Solid-state optical refrigeration to sub-100 Kelvin regime

Abstract

Since the first demonstration of net cooling twenty years ago, optical refrigeration of solids has progressed to outperform all other solid-state cooling processes. It has become the first and only solid-state refrigerator capable of reaching cryogenic temperatures, and now the first solid-state cooling below 100 K. Such substantial progress required a multi-disciplinary approach of pump laser absorption enhancement, material characterization and purification, and thermal management. Here we present the culmination of two decades of progress, the record cooling to ≈ 91 K from room temperature.

First demonstration of an all-solid-state optical cryocooler

“…We used the YLF:10%Yb laser-cooling crystal that was cooled to 91 K in a previous study 10 . This  crystal was cut so that the linearly polarized pump laser has because the absorption cross section of the E4→E5 crystal-field transition in a YLF:Yb crystal at 1020 nm is ~1.7× greater in this geometry than that with polarization 12 .…”

“…However, laser cooling of Yb 3+ -doped solids to cryogenic temperatures is only practical if η ext η abs exceeds 0.98–0.99, a challenging requirement that necessitates a cooling crystal with exquisite purity 9 . High-purity YLiF 4 crystals doped with 10 mol% Yb 3+ (YLF:10%Yb) have recently been cooled to 91 K by exciting them with a laser tuned to the E4→E5 crystal-field transition of Yb 3+ at 1020 nm 10 .…”

Photothermal Heating and Cooling of Nanostructures

“…In particular, they reported that Yb 3+ :YLF bulk crystals can be cooled in vacuum to 91 K from room temperature with tuning the excitation of the crystal to Yb 3+ E 4 ‐E 5 resonance at 1020 nm, shown in Figure . The energy levels of Yb 3+ are shown in Figure b …”

“…(Inset) Illustration of the cooling cavity, which enables multiple passes of the cooling laser through the crystal. (b) The crystal temperature (blue) reaches 91 K after ≈ 12 minutes of pumping while the clamshell temperature is maintained at ≈ 265 K. Figure reproduced with permission from reference copyright Nature Publishing Group.…”

“…Solid state laser refrigeration may also lead to cooling to temperatures below 100 K, beyond what is possible with thermoelectric coolers (175 K). This opens up applications for solid state laser refrigeration in cooling laser reference cavities used in the detection of gravitational waves, gamma‐ray detectors, and high‐temperature superconductors …”