Nonlinear total internal reflection through the thermoplastic effect.

We report on the observation of strong nonlinear optical behavior of SF59 Schott glass. An unfocused Ar' laser beam undergoing total internal reflection in this glass shows extremely strong selffocusing. This effect is ~ 1 order of magnitude higher than the usual thermal lensing, and, by means of an interferometric technique, we demonstrate that this effect is due to the thermoplastic effect. Focal lengths that are shorter than 10 cm are observed for the beam intensity I asymptotically equal to 30 W/cm(2). The use of this glass as a self-limiter is shown experimentally.