Impact of foreign rare-earth impurities on optical refrigeration of Yb:YLF

We quantitatively investigated the impact of rare-earth ions Ho3+, Er3+, and Tm3+ on Yb:YLF laser cooling performance. Four Yb(5%):YLF crystals co-doped with these ions were characterized for fluorescence lifetime, intensity-dependent laser-cooling performance, and high-power optical refrigeration. Results show that all three ions negatively impact laser cooling, contrary to previous suggestions about a positive impact of Tm3+. Tm3+ and Ho3+ reduced cooling efficiency regardless of pump intensity, while Er3+ co-doping showed comparable cooling efficiency to Yb:YLF at low pump intensities but lower cooling efficiency at higher pump intensities. High-power cooling experiments confirmed that Ho3+ and Tm3+ restricted the minimum achievable temperature. Despite Er3+ quenching Yb3+ fluorescence, the Er3+ co-doped crystal achieved the same minimum temperature as Yb:YLF under low-intensity pumping.