Saturation Effects and Thermal Balance in Laser-Cooled Solids

The role of saturation of background absorption by unintended impurities has been investigated for the first time theoretically and experimentally in the context of laser cooling of crystalline solids and the operation of radiation-balanced lasers (RBLs). It’s shown that when saturation of the background takes place at an intensity lower than the saturation intensity of dopant ions responsible for cooling, cooling efficiency can be improved by using elevated input intensity. The measured cooling efficiency in 10% Yb³⁺:LiYF­₄ crystal was doubled at 1020 nm by partially saturating the background impurity absorption. This is effective not only as a post-growth method of enhancing cooling efficiency but offers a new way of reaching lower temperatures than were previously possible with anti-Stokes fluorescent refrigeration. Prior to this work, cooling efficiency was thought to be independent of intensity and crystal purification was considered as the only way to improve the cooling efficiency.

It is further demonstrated that background saturation plays a key role in RBL operations. When the limitations imposed by impurity is included, quantitative predictions of radiation-balanced condition become possible. Laser operation can then be made more efficient and can be extended to impure materials. 3% Yb:YAG RBL achieved 30.5% efficiency and agreed with predictions within 2%, which is the highest efficiency level of any RBLs operated to date. Yb:KYW achieved 1.6% efficiency as the result of non-optimal optical components, high Yb concentration and high impurity content. However, this is the first demonstration of an Yb³⁺ RBL in the tungstate host.

Chair: Professor Stephen C. Rand