# Gigantic Vortical Dichroism and Handedness-Dependent Optical Response in Spiral Metamaterials

## TL;DR

This paper introduces spiral metamaterials that strongly interact with twisted light, creating a large optical asymmetry based on the handedness of the light.

## Contribution

The study introduces three-dimensional spiral metamaterials that achieve a gigantic vortical dichroism effect through unique electromagnetic mode interactions.

## Key findings

- Spiral metamaterials exhibit a vortical dichroism (VD) value of 0.69 due to asymmetric scattering of opposite-handed OAM beams.
- Opposite-handed OAM beams cause spatial mode mismatch and enhanced scattering, while matching handedness suppresses scattering via guided spiral modes.
- The work provides a new platform for advanced chiral photonic devices and OAM-based light–matter interactions.

## Abstract

Light carrying orbital angular momentum (OAM) has emerged as a promising tool for manipulating light–matter interactions, providing an additional degree of freedom to explore chiral-optical phenomena at the nanoscale. When such vortex beams interact with chiral metamaterials, a unique phenomenon of optical asymmetry known as vortical dichroism (VD) arises. Nevertheless, most existing chiral metamaterials exhibit limited VD responses, and the underlying physical mechanisms are yet to be fully clarified. In this work, we propose three-dimensional spiral metamaterials that achieve gigantic VD effect. This pronounced VD effect originates from the intrinsic coupling between the spiral structure and the chirality inherent to optical vortices, which leads to strongly asymmetric scattering intensities for left- and right-handed OAM beams of opposite topological charges. Numerical simulations confirm a remarkable VD value of 0.69. Further analysis of electric field distributions reveals that the asymmetric VD response stems from a handedness-dependent excitation of distinct electromagnetic modes. For opposite handedness, spatial mode mismatch results in enhanced scattering. In contrast, matching handedness enables efficient energy coupling into a guided spiral mode, which suppresses scattering. These findings not only deepen the physical understanding of VD mechanisms but also establish a versatile platform for developing advanced chiral photonic devices and enhancing OAM-based light–matter interactions.