Determination of stress waves and their effect on the damage extension induced by surface defects of KDP crystals under intense laser irradiation

The residual crack defects on the surface of potassium dihydrogen phosphate (KDP) crystals are the bottleneck that limits the improvement of laser damage resistance in the application of high-power laser devices. The multiple stress waves introduced by these residual surface lateral cracks on crystals under laser irradiation are the main inducement for damage extension and reduction of laser damage resistance. However, the coupling of these stress waves complicates their propagation in the crystal, and the interaction mechanism between each stress wave and laser damage has not been quantitatively characterized. Herein, a laser damage dynamic model for surface lateral cracks is constructed to reproduce the dynamic behaviors of the evolution of micro-defects to sub-millimeter damage pits under laser irradiation. Combined with the time-resolved pump and probe technique, the distribution of stress waves induced by lateral cracks was detected in situ to determine the type of stress waves. Then, the initiation and extension of laser damage were analyzed quantitatively to establish the correlations between different stress waves and damage extension. It is found that the longitudinal, shear, and Rayleigh waves induced by lateral cracks lead to large crush zones on the surface of KDP crystals, as well as butterfly-like damage sites accompanied by a large number of cracks at the bottom in the longitudinal section. The scale of the damage site can reach up to approximately 150 µm for lateral crack defects with large surface widths. This study ultimately reveals the physical mechanism of damage evolution induced by lateral cracks, providing effective guidance for developing control standards of surface crack defects during optical ultra-precision machining processes. This is of great significance for the improvement of laser damage resistance of KDP crystals in high-power laser systems.