Degradation mechanisms of optical properties of KDP crystals induced by nanosecond laser

Potassium dihydrogen phosphate (KH2PO4, KDP) crystals are vital optical materials in Inertial Confinement Fusion (ICF) engineering. Optical properties of KDP crystals induced by nanosecond laser with two wavelengths at different energy densities were investigated by means of ultraviolet-visible (UV-Vis) transmission spectra, infrared (IR) transmission spectra and Raman scattering spectra. Results from UV-Vis spectra showed that transmittance of KDP crystals increased, accompanied by a general reduction in refractivity index, extinction coefficients and photoconductivities after irradiation. Infrared spectra revealed that the KDP crystal successively underwent a breakage of hydrogen bonds, dehydration and formations of new phosphates, as well as recombination of broken segments with an increase in laser energy densities. Raman spectra analysis primarily exhibited that vibration modes of PO43- and H2PO4- groups became disorder, and a new vibration mode, related to P=O bonds appeared with respect to energy densities after irradiation with two-wavelength laser. Furthermore, the results from three analysis techniques consistently suggests that KDP crystals were damaged firstly, then repaired, and finally damaged again, with an increase of energy density. And 355 nm laser induced more serious damages than that of 1064 nm laser at the same energy density. Similar damage processes were found after twowavelength laser irradiation. Moreover, underlying degradation mechanisms in optical properties of KDP crystals were discussed in detail. This work provided insights into the full use of KDP crystal in ICF engineering.