Nonlinear chiral response from linearly achiral membrane metasurfaces

TL;DR

This paper demonstrates that linearly achiral silicon membrane metasurfaces can exhibit strong nonlinear chiral responses, such as circular dichroism in third-harmonic generation, by breaking in-plane symmetry.

Contribution

It introduces a novel method to achieve nonlinear chiral responses in metasurfaces that are linear achiral, expanding the design possibilities for chiral photonic devices.

Findings

Strong nonlinear circular dichroism observed in third-harmonic generation.

In-plane symmetry breaking reverses the sign of nonlinear circular dichroism.

Achiral metasurfaces can exhibit significant nonlinear chiral effects.

Abstract

Chiral photonics aims to control and engineer light handedness for many applications in optical communications, biological and chemical sensing, and quantum technologies. While traditional approaches focus on engineering strong linear chiroptical response, nonlinear chiral phenomena remain largely unexplored. Here, we demonstrate experimentally a pronounced nonlinear chiral response in free-standing silicon membrane metasurfaces that are effectively achiral in the linear regime. By employing patterned membranes with both $C_4$-symmetry and intentionally broken in-plane symmetry, we reveal that strong nonlinear circular dichroism can be observed in third-harmonic generation. An unperturbed metasurface exhibits strong cross-polarized third-harmonic signal with nonlinear circular dichroism of the value $-0.83$, whereas in-plane symmetry breaking enables a co-polarized channel, and it reverses the sign of nonlinear circular dichroism that may be as large as the value $0.41$. Our findings suggest a novel approach for engineering nonlinear chiral responses in metasurfaces, complementing traditional approaches and paving the way towards advanced chiral metadevices.