Crystalline materials with suppressed impurity concentrations are essential elements for efficient solid-state laser cooling based on anti-Stokes fluorescence. So far, fluoride single crystals doped with rare earth ions have been demonstrated as efficient laser cooling media. We report on our growth activities on high purity rare-earth-doped fluoride single crystals for this specific application. We grew a variety of fluoride crystals doped with ytterbium by the Czochralski method. These crystals are studied by temperature-dependent spectroscopy to fully reveal their potential as laser cooling media. The cooling efficiency of the grown crystals is directly evaluated in a laser-induced cooling setup in vacuo.

Optical refrigeration of Yb:YLF is used to cool an arbitrary payload. An astigmatic Herriott cell enhances the total pump laser absorption by keeping the average pump intensity below the saturation while minimizing the leakage from the cavity. A spectrally-selective reflection coating mitigates the effects of amplified spontaneous emission and parasitic lasing, which limit the power scaling for temperatures <140 K. Direct cooling of the entire clamshell and shielding stray fluorescence prevents adverse heating of the crystal from its surroundings. Finally, an improved Differential Luminescence Thermometry (DLT) technique is used to measure the crystal temperature with higher accuracy and precision.

The H3 center in diamond has been shown to exhibit several promising characteristics for laser cooling applications including a neutral charge state, high radiative quantum yield, and efficient anti-Stokes photoluminescence. In this work, we show that upon excitation with a 532 nm laser, bulk diamond crystals doped with H3 centers emit efficient up-conversion photoluminescence and also show significantly reduced photothermal heating relative to crystals doped with NV centers. These results encourage future exploration of techniques for H3 enrichment in diamonds at high-pressure, high-temperature conditions for the simultaneous anti-Stokes fluorescence cooling and radiation balanced lasing in semiconductor materials.

Record laser cooling of Yb-doped silica by 18.4 K and 6.3 K was observed in vacuum and atmospheric pressure conditions, respectively. We present a detailed investigation into the optical refrigeration of ytterbium doped silica glass for both in-air and in-vacuum conditions using various pump powers. Temperature measurements were made relative to the room temperature using thermal camera imaging and differential luminescence thermometry. Through analysis of the temporal behavior of the temperature differential at the start of the in-vacuum experiments, we find the cooling efficiency of the studied silicates to be 0.66 ± 0.07%.

In this work we demonstrate laser cooling in liquids on a micrometric scale by using colloidal microparticles doped with ytterbium ions. These microparticles behave as cooling elements thanks to their anti-stokes luminescence. The novel point of our work is that laser radiation produces laser refrigeration and linear and angular transfer of momentum that allows cooling during remote particle manipulation and rotation. This dual function of laser radiation opens the door to new applications based on the use of portable micro-refrigerants. In addition, we demonstrate for the first time how analyzing rotation dynamics is a new and simpler way to determine, in real time, the magnitude of cooling. In other words, we show how laser light can heat, cool, move and rotate our particles simultaneously. All this without requiring complicated experimental systems.

Hexagonal sodium yttrium fluoride (β-NaYF) is a promising material for optical refrigeration due to the narrow crystal field splitting of the Yb(III) ion. However, growing single crystals of β-NaYF remains a challenge due to thermal expansion stresses during melt growth. We demonstrate a hydrothermal synthesis of β-NaYF with widely tunable aspect ratios that match computationally predicted cavity resonances. The β-NaYF microcrystals contain 10% Yb(III) cations and are used to build optomechanical laser-refrigeration cantilever devices. Laser refrigeration of these devices shows cooling up to 12.5°C, which is measured using the cantilever’s fundamental eigenfrequency and photoluminescence from the Yb(III) ions.

Heat engines for space applications and radiation balanced lasers need to withstand radiation damage. The lasere cooling function of Yb:YLF microcrystals after X-ray radiation was studied. F-centers stable at room temperature were induced after X-ray irradiation. Such point defects were confirmed with electron paramagnetic resonance spectroscopy and thermal luminescence. The cooling performance of the YLF microcrystals deteriorated after irradiation, which is caused by F-center induced increased background absorption and/or higher non-radiative relaxation rate. The absorption spectrum and Yb3+ excited state lifetime were measured to investigate the failure of optical refrigeration in X-ray irradiated Yb:YLF microcrystals.