Spontaneous symmetry breaking in an optical microcavity

Spontaneous symmetry breaking is a ubiquitous property in diverse fields of modern physics, which is, however, elusive in optical domain. As a prominent photonic platform, the ultrahigh-Q whispering-gallery (WG) microcavity supports clockwise (CW) and counterclockwise (CCW) propagating waves with balanced amplitudes. Such chiral symmetry in a WG microcavity can be explicitly broken by removing the parity or time-reversal symmetry in previous works [1,2]. In this work, we report the experimental demonstration of the spontaneous chiral symmetry breaking in a single WG microcavity. The Kerr nonlinearity gives rise to an intensity-dependent CW-CCW coupling that becomes zero beyond a threshold, causing the original standing-wave mode to be unstable and spontaneously evolve into two chiral states with the unbalanced CW and CCW components [3,4]. Furthermore, we extended such the spontaneous symmetry breaking to the active regime and realized a unidirectional microlaser with the controllable chirality.
[1] B. Peng et al., Proc. Natl. Acad. Sci. USA 113, 6845 (2016).
[2] S. Maayani et al., Nature 558, 569 (2018).
[3] Q.-T. Cao et al., Phys. Rev. Lett. 118, 033901 (2017).
[4] L. Del Bino, J. M. Silver, S. L. Stebbings, P. Del'Haye, Sci. Rep. 7, 43142 (2017).