Deep-UV optical properties of KDP-family crystals: a comprehensive characterization

As a class of widely used optical materials, KH2PO4 (KDP)-family crystals play an important role in the generation of infrared, visible, UV, and deep-UV lasers, but their deep-UV optical characteristics have been lacking in systematic research. Aiming at the typical KDP-family nonlinear crystals, including NH4H2PO4, KDP, K(H1-xDx)(2)PO4 (partially deuterated KDP), and KD2PO4 crystals, we systematically investigated their transmittance, cut-off edge, band-gap energy, linear absorption, and nonlinear two-photon absorption (TPA) properties in the deep-UV spectral region. By using a vacuum-UV spectrophotometer, the UV absolute transmittances were measured in the range of 160-290 nm. Based on the measured results, the UV cut-off edges of these crystals were obtained and band-gap energies were calculated. By using an Nd:YLF picosecond laser (1053 nm, 9 ps, 10 Hz), the fourth and fifth harmonics (263.2 and 210.5 nm) were generated respectively via a LiB3O5 crystal and two beta-BaB2O4 crystals. The anisotropic linear absorption and nonlinear TPA coefficients of the above KDP-family crystals were obtained in detail from intensity-dependent transmittance measurements at 263.2 and 210.5 nm with ordinary and extraordinary polarization states. This comprehensive investigation of the deep-UV optical properties of commonly used KDP-family optical materials and the obtained new data can provide an important parameter basis for crystal growth with higher optical purity, deep-UV laser generation, and applications.