Nonlinear Optical Rotational Anisotropy

When light of a fixed frequency illuminates a material, the nonlinear response of the material can produce reflected light at higher harmonics of the incident frequency. By measuring how this nonlinear harmonic generation depends on the angles of incidence and reflection of the light, one can determine the full nonlinear optical susceptibility tensor of the material, which encodes information about the symmetries of the material under study and the breaking of those symmetries at phase transitions.