Decomposition of KH2PO4 crystals during laser-induced breakdown

Abstract

The structure of KH2PO4 single crystals (so-called KDP) irradiated with ∼3ns, 355 nm laser pulses with fluences above the laser-induced breakdown threshold is studied by a combination of Raman scattering, photoluminescence, and soft x-ray absorption spectroscopies. We compare spectra from the as-grown material, surface and bulk laser-induced damage sites, as well as from KPO3 references. Results show that irradiation with fluences above the laser-induced breakdown threshold leads to decomposition of KDP at surface damage sites but not at bulk damage sites. New spectroscopic features are attributed to dehydration products. For the laser irradiation conditions used in this study, the decomposed near-surface layer absorbs photons at ∼3.4eV (364 nm). These results may explain the recently reported fact that surface laser damage sites in KDP crystals tend to grow with subsequent exposure to high-power laser pulses, while bulk damage sites do not.