Photonic bands, superchirality, and inverse design of a chiral minimal metasurface

Simone Zanotto,Giacomo Mazzamuto,Alessandro Tredicucci,Alessandro Pitanti,Francesco Riboli,Giorgio Biasiol,Giuseppe C La Rocca

doi:10.1515/nanoph-2019-0321

Abstract

Abstract Photonic band structures are a typical fingerprint of periodic optical structures, and are usually observed in spectroscopic quantities such as transmission, reflection, and absorption. Here we show that the chiro-optical response of a metasurface constituted by a lattice of non-centrosymmetric, L-shaped holes in a dielectric slab shows a band structure, where intrinsic and extrinsic chirality effects are clearly recognized and connected to localized and delocalized resonances. Superchiral near-fields can be excited in correspondence to these resonances, and anomalous behaviors as a function of the incidence polarization occur. Moreover, we have introduced a singular value decomposition (SVD) approach to show that the above mentioned effects are connected to specific fingerprints of the SVD spectra. Finally, by means of an inverse design technique we have demonstrated that the metasurface based on an L-shaped hole array is a minimal one. Indeed, its unit cell geometry depends on the smallest number of parameters needed to implement arbitrary transmission matrices compliant with the general symmetries for 2d-chiral structures. These observations enable more powerful wave operations in a lossless photonic environment.

Highlights

Electromagnetic fields which carry chirality – in their simplest form, left- and right-circularly polarized plane waves – deserve huge interest as they interact with matter chirality, enabling for instance to discriminate enantiomers in chemistry, which are connected with key features of living organisms
The spectral features present on σ1,2 and on clearly recall what noticed on the observables that have been studied in the previous sections: we have attributed this fact to the presence of a common ground – i.e. metasurface resonances – standing behind all these phenomena
Circular polarization differential transmittance is arranged in dispersive bands, and shows the fingerprints of localized and delocalized photonic resonances

Summary

Introduction

Electromagnetic fields which carry chirality – in their simplest form, left- and right-circularly polarized plane waves – deserve huge interest as they interact with matter chirality, enabling for instance to discriminate enantiomers in chemistry, which are connected with key features of living organisms. There is a wide interest in developing subwavelength-patterned high-index dielectrics, both for applications and for fundamental research: from one side, they enable the synthesis of flat lenses, polarimeters, spectrometers, nonlinear components and computer-generated holograms [26,27,28,29,30,31,32,33]; from another side, they exhibit a variety of intriguing phenomena such as Fano lineshapes, perfect forward scattering, geometric phase effects, and bound states in the continuum resonances [34,35,36,37,38,39,40] Most of these effects arise from complex electromagnetic behaviors whose essence can be grasped understanding the interplay between localized and delocalized resonances. We approached the solution of the inverse design problem of 2d-chiral metasurfaces [43]

Chiral response at normal incidence

Chiral response at oblique incidence and band structure

Superchiral near-fields

Singular value decomposition analysis of the metasurface operation

Inverse problem and metasurface minimality

Conclusions