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 field1,2. 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 fluorescence4-6. Since its initial obser-vation in 1995, research7-15 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. The process of optical refrigera