Impact of Ho3+, Er3+, and Tm3+ on laser cooling of Yb:YLF

Abstract

Ytterbium-doped LiYF4 (Yb:YLF) is a promising material for all-solid-state optical cryocoolers, but the impact of foreign rare-earth impurities on the laser-cooling performance is not completely understood. In particular, Tm3+ has been reported to reduce the background absorption. This study quantitatively assesses the impact of Ho3+, Tm3+, and Er3+ impurities on laser-cooling of Yb:YLF by anti-Stokes fluorescence. We grew five Yb(5%):YLF crystals intentionally doped with tens of ppm levels of these impurities. Laser-induced thermal modulation spectroscopy tests confirmed that these rare-earth impurities reduce the external quantum efficiency of Yb:YLF without affecting the background absorption coefficient. Although Er3+ is a well-known quencher for Yb3+, Er3+ co-doping only slightly decreases the laser-cooling efficiency at low pump intensities but becomes detrimental at high pump intensities (>5 kW cm-2). However, this detrimental effect diminishes at lower temperatures, as evidenced by cooling an Er3+ co-doped crystal to the same minimum temperature of 144 K as a solely Yb3+-doped crystal. Contrary to previous reports, Tm3+ proved to be the most detrimental among the three impurities.