Hybrid density functional theory calculations for surface damaged phosphate products of laser irradiated KDP crystals

Abstract

Laser induced damage sites on the surface of a KDP crystal component tend to grow with subsequent laser irradiation, which substantially decrease the lifetime of these optics. The structural investigation suggests that this may be related to the surface damage products of the crystals. In this work, the differences in the crystal structures, electronic properties and optical absorption between the KDP crystal and its surface laser-induced decomposition products K₂H₂P₂O₇ and KPO₃ are investigated by using first-principles calculations. The theoretical results show that the nonlinear optical active units of KDP crystals are disrupted after irradiation dehydration to K₂H₂P₂O₇ and KPO₃, which leads to the optical properties at the surface damage being deteriorated. In terms of the electronic structure, the dehydration products K₂H₂P₂O₇ and KPO₃ have a shorter band gap compared with the KDP crystal. In addition, K₂H₂P₂O₇ and KPO₃ have a wider optical absorption band than that of the KDP crystal and introduce more ultraviolet absorption in the optical elements. Compared to polycrystalline KDP at the bulk damage sites, the dehydration products at the surface damage increase the ultraviolet absorption of the crystals and cause the surface damage to continually grow under subsequent irradiation, whereas the bulk damage does not continue to increase in subsequent laser irradiation.