Photoluminescent cooling with incoherent light

TL;DR

This paper demonstrates that incoherent light sources like LEDs and sunlight can achieve optical cooling efficiencies comparable to lasers, broadening the potential applications of optical refrigeration.

Contribution

It introduces a theoretical framework showing incoherent, unidirectional light can be used effectively for optical cooling, challenging the previous focus on coherent laser sources.

Findings

Incoherent light sources can achieve similar cooling efficiencies as lasers.

The cooling efficiency depends on wavelength, coherence, and directionality of the light.

Thermodynamics laws support optical cooling with incoherent sources.

Abstract

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.