Solid-state Laser Cooling

TL;DR

The primer surveys the field of solid-state laser cooling, detailing the physical principles, materials, and measurement strategies that enable photoluminescence-based refrigeration. It synthesizes milestones in RE3+-doped glasses and crystals and in semiconductors, clarifying the conditions under which net cooling is achieved and how gMAT and MAT govern ultimate temperatures. The work also outlines practical applications—from optical cryocoolers and radiation-balanced lasers to metrology and quantum sensing—while emphasizing reproducibility through reporting standards and data deposition. By highlighting remaining challenges, including material purity, background absorption, and scaling cooling power, the paper signals a path toward practical, vibration-free cooling technologies with broad scientific and technological impact. Overall, the primer serves as a rigorous, tutorial reference to unify methodology, enable cross-lab comparability, and guide future material discovery and device integration in solid-state laser cooling.

Abstract

Since the first proof-of-concept demonstrations of photoluminescence-based optical refrigeration, solid-state laser cooling has developed into a credible competitor to conventional cryogenic technologies. Solid-state laser cooling continues to advance as new materials push cooling limits. These developments have created a need to consolidate progress made to date as well as standardize critical experimental considerations needed for reliable and verifiable cooling measurements. This primer therefore outlines essential concepts and requirements, which underpin solid-state laser cooling. The primer summarizes key milestones achieved with cooling-grade, rare-earth-doped glasses and crystals as well as with semiconductors. It additionally highlights emerging applications of solid-state optical refrigeration. To strengthen the consistency and reproducibility of cooling results going forward, two reporting checklists are introduced. They cover materials, cooling metrics, and thermometry. This primer is intended to serve as both a tutorial and a practical reference for incoming and existing researchers involved in solid-state laser-cooling.

Figures (4)

• Figure 1: Schematic of laser cooling cycles in RE3+-doped glasses/crystals (left) and semiconductors (right).
• Figure 2: Crystal-field energies (in wavenumbers relative to the crystal-field ground state, not to scale) for the ground and first excited state multiplets of Yb3+demirbas2021detailedpuschel2021temperature, Er3+gruber2006modeling, Tm3+klimin2010high, Ho3+walsh2004spectroscopy, and Dy3+rana1988optical in crystal hosts relevant to solid-state laser cooling. Energies in parenthesis are calculated. The arrows show $E_\textrm{exc}$ for the lowest-energy crystal-field excitations.
• Figure 3: (a) Photograph of a cryocooler. The cooling crystal (YLF:Yb3+ 10%) is attached to a copper cold finger (CF) via a textured MgF2 thermal link. Note that the clamshell has been removed for this photograph. (b) Measured temperature of YLF (via DLT) and CF (via silicon diode) during payload cooling to 125 K.kock2022ol
• Figure 4: Simulated (a) $\eta_\textrm{c}$ and (b) $\eta_\textrm{abs}$ for CsPbBr$_3$ nanocrystals ($\eta_\textrm{EQE}=0.991$, $\alpha_\textrm{b}=3\times10^{-4}$ cm$^\textrm{-1}$). Indicated are cooling and heating regimes as well as corresponding transition points.