A Toroidal‐Fano‐Resonant Metasurface with Optimal Cross‐Polarization Efficiency and Switchable Nonlinearity in the Near‐Infrared

Abstract

The recent progress in plasmonic metasurfaces gives rise to an intense evolution of controlling light properties such as phase, amplitude, polarization, and frequency. In this work, a new paradigm is established to control the light properties centered on low‐loss toroidal multipoles with high field enhancement in contrast to most of the previous plasmonic metasurfaces that are optimized through electric and magnetic multipolar resonances. Through a proof‐of‐concept demonstration, a linear cross‐polarization conversion efficiency reaching 22.9%, remarked as the optimal value that can exist in a single‐layer plasmonic metasurface in the near‐infrared spectrum, is experimentally realized. A polarization‐insensitive toroidal response, that previously was accessible only in isotropic high‐index metasurfaces, is also observed. Furthermore, a giant anisotropic (polarization‐sensitive) generation of the second‐harmonic frequency is demonstrated with the proposed polarization‐independent toroidal metasurface that provides different levels of electric energy storage within the metasurface. These findings open a new path for keeping low‐efficiency plasmonic components on track when one engineers a metasurface based on the toroidal multipole family.

Vertically‐Stacked Discrete Plasmonic Meta‐Gratings for Broadband Space‐Variant Metasurfaces

“…[41] In Figure 2d, the two strongest coupling terms are calculated. [13,44,45] The dominant coupling takes place between electric dipole (ED) and toroidal dipole (TD) and then coupling admits ED and electric quadrupole (EQ) in the second place. Please note that the couplings among other multipoles were not included in the plot because of their negligible contributions.…”

“…Nevertheless, the ease of fabrication in these metallic nanostructures persuades researchers to tackle the efficiency limitation and to further enhance the performance, particularly at optical frequencies. There are reports that address a thin single‐layer plasmonic metasurface [ 13 ] whose performance can reach its theoretical limit (25%), [ 14,15 ] Huygens meta‐atoms with a transmission efficiency of 42.3%, [ 16 ] and highly transmissive (50.7%) toroidal‐assisted plasmonic metasurface [ 17 ] which can theoretically transmit the light with an efficiency of 77.2% when the nanostructures are made of silver. Recent developments on plasmonic metasurfaces reveal that metallic meta‐atoms now are able to perform on the verge of their dielectric counterparts, thus utilization of these plasmonic systems in a wide range of applications is feasible.…”

Toroidal‐Assisted Generalized Huygens’ Sources for Highly Transmissive Plasmonic Metasurfaces

“…One should note that different from dielectric metasurfaces that possess essential high refractive index to generate out-of-plane displacement current oscillations, high-order multipolar modes in plasmonic nanostructures are often excited via near-field coupling. [33,34] Also, we have to consider the theoretical limit (25%) of single-layer plasmonic nanostructures (see Figure S2, Supporting Information) [35,36] that remains uncompetitive with those of dielectric metasurfaces. Therefore, efficient HPMA with well-balanced multipoles is proposed to better fulfill the GKC.…”

Superhybrid Mode-Enhanced Optical Torques on Mie-Resonant Particles

“…For small particles in an electromagnetic wave, the multipole expansion of scattered power in coupled multipoles can be expressed as , where P individual and P irreducible represent the power from individual dipole/multipoles and interactions, respectively. P p α , P m α , P T α (e) , P T α (m) , P Q α,β (e) , P Q α, β (m) and P Q α β ( T e ) correspond to the scattering power from the electric dipole, magnetic dipole, ETD, magnetic toroidal dipole, electric quadrupole (EQ), magnetic quadrupole, and electric toroidal quadrupole, respectively; P p α –T α (e) , P m α –T α (m) , and P Q e –Q T (e) correspond to the scattering power from interactions of electric dipole and toroidal dipole, magnetic dipole and toroidal dipole, and electric quadrupole and toroidal dipole, respectively.…”

“…For small particles in an electromagnetic wave, the multipole expansion of scattered power in coupled multipoles can be expressed as 60,61…”