Experimental demonstration of spontaneous chirality in a nonlinear microresonator

TL;DR

This paper reports the first experimental observation of spontaneous chirality in a single ultrahigh-Q microresonator, caused by Kerr nonlinearity, with potential applications in fundamental physics and photonic devices.

Contribution

It demonstrates spontaneous chiral symmetry breaking in a single microresonator, a phenomenon previously unobserved in such simple systems.

Findings

Chiral field emerges spontaneously in a microresonator.

Chirality occurs at ultra-weak input power levels.

CW-to-CCW output ratio reaches 20:1.

Abstract

Chirality is an important concept that describes the asymmetry property of a system, which usually emerges spontaneously due to mirror symmetry breaking. Such spontaneous chirality manifests predominantly as parity breaking in modern physics, which has been studied extensively, for instance, in Higgs physics, double-well Bose-Einstein condensates, topological insulators and superconductors. In the optical domain, spontaneous chiral symmetry breaking has been elusive experimentally, especially for micro- and nano-photonics which demands multiple identical subsystems, such as photonic nanocavities, meta-molecules and other dual-core settings. Here, for the first time, we observe spontaneous emergence of a chiral field in a single ultrahigh-Q whispering- gallery microresonator. This counter-intuitive effect arises due to the inherent Kerr nonlinearity-modulated coupling between clockwise (CW) and counterclockwise (CCW) propagating waves. At an ultra-weak input threshold of a few hundred microwatts, the initial chiral symmetry is broken spontaneously, and the CW-to-CCW output ratio reaches 20:1. The spontaneous chirality in microresonator holds great potential in studies of fundamental physics and applied photonic devices.