Laser induced damage due to scratches in the surface of nonlinear optical crystals KH₂PO₄ (KDP)

Background: Potassium dihydrogen phosphate (KDP, KH₂PO₄) crystal is an important kind of nonlinear optical crystal because of widespread use in high-intensity laser systems. A critical issue in high-intensity lasers is the laser induced damage to dielectrics since the laser fluence that optical components can stand sets an upper limit to operational fluence of laser systems. Methods: Surface scratches on finished KDP surface were artificially generated and characterized with nano-indenter and atomic force microscope (AFM), respectively. The scratches were characterized with varied depth, directions and shapes. The morphology of scratches in different directions is quite different resulting from anisotropy of KDP crystals. The scratched surface along with surfaces free of scratches were irradiated with a 355 nm pulsed laser to acquire the damage threshold of KDP. Results: The experimental results show that scratches can reduce the laser induced damage threshold (LIDT) of KDP surface by >30%. Comparing the LIDTs of scratches of different depth, directions and shapes, no significant difference is observed, but they are much lower than scratch-free surface. The morphologies vary from scratches depending on the orientation of scratches relative to crystals. Moreover, the numerical simulation of light intensity shows that cross-section shape of scratches also influences the local electrical field of incident laser light. Conclusions: Scratches with smaller width-to-depth ratio enhance laser light intensity more significantly and more harmful to the improvement of laser damage threshold of KDP. The results suggest that the mechanical scratches must be avoided to achieve high-quality surface of strong resistance to UV laser pulses, especially those with smaller width-to-depth ratios.