Laser cooling in Yb:KY3F10: a comparison with Yb:YLF

Laser cooling by anti-Stokes fluorescence is a technology to realize all-solid-state media and compare their cooling performance to Yb3+-doped LiYF4 (Yb:YLF) crystals also grown in our institute. We present temperature-dependent absorption and emission cross sections as well as the fluorescence lifetime of Yb:KYF, and calculate its material figure-of-merit for laser cooling. Yb:KYF exhibits a higher figure-of-merit than Yb:YLF at temperatures below 200 K. This is because, in contrast to Yb:YLF, the excitation transition from the second-highest Stark level of the ground state is best-suited for cryogenic cooling in Yb:KYF. Thus, it has the potential to achieve unprecedentedly low temperatures below the boiling point of liquid nitrogen. In this work, we observe the first laser cooling of Yb:KYF, and obtain a background absorption coefficient of similar to 10-4 cm-1, which is among the lowest ever reported for Yb3+-doped fluoride crystals. A simple model calculation predicts that our Yb:KYF and Yb:YLF crystals can potentially be cooled down to approximate to 100 K in a high-power cooling setup. Our Yb:KYF crystals still leave room for further improvement through the optimization of the growth process and the use of purer raw materials.

Automated summary

This study compares the cooling performance of Yb3+-doped LiYF4 crystals with Yb:KYF crystals grown in our institute using laser cooling by anti-Stokes fluorescence. Yb:KYF exhibits better cooling performance at temperatures below 200K. Further improvement is possible for Yb:KYF crystals through optimizing the growth process and using purer raw materials.