Defect-related optical properties of KDP crystals caused by nanosecond laser

Potassium dihydrogen phosphate (KH₂PO₄, KDP) crystals are critical nonlinear optical materials in Inertial Confinement Fusion (ICF) systems. Optical properties and defect evolutions of KDP crystals induced by nanosecond laser with two wavelengths at different energy densities were investigated by means of photoluminescence (PL) spectroscopy and ultraviolet–visible (UV–Vis) transmission spectroscopy. The PL spectrum revealed that a broad emission band of KDP crystals ranging from 280 to 450 nm presented. After irradiation with two-wavelength laser, its intensity first decreased and then increased generally with respect to laser energy densities. Moreover, this emission band primarily consists of three types of defects. They are A radicals ([HPO₄]^-), B radicals ([H₂PO₄]⁰), and D defects ([H₃PO₄]^2-). After laser irradiation, defect types undergo mutual transformations with energy densities, and the transformation processes strongly depend on the laser wavelength. Meanwhile, the band-gap energy (E_g) and Urbach energy (E_u) of KDP crystals were calculated out from UV–Vis transmission spectra after irradiation. As the laser energy density increases, the band-gap energy (E_g) exhibits a trend of first decrease, then increase, followed by a reduction again, while an opposite tendency was observed for Urbach energy (E_u). At the same energy density, 355 nm laser induced more significant changes than that from 1064 nm. After 10 J/cm² irradiation, the crystal undergoes remarkable changes for both wavelengths. Mechanisms underlying were discussed in detail. This study provided the insights for full applications of KDP crystals in ICF engineering.