Improving third harmonic generation performance by elevating the operation temperature of KDP and DKDP crystals

Potassium dihydrogen phosphate (KH₂PO₄, KDP) family crystals are the only nonlinear optical materials that can satisfy the aperture requirement of the inertial confinement fusion (ICF) laser driver. As a major ultraviolet (UV) generation scheme in service, the type-II phase-matched (PM) third harmonic generation (THG) performance of KDP and DKDP crystals is critical to ICF equipment. All the time, the largest efforts from all aspects have been made to achieve the highest output energy, conversion efficiency, and anti-damage ability. For this purpose, an effective route is developed and analyzed in this paper, i.e. substantial elevating the operation temperature of KDP and DKDP crystals. When the crystal temperature was set at 130 °C, the THG energy, conversion efficiency, and laser-induced damage threshold (LIDT) were increased by 11.9%-16.5%, 5.3%-5.4%, 17.1%-24.5% for KDP crystals, and 4.2%-7.6%, 4.2%-6.6%, 14.2%-26.5% for DKDP crystals, respectively. Variable-temperature Z-scan tests reveal that increasing the temperature of KDP-like crystals effectively suppresses the two-photon absorption effect of the crystals and thus improves their UV frequency conversion performance. This research offers a valuable technique for the future development of ultraviolet, deep-ultraviolet lasers in ICF facilities.