnetic wave absorber for far-field wireless power transmission (WPT) is the subject of this research study. The goal of the research is to effectively absorb energy from ambient RF electromagnetic waves without the usage of a ground plane by employing metasurfaces with chiral components.By integrating trioidal moments into the design theory, the objective is to create a metasurface that functions in two frequency bands and produces high-quality resonance. The study also explores the dual non-homogeneity property of structures, polarization tensor coefficients, and the electromagnetic response of non-homogeneous metasurfaces. Based on the relative orientation of induced fields and moments, it delves deeper into the two basic possibilities for dual non-homogeneous elements. The development of chiral metasurfaces and the notion of electromagnetic chirality and its implications for polarization properties are introduced.

Chirality in Light-Matter Interaction

The scientific effort to control the interaction between light and matter has grown exponentially in the last 2 decades. This growth has been aided by the development of scientific and technological tools enabling the manipulation of light at deeply sub-wavelength scales, unlocking a large variety of novel phenomena spanning traditionally distant research areas. Here, the role of chirality in light-matter interactions is reviewed by providing a broad overview of its properties, materials, and applications. A perspective on future developments is highlighted, including the growing role of machine learning in designing advanced chiroptical materials to enhance and control light-matter interactions across several scales.

Photonic spin-selective perfect absorptance on planar metasurfaces driven by chiral quasi-bound states in the continuum

Optical metasurfaces with high-quality-factor resonances and selective chirality simultaneously are desired for nanophotonics. Here, an all-dielectric planar chiral metasurface is theoretically proposed and numerically proved to support the astonishing symmetry-protected bound state in the continuum (BIC), due to the preserved π rotational symmetry around the z axis and up-down mirror symmetry simultaneously. Importantly, such BIC is a vortex polarization singularity enclosed by elliptical eigenstate polarizations with non-vanishing helicity, owing to the broken in-plane mirror symmetry. Under the oblique incidence, companied by the BIC transforming into a quasi-BIC (Q-BIC), the strong extrinsic chirality manifests. Assisted by the single-port critical coupling, the planar metasurface can selectively and near-perfectly absorb one circularly polarized light but non-resonantly reflect its counterparts. The circular dichroism (CD) approaching 0.812 is achieved. Intriguingly, the sign of CD (namely, the handedness of the chiral metasurface) can be flexibly manipulated only via varying the azimuthal angle of incident light, due to the periodic helicity sign flip in eigen polarizations around the BIC. Numerical results are consistent with the coupled-mode theory and multipole decomposition method. The spin-selective metasurface absorber empowered by the physics of chiral Q-BICs undoubtedly may promise various applications such as optical filters, polarization detectors, and chiral imaging.

Metric-Torsion Duality of Optically Chiral Structures

We develop a metric-torsion theory for chiral structures by using a generalized framework of transformation optics. We show that the chirality is uniquely determined by a metric with the local rotational degree of freedom. In analogy to the dislocation continuum, the chirality can be alternatively interpreted as the torsion tensor of a Riemann-Cartan space, which is mimicked by the anholonomy of the orthonormal basis. As a demonstration, we reveal the equivalence of typical three-dimensional chiral metamaterials in the continuum limit. Our theory provides an analytical recipe to design optical chirality.

Meta-Optical Chirality and Emergent Eigen-polarization Modes via Plasmon Interactions

The response of an individual meta-atom is often generalized to explain the collective response of a metasurface in a manner that neglects the interactions between meta-atoms. Here, we study a metasurface composed of tilted achiral meta-atoms with no spatial variation of the unit cell that derives appreciable optical chirality solely from the asymmetric interactions between meta-atoms. The interactions between meta-atoms are considered to stem from the Lorentz force arising from the Larmor radiation of adjacent plasmonic resonators because their inclusion in a simple model accurately predicts the bonding/anti- bonding modes that are measured experimentally. We also experimentally observe the emergence of multiple polarization eigenmodes, among other polarization-dependent responses, which cannot be modeled with the conventional formalism of transmission matrices. Our results are vital to the precise characterization and design of metasurfaces.

Gain properties of an uncoated and chiral coated slotted sphere embedded in a chiral background

The gain properties of an uncoated and a chiral coated slotted sphere embedded in a chiral background have been investigated using numerical simulations. In this paper, it is found that a chiral background medium enhances the gain of an uncoated slotted sphere in the forward direction as compared to the free space background. It is shown that the forward direction gain of a chiral coated slotted sphere embedded in a chiral background increases with the increase in the background chirality. It is further determined that the maximum gain moves away from the polar direction toward the forward direction as the chirality of the coating increases for a fixed background chirality. Also, this maximum gain gradually decreases as the chirality of the coating increases. An interesting feature of an angular window is introduced for a chiral coated slotted sphere embedded in a chiral background where the gain is nearly constant for a specific range of angles.

Gain of an axially slotted cylinder covered with a chiral coating and embedded in a chiral medium

The gain characteristics of an axially slotted cylinder coated with a chiral layer and placed in another chiral background have been investigated using numerical simulations. The effects of various types of chiral coatings and chiral backgrounds upon the gain pattern have been studied. It is shown that an increase in the chirality of the coating enhances the gain in the forward direction and reduces the gain in the backward direction for the fixed chirality of the background. It is also studied that, by increasing the chirality of the background medium, the gain in the backward direction also increases. It is further found that the chiral nihility coating makes the gain pattern nearly isotropic, and this gain is almost independent of the chirality of the background chiral medium.

Large-area 3D chiral plasmonic structures

We manufacture large-area plasmonic structures featuring 3-dimensional chirality by colloidal nanohole lithography. By varying the polar rotating speed of the samples during gold evaporation, we can fabricate spiral-type ramp nanostructures. The optical properties show chiroptical resonances in the 100 to 400 THz frequency region (750 to 3000 nm), with circular dichroism values of up to 13%. Our method offers a simple low-cost manufacturing method of cm(2)-sized chiral plasmonic templates for chiroptical applications such as stereochemical enantiomer sensors.

Polarization eccentricity of the transverse field for modes in chiral core planar waveguides

The general solution for modes in an asymmetric planar waveguide with a homogeneous and isotropic chiral core is given in terms of a pair of parameters related to the eccentricity of the polarization ellipse for the transverse electric field. This formulation provides insight into the transition, with increasing chirality of the core, from TE/TM modes to right-handed and left-handed circular polarization modes. Mode polarization as a function of waveguide thickness and of frequency is discussed in detail. Beyond a mode-dependent maximum thickness (or frequency), the left-handed elliptical modes consist of a slow-wave component whose cutoff properties are examined. The limiting case of a symmetric waveguide is also discussed.

Effect of chirality on the transient signal wave front

We analyze the behavior of the wave front of a transient pulse propagating in an isotropic homogeneous chiral material and discuss the role of chirality and its underlying physics on the pulse's rise time, ringing frequency at the early time of arrival, and state of polarization at the wave front. We use the one-resonance model of dispersion for the material parameters. It is shown that the chirality of the medium decreases the ringing frequency at the early time as compared with the nonchiral case and causes the rotation of the plane of polarization and the evolution of the cross-polarized component at the transient signal wave front. Potential applications and motivations for the present study are also mentioned.