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 Ta2O5 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 Ta2O5 layer is nearly 10−15 m2/W.
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