Concentration characterization of underlying intrinsic defects accompany with surface structural defects and their effect on laser damage resistance

During ultra-precision processing of potassium dihydrogen phosphate (KDP) crystals, various macro-structural surface defects are inevitably introduced, but there is no monotonic relationship between different macro-defects and laser-induced damage threshold (LIDT). Therefore, a single macro-perspective cannot fundamentally reveal the causation for low LIDT. In this work, energy, photoelectron spectra characterization and photoluminescence characteristics of surface were adopted to determine the type and concentration of intrinsic defects accompany with macro-structural defects. First-principles calculation was applied to explore the influence of oxygen vacancy (V_O) defect concentration on performance in terms of crystal structure, electronic structure, and optical properties. The results show that the causation for low LIDT of cracks is V_O defects. With the increase of V_O defect concentration, PO₄^3- group would collapse, leading to a decrease in the stability of crystal structure. High concentration of V_O defects would introduce defective-energy level, causing irreversible alteration in energy level. Free electrons are susceptible to absorb photons and transition due to defective-energy level, severely reducing the laser damage resistance of components. This work reveals the influence of surface defects on laser damage from intrinsic defects at atomic-level, which is significance for the suppression of surface defects during ultra-precision processing and effective improvement of laser damage resistance of KDP.