Mechanism of laser damage threshold reduction induced by radical crack defects on KDP crystal surfaces

The radical crack defects on the KDP crystal surfaces would produce severe damage propagation and greatly reduce the service life and laser damage resistance of the component under high flux laser irradiation. The internal mechanism of laser-induced damage in KDP crystals remains unclear due to the random distribution of its surface defects, the complex response of KDP materials to the laser irradiation, and the modulation effect of the defect morphology on the laser. In this work, the morphology, element content and photoluminescence (PL) properties of the radial crack defect zones on the KDP crystal surface were characterized. By integrating laser-induced damage threshold (LIDT) experiments, it revealed a direct positive correlation between PL intensity in defect zones and intrinsic defect concentration, whereas an inverse correlation was observed between PL intensity and LIDT. Specifically, higher intrinsic defect concentrations enhance PL emission by promoting defect-related radiative transitions, while this defect-driven increase in PL intensity is concomitantly associated with a reduction in LIDT. The radical crack defect zones would contain a large number of invisible intrinsic defects (such as hydrogen vacancy and oxygen vacancy defects), reducing the LIDT of KDP crystals to approximately one-fifth to one-third of that of regions with only plastic deformation. More interestingly, due to the anisotropy of the crystal, the concentration of the intrinsic defect accompanying to the radial crack defect zones introduced by the scratch with linear load was obviously lower than that of the scratch with the same constant load, and the additional hydrogen defects with a PL peak of 655 nm were introduced. For example, when the load of scratches was 100 mN, the concentration of the intrinsic defect of the scratch with constant load was 3 times than that of the scratch with linear load, while the LIDT of the zones of the scratch with constant load (4.85 J/cm²) was lower than that of the scratch with linear load (5.96 J/cm²), which meant that the LIDT exhibited a reduction exceeding 20%. It is revealed that the LIDT of the KDP crystal exhibits an exponential attenuation relationship with the increase in defect-induced PL intensity. This work could provide theoretical basis and data support for the suppression of surface defects of the KDP crystal elements.