True Circular Dichroism in Optically Active Achiral Metasurfaces and Its Relation to Chiral Near-Fields

TL;DR

This paper investigates the true circular dichroism in achiral metasurfaces with U-shaped resonators, analyzing the contributions to optical activity and exploring how near-field properties relate to chiral molecule detection.

Contribution

It provides a quantitative analysis of the optical activity contributions in achiral metasurfaces and links near-field magneto-electric modes to far-field chiroptical responses.

Findings

Linear birefringence and retardance have minimal impact on apparent circular dichroism.

Magneto-electric modes exhibit unique near-field properties.

Potential for improved chiral molecule detection using plasmonic resonators.

Abstract

Optically active achiral metasurfaces offer a promising way to detect chiral molecules based on chiroptic methods. The combination of plasmonic enhanced circular dichroism and reversible optical activity would boost the sensitivity and provide enantiomerselective surfaces while using a single sensing site. In this work, we use metasurfaces containing arrays of U-shaped resonators as a benchmark for analyzing the optical activity of achiral materials. Although the peculiar optical activity of these metasurfaces has 1 been quite well described, we present here an experimental and numerical quantitative determination of the different contributions to the measured optical activity. In particular, it is shown that linear birefringence and retardance contribute, but only marginally, to the apparent circular dichroism of the metasurface associated with the excitation of magnetoelectric modes. We then numerically demonstrate the peculiar near-field properties of the magneto-electric modes and explain how these properties could be reflected in the far-field polarimetric properties in the presence of chiral molecules. This work provides alternatives for the detection scheme of chiral molecules using plasmonic resonators.