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 observation 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 refrigeration in solids is based on antiStokes fluorescence (Fig. 1). Laser light of frequency n tuned below the mean emission frequency (n, n̄f ) produces a non-equilibrium electron distribution in the manifolds of the initial and final states. The interaction of these excitations with the lattice leads to phonon absorption followed by blueshifted fluorescence. Heat and entropy are carried away by the fluorescence photons, resulting in net cooling of the material17. Research in the field of solid-state laser cooling has primarily focused on rare-earth-doped materials. Semiconductors have also been investigated because of their potential to achieve lower temperatures (,20 K) and their higher cooling capacity6,18–20. However, research in this area is still at an early stage6. Optical refrigeration is distinguished from opto-mechanical cooling of microscale objects, in which Brownian motion along only one dimension is reduced by use of radiation pressure. Recently, micromechanical resonators have been cooled along the cavity axis, leading to very low ‘effective’ temperatures21. The two essential conditions for achieving net cooling in solids are (i) high external quantum efficiency (EQE) transitions and (ii) extremely high-purity materials with low parasitic loss6. The EQE (hext) describes the probability that an excited atom (ion) will emit a photon that exits the material. It is given by the ratio heWrad/(heWradþWnr), where Wrad and Wnr are the radiative and non-radiative recombination rates, respectively. The extraction efficiency he represents the fraction of photons that are not lost to total internal reflection and reabsorption and escape the material. A high EQE can be achieved with rare-earth ions in hosts such as fluoride or chloride glasses and crystals that have low phonon energy. The parasitic loss can be reduced in laser cooling materials by using high-purity starting components in an ultra-clean environment. These requirements are captured in an expression for the cooling efficiency, defined as the ratio of heat lift power to the absorbed laser power6: