Experimental evidence of spontaneous symmetry breaking in intracavity   type-II second harmonic generation with triple resonance
  We describe an experiment showing a spontaneous symmetry breaking phenomenon between the intensities of the ordinary and extraordinary components of the fundamental field in intracavity type-II harmonic generation. It is based on a triply resonant dual cavity containing a type II phase matched $\chi^{(2)}$ crystal pumped at the fundamental frequency $\omega_0$. The pump beam generates in the cavity a second harmonic mode at frequency $2\omega_0$ which acts as a pump for frequency degenerate type II parametric down conversion. Under operating conditions which are precisely symmetric with respect to the ordinary and extraordinary components of the fundamental wave, we have observed a breaking of the symmetry on the intensities of these two waves in agreement with the theoretical predictions. 
Laurent Longchambon, Nicolas Treps, Thomas Coudreau, Julien Laurat,   Claude Fabre