Abstract

The phenomena of optical breakdown (damage) of normally transparent optical materials such as fused silica involve ionization of valence band electrons by the intense incident laser field. Subsequent electron heating, acceleration, and impact ionization of secondary electrons in the laser field result in an exponential multiplication of free electrons, which causes the material breakdown. Laser-induced breakdown has been studied for three decades, and two major theories, the avalanche model and the photoionization model, exist for the breakdown process.^1,2 In an avalanche- dominated process, a few initial electrons are generated through photoionization or thermal excitation from defects. These free electrons are accelerated by the intense laser field and create secondary electrons via collisions with atoms. The process repeats itself and eventually results in breakdown. In the photoionization theory, on the other hand, free electrons are created by multiphoton ionization or tunneling ionization only, and electron avalanche is negligible. In both models, the breakdown threshold is dependent on the laser pulse width.

© 1994 Optical Society of America