Laser cooling in solids: Demonstration of 115K all-solid-state cryocooler

Optical refrigeration is based on anti-Stokes fluorescence, a process in which a material absorbs light near one frequency and then fluoresces at a higher mean frequency. In a solid this process removes phonons to furnish the energy difference between the absorbed and emitted light and thereby cools the material. The best optical refrigeration results have been achieved by using transparent solids doped with rare earth ions such as ytterbium, thulium and erbium. The cooling cycle is illustrated in Fig. 1, which shows the ground state and first excited state energy manifolds of a dopant ion. To produce cooling, laser light tuned to the energy difference between the top of the lower manifold and the bottom of the upper manifold pumps the cooling element and excites ions to the upper manifold. The excited ions absorb thermal energy from the solid thereby raising their excitations to somewhere in the middle of the upper manifold. These ions then radiatively decay to the middle of the lower-energy manifold, emitting photons with mean energies greater than that of the pump radiation. The cycle concludes when the ions in the ground state manifold absorb additionally lattice energy to re-establish a thermal distribution of excitations.