  Optical refrigeration using anti-Stokes photoluminescence is now well
established, especially for rare-earth-doped solids where cooling to cryogenic
temperatures has recently been achieved. The cooling efficiency of optical
refrigeration is constrained by the requirement that the increase in entropy of
the photon field must be greater than the decrease in entropy of the sample.
Laser radiation has been used in all demonstrated cases of optical
refrigeration with the intention of minimizing the entropy of the absorbed
photons. Here, we show that as long as the incident radiation is
unidirectional, the loss of coherence does not significantly affect the cooling
efficiency. Using a general formulation of radiation entropy as the von Neumann
entropy of the photon field, we show how the cooling efficiency depends on the
properties of the light source such as wavelength, coherence, and
directionality. Our results suggest that the laws of thermodynamics permit
optical cooling of materials with incoherent sources such as light emitting
diodes and filtered sunlight almost as efficiently as with lasers. Our findings
have significant and immediate implications for design of compact,
all-solid-state devices cooled via optical refrigeration.
