Lattice-plasmon-induced asymmetric transmission in two-dimensional chiral arrays

Abstract

Asymmetric transmission—direction-selective electromagnetic transmission between two ports—is a phenomenon exhibited by two-dimensional chiral systems. The possibility of exploiting this phenomenon in chiral metasurfaces opens exciting possibilities for applications such as optical isolation and routing without external magnetic fields. This work investigates optical asymmetric transmission in chiral plasmonic metasurfaces supporting lattice plasmon modes and unveils its physical origins. We show numerically and experimentally that asymmetric transmission is caused by an unbalanced excitation of such lattice modes by circularly polarized light of opposite handedness. The excitation efficiencies of the lattice modes, and hence, the strength of the asymmetric transmission, are controlled by engineering the in-plane scattering of the individual plasmonic nanoparticles such that the maximum scattering imbalance occurs along one of the in-plane diffraction orders of the metasurface. Furthermore, we show that only the nonzero diffraction orders contribute to this effect. By highlighting the role of the localized plasmon modes supported by the nanoparticle and their radiative coupling to the lattice structure, our study provides a guideline for designing metasurfaces with asymmetric transmission enabled by lattice plasmons.

Chiral Lattice Resonances in 2.5-Dimensional Periodic Arrays with Achiral Unit Cells

“…More recently, it has been shown that the combination of a rectangular lattice with an achiral unit cell results in arrays capable of supporting lattice resonances with some degree of chirality, which can be enhanced by positioning the array in an inhomogeneous dielectric environment . Other alternatives that have been explored to obtain chiral lattice resonances in arrays with achiral unit cells involve the use of higher-order lattice resonances or the introduction of extrinsic chirality , through the excitation of the system with tilted illumination. − …”

Tunable circular conversion dichroism of single-layer twisted graphene-patterned metasurface

“…Considering the thickness of monolayer graphene, the circular polarization conversion efficiency here is comparable to or greater than in previously reported metallic metamaterials. 23 , 24 , 29 , 30 The measurable chiral response origins from the twist of single-layer graphene pattern and is enhanced by the graphene SPPs. Compared with the bilayer chiral graphene metasurface with 3.504% asymmetric transmission in Ref.…”

“… 23 Similarly, lattice-plasmon-induced asymmetric transmission can be achieved in a planar array of achiral tetramer nanostructures. 24 However, the aforementioned studies about intrinsic structural 2D chirality focus on metallic systems, which usually work in visible-infrared regions, and suffer from nontunable characteristics after fabrication. In addition, previously, 2D chiral response such as CCD and asymmetric reflection has rarely been discussed in the THz region, resulting in the lack of planar chiral polarization components and devices in the THz realm.…”

High‐Q and Strong Chiroptical Responses in Planar Metasurfaces Empowered by Mie Surface Lattice Resonances

“…[ 41 ] By adopting planar [ 42–44 ] or 3D [ 45,46 ] chiral nanostructures as the unit cell, SLRs have shown to be capable to improve the Q factors and the magnitudes of the chiroptical responses. For metasurfaces composed of periodic achiral nanostructures, SLR‐assisted extrinsic chirality under oblique incidence [ 47–50 ] and SLR‐induced planar chirality under normal incidence [ 51,52 ] have been numerically or experimentally demonstrated. These SLR‐enhanced, ‐assisted or ‐induced chiroptical responses, however, are all restricted to plasmonic metasurfaces, and thus have relatively small Q factors (measured $$Q<100$$) and sometimes weak chirality because of inevitably high ohmic loss of metal in the visible and near‐infrared regimes.…”

“…We will demonstrate SLR‐induced strong planar chirality under normal incidence with large measured CD maxima of −0.37 and high Q factor of 1220, and SLR‐assisted strong extrinsic 3D chirality with large measured CD maxima of 0.7 and high Q factor of 872 by slightly tilting the incidence angle. By introducing asymmetric lattice periods of the nanostructures rather than introducing tiny symmetry defects of meta‐atoms, [ 53 ] these measured Q factors, which are obtained together with strong chirality, exceed those of the reported SLR‐based chiral metasurfaces by an order of magnitude, [ 47–52 ] and are several times of those of the BIC‐based planar metasurfaces without the out‐of‐plane asymmetry. [ 37 ] Compared with the asymmetry meta‐atoms, the asymmetric lattice periods are more likely to be retained after pattern transfers for multiple times by lithography, lift‐off, and etching during the nanofabrication of silicon metasurfaces (see Experimental Section).…”