Principles for demonstrating condensed phase optical refrigeration

Abstract

Optical refrigeration, or condensed phase laser cooling, uses lasers to remove thermal energy from solids through anti-Stokes photoluminescence. This non-contact, vibration-free, optically addressable cooling technique opens up many application possibilities, ranging from high-resolution space-based imaging to the stabilization of ultraprecise frequency combs. The field has seen rapid progress in the past 25 years, from the first cooling of a rare-earth-doped glass by 0.3 K in 1995 to reaching cryogenic temperatures around 90 K in ytterbium-doped fluoride crystals in 2018. Attention has now shifted to semiconductors with higher cooling power densities and predicted cooling floors as low as 10 K. This has stimulated a race to demonstrate the optical refrigeration of a semiconductor. It is therefore timely to systematize the necessary and sufficient experimental minimum criteria for reporting optical refrigeration results to elevate the reliability and reproducibility of current and future optical refrigeration claims. In this Expert Recommendation, we propose four principles and provide guidelines for verifying and reporting new cooling results: optical cooling metrics, demonstrations of explicit heating versus cooling, thermodynamic consistency and reliable temperature measurements. We further propose that these principles serve as a guide for reviewing literature claims in the field. Optical refrigeration of semiconductors has the potential to reach temperatures as low as 10 K for applications in non-contact cooling and high-precision metrology. This Expert Recommendation outlines four criteria for the standardized reporting of new cooling results towards these goals.