Characterization-method and repair-mechanism of atomic defects in potassium dihydrogen phosphate

Potassium dihydrogen phosphate (KDP), a frequency-doubling crystal, is typically characterized by destructive laser-induced damage threshold (LIDT) testing for performance evaluation. Atomic defects on the component surface constrain its service lifetime. To address these challenges, this work proposes a non-destructive evaluation method for the LIDT of optical components based on the photoluminescence (PL) mechanism associated with defect energy levels. A deconvolution analysis model for steady-state PL spectra of KDP materials is developed to extract characteristic parameters of atomic defects on the micro-milled surface. The microscopic mechanism through which micro-milling enhances the LIDT of flawed KDP optics is revealed. The results indicate that the PL intensity of micro-milled surfaces exhibits a nonlinear positive correlation with the relative concentration of atomic defects (c_a) in the KDP materials, while inversely correlating with the LIDT of optical components. Hydrogen/oxygen vacancies, interstitial hydrogen, and hydrogen defects are present on the KDP optical surface. During the removal of surface imperfections, oxygen vacancies with high radiative recombination capability can serve as key characterization parameters for the non-destructive evaluation of the repair quality. For micro-milling repair, a larger tool edge radius diminishes the shear deformation capacity of KDP materials, resulting in elevated c_a. On the initial imperfection surface (Sa = 116 nm), the removal ratio of c_a achieves 95.2 % through micro-milling repair. This work offers significant theoretical and practical implications for the identification/repair of enriched atomic defects, as well as the non-destructive evaluation of the LIDT.