Quantitative identification of deposited energy in UV-transmitted KDP crystals from perspectives of electronic defects, atomic structure and sub-bandgap disturbance

The laser-induced damage threshold (LIDT) of ultra-precision machined potassium dihydrogen phosphate (KDP) crystal is always lower than the intrinsic threshold. However, as the interaction between intense laser and transparent dielectric materials involves complex physical phenomena, it is difficult to essentially quantify the influence of defects on laser damage. Herein, the defective energy levels caused by bandgap disturbance were calculated to explore the nonlinear ionization of free electrons and reveal the essential cause that triggers energy deposition. According to first principles calculations and photoluminescence characteristics, it was found that manufacturing-induced intrinsic defects lead to defective energy levels. Then, the sub-bandgap disturbed energy deposition model was developed to understand the influence of defects on laser damage. The results revealed that under the excitation of an intense laser, the free electron density of manufacturing-induced defects is 3.73 times that of a defect-free surface, causing a significant decrease in LIDT, exacerbating energy deposition inside the crystal. The energy deposited within crystals at 400 ps is ∼1.26 times that of the surface without structural defects, which leads to severe extension of subsequent damage. This work offers a standard for controlling surface defects during ultra-precision processing and is of great help to promote the performance of optics applied in high-power laser facilities.