Laser cooling by collisional redistribution of radiation

The general idea that optical radiation may cool matter was put forward 80 years ago(1). Doppler cooling of dilute atomic gases is an extremely successful application of this concept(2,3). More recently, anti-Stokes cooling in multilevel systems has been explored(4,5), culminating in the optical refrigeration of solids(6-9). Collisional redistribution of radiation has been proposed(10) as a different cooling mechanism for atomic two-level systems, although experimental investigations using moderate-density gases have not reached the cooling regime(11). Here we experimentally demonstrate laser cooling of an atomic gas based on collisional redistribution of radiation, using rubidium atoms in argon buffer gas at a pressure of 230 bar. The frequent collisions in the ultradense gas transiently shift a highly red-detuned laser beam(that is, one detuned to a much lower frequency) into resonance, whereas spontaneous decay occurs close to the unperturbed atomic resonance frequency. During each excitation cycle, kinetic energy of order k(B)T-that is, the thermal energy (k(B), Boltzmann's constant; T, temperature)-is extracted from the dense atomic sample. In a proof-of-principle experiment with a thermally non-isolated sample, we demonstrate relative cooling by 66 K. The cooled gas has a density more than ten orders of magnitude greater than the typical values used in Doppler-cooling experiments, and the cooling power reaches 87 mW. Future applications of the technique may include supercooling beyond the homogeneous nucleation temperature(12,13) and optical chillers(9).