Laser cooling of solids to cryogenic temperatures

Laser radiation has been used to cool matter ranging from dilute gases to micromechanical oscillators. In Doppler cooling of gases, the translational energy of atoms is lowered through interaction with a laser field^1,2. Recently, cooling of a high-density gas through collisional redistribution of radiation has been demonstrated³. In laser cooling of solids, heat is removed through the annihilation of lattice vibrations in the process of anti-Stokes fluorescence^4,5,6. Since its initial observation in 1995, research^{7,8,9,10,11,12,13,14,15} has led to achieving a temperature of 208 K in ytterbium-doped glass¹⁶. In this Letter, we report laser cooling of ytterbium-doped LiYF₄ crystal to a temperature of ∼155 K starting from ambient, with a cooling power of 90 mW. This is achieved by making use of the Stark manifold resonance in a crystalline host, and demonstrates the lowest temperature achieved to date without the use of cryogens or mechanical refrigeration. Optical refrigeration has entered the cryogenic regime, surpassing the performance of multi-stage Peltier coolers.