Investigation of processes leading to damage growth in optical materials for large-aperture lasers.

Damage growth in optical materials used in large-aperture laser systems is an issue of great importance to determine component lifetime and therefore cost of operation. Small size damage sites tend to grow when exposed to subsequent high-power laser irradiation at 355 nm. An understanding of the photophysical processes associated with damage growth is important to devise mitigation techniques. We examine the role of laser-modified material and cracks formed in the crater of damage pits in the damage growth process using fused-silica and deuterated KDP samples. Experimental results indicate that both of the above-mentioned features can initiate plasma formation at fluences as low as 2 J/cm2. The intensity of the recorded plasma emission remains low for fluences up to approximately 5 J/cm2 but rapidly increases thereafter, accompanied by an increase of the size of the damage crater.