Light-induced refractive-index modifications in dielectric thin films: experimental determination of relaxation time and amplitude.

A two-beam setup based on the totally reflecting prism coupler is shown to be a powerful means of characterizing light-induced refractive-index modifications in dielectric thin films. Rise and relaxation times and amplitudes of thin-film refractive-index variations can be measured. Some developments of the electromagnetic theory of prism coupling are presented for Gaussian incident beams. Measurements made on a single Ta(2)O(5) layer deposited on a silica glass are presented. Relaxation times of a few milliseconds reveal the thermal origin of the phenomena. The thermal nonlinear coefficient of this Ta(2)O(5) layer is nearly 10(-15) m(2)/W.