Abstract

We have experimentally demonstrated nanosecond transmission to total internal reflection (TIR) switching and optical limiting resulting from laser induced cavitation in films of carbon microparticles suspended in ethanol. The films are bounded by two glass windows separated by mylar spacers. Laser-induced cavitation occurs when the fraction of the incident energy absorbed by a carbon particle is sufficient to heat up and vaporize a small volume of the surrounding liquid. These small vapor bubbles coalesce to form larger vapor bubbles. Total internal reflection switching/limiting occurs at the glass-vapor interface that is formed when these bubbles meet the glass substrate. Nonlinear refractive-index changes as large as 0.3 have been obtained using a single 5-ns Q-switched doubled (532-nm) Nd:YAG laser pulse. Switching and limiting thresholds as low as 0.2 J/cm² have been measured. More than 90% of the incident energy has been reflected using the cavitation/TIR switching mechanism. The observed limiting results from two mechanisms: energy removal via TIR and scattering from microplasmas that form when the carbon particles are thermally ionized by the incident laser energy. We have investigated the effects that particle concentration, film thick ness and film bias temperature have on the cavitation/TIR process and limiting behavior.

© 1991 Optical Society of America