Multi-wavelength study of nanosecond laser-induced bulk damage morphology in KDP crystals

This study reports on experimental results on the morphology of laser-induced damage in the bulk of potassium dihydrogen phosphate (or KDP) crystals by nanosecond pulses. The resulting morphology of bulk laser damage is structured in four different zones whose characteristics depend on laser parameters such as wavelength, laser polarization, and fluence. To have a better understanding of bulk formation and structure of KDP laser damage, accessing the mechanical properties of KDP is a promising way which departs from a thermal approach which is mainly used in various models well documented in the literature. In that way, we have revisited various mechanical models of fracturation frequently used to evaluate the toughness of optical materials. Our experiments are compared to these models where the toughness is the criterion, which has been chosen to validate the modeling part of this study. We conclude that experiments are in agreement with the common values obtained by indentations tests proposed in the literature, allowing us further developments with one of these models to describe the damage formation in KDP crystals. Also, it appears that laser damage events split into two separated groups depending on the wavelength, with on one side 351-nm damage sites and on the other side 1064- and 532-nm damage sites. That suggests the existence of different physical mechanisms responsible for bulk damage initiation in KDP crystals with respect to the wavelength.