Evolution of intrinsic defect density in UV laser conditioning at the KDP crystal stress affected zone and its role in improving the laser induced damage threshold

UV laser conditioning, described as pre-exposure to subthreshold laser pulses, is an extremely promising method for improving the laser induced damage threshold (LIDT) of KDP crystals via reducing the density of intrinsic defects. Therefore, the evolution of intrinsic defect density during UV laser conditioning is to be researched. In this research, first, photoluminescence characterization was carried out to analyse the evolution of intrinsic defect densities in the UV laser conditioning process of KDP crystals in a stress affected zone. It was found that the density of hydrogen vacancy (V+H) and oxygen vacancy (V2-O) intrinsic defects decreases in the UV laser conditioning process, while the PO4 radicals formed while cracking was barely influenced by UV laser conditioning. Second, first-principles calculations were introduced to reveal the role of the intrinsic defects in the laser induced damaging process. Two defect energy levels that could initiate the laser induced damaging process were found, one of which was introduced by the V+H defect, the other introduced by V2-O defects and PO4 radicals formed while cracking. Finally, LIDT tests were carried out. The PO4 radicals formed while cracking were found to be the main reason that limits the effectiveness of UV laser conditioning in increasing the LIDT in the stress affected zone of KDP crystals. Overall, this work reveals the evolution of defect densities of KDP crystals via UV laser conditioning and its role in improving laser induced damage threshold, providing a new understanding of the mechanism of UV laser conditioning and guidance on the manufacturing of KDP optics.