Optical properties of Ba-impurity defect clusters in KH2PO4 crystals

Abstract

Potassium dihydrogen phosphate (KDP) is widely used in electro-optical switches and inverter crystals in inertial confinement fusion (ICF). The early studies suggested that defect clusters are one of the main reasons why the laser-induced damage threshold (LIDT) is far lower than the theoretical value. However, it is not clear what type of defect clusters cause this phenomenon. The BaK defect cluster models have been determined by simulation using General Utility Lattice Programs (GULP). We found that BaK+VH and BaK+VK cause a dramatic distortion on H-O bonds with maximum values of 29.5 % and 27.12 %, respectively. By analyzing the configuration coordinate diagram and optical spectra, we believe that both defect clusters can promote ionization through defect-assisted "2+1" photon absorption, which leads to free electrons more easily exceeding the critical value for electron avalanche breakdown. Our study will offer insight into the effects of defect clusters in KDP crystals.