Symmetry breaking of counterpropagating Raman waves in optical microresonators: unexpected findings

Recently, symmetry breaking and related issues in high-Q nonlinear optical microresonators have been of growing interest for fundamental research and photonic device development. Most studies employ Kerr nonlinearity, but this is not the only way. We present our recent results on symmetry breaking of counterpropagating Raman waves at Stokes frequencies in a bidirectionally pumped silica microresonator. We experimentally demonstrate symmetry-broken Raman lasing with previously unreported features such as intensity switching of counterpropagating waves and symmetry restoring during pump frequency sweeping. Symmetric and asymmetric stationary states have been found theoretically, but asymmetric states after a pitchfork bifurcation have proven to be unstable. Such behavior is surprising, since in the case of pure Kerr nonlinearity (without stimulated Raman scattering), a pair of stable asymmetric states is born after a pitchfork bifurcation, and the symmetric solution becomes unstable. The explanation of our experimental findings is based on a weak asymmetry in the scheme, which dramatically changes the dynamic behavior of the system. This is confirmed by numerical simulations with added back reflection for a single Raman wave.