The Game of Light & Heat: Cryogenic Optical Refrigeration and Athermal Lasers

Laser cooling of solids, also known as optical refrigeration, is based on anti-Stokes fluorescence upconversion where low-entropy laser light with photon energy less than mean fluorescence energy is absorbed followed by highly efficient fluorescence that carries heat and entropy away from the material [1] , [2] . Much progress has been made recently by laser cooling high-purity Yb:YLF crystals to cryogenic temperatures (~90K) [3] , [4] . More recently, Ho and Tm-doped crystals have also been cooled with cryogenic prospects [5] . On the applications front, optical refrigeration has been touted as the only promising technology for realizing an all-solid-state and vibration-free cryocooler. It was also shown that the cooling process when combined with stimulated emission can lead to radiation-balanced lasers (RBL) that are free from thermal instabilities and beam distortion [6] . We report on the most recent advances in this field which includes the realization of the first optical cryocooler prototype, and the progress towards its application in ultra-stable lasers for precision metrology. Furthermore, the feasibility of high-power "athermal" radiation-balanced lasers in disks, rods and fibers are discussed.