Angle and time-resolved polarization change induced by the Kerr effect in amorphous and crystalline SiO2.

We measure the polarization change of a beam reflected from the surface of both crystalline α and amorphous SiO2 samples while they are photo-excited by an intense light pulse, at intensities above the nonlinear excitation threshold yet below the damage threshold. The polarization change varies with the angle between the polarization of the pump and probe light, but is found to be independent of their orientation relative to the crystal axes. This behavior differs between the reflected and transmitted beams, and can be modeled by taking into account a birefringence induced by the electric field of the pump. These polarization-change effects can be very strong, with polarization rotation exceeding 90°, at pump intensities well below the damage threshold. We also observe a markedly different behavior of the reflected beam depending on whether the material is crystalline or amorphous. We aim to reproduce our results by an anisotropic modelization, taking into account self-trapped excitons for the crystalline SiO2.