Spontaneous symmetry breaking in an ultrahigh-Q microcavity

Spontaneous chiral symmetry breaking is a ubiquitous property in nature and diverse fields of modern physics. However, such symmetry breaking has been elusive experimentally in the optical systems, which usually demands multiple identical subsystems [1]. As a prominent photonic device, the ultrahigh-Q whispering-gallery mode (WGM) microresonator supports clockwise (CW) and counterclockwise (CCW) propagating waves coupled to each other, leading to symmetric and antisymmetric standing-wave modes with equal CW and CCW amplitudes. The demonstrations of such overall chirality have to rely on external perturbations to a resonator, either by breaking the parity or time-reversal symmetry [2,3]. The chirality with unbalanced CW and CCW components not only attracts general interest in physics, but also is of importance in novel devices [2,3]. Here, we experimentally demonstrate the spontaneous chirality in a single WGM microresonator (Fig. 1) without any explicit breaking of parity or time-reversal symmetry.