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Solid-state laser refrigeration of semiconductors remains an outstanding experimental challenge. In this work, we show that, following excitation with a laser wavelength of 532 nm, bulk diamond crystals doped with H3 centers both emit efficient up-conversion (anti-Stokes) photoluminescence and also show significantly reduced photothermal heating relative to crystals doped with nitrogen–vacancy (NV) centers. The H3 center in diamond is a highly photostable defect that avoids bleaching at high laser irradiances of 10–70 MW/cm[Formula: see text] and has been shown to exhibit laser action, tunable over the visible band of 500–600 nm. The observed reduction of photothermal heating arises due to a decrease in the concentration of absorbing point defects, including NV-centers. These results encourage future exploration of techniques for H3 enrichment in diamonds under high-pressure, high-temperature conditions for the simultaneous anti-Stokes fluorescence cooling and radiation balanced lasing in semiconductor materials. Reducing photothermal heating in diamond through the formation of H3 centers also opens up new possibilities in quantum sensing via optically detected magnetic resonance spectroscopy at ambient conditions. 

Rare earth doped lithium fluorides are a class of materials with a wide variety of optical applications, but the hazardous reagents used in their synthesis often restrict the amount of product that can be created at one time. In this work, 10%Yb3+:LiLuF4 (Yb:LLF) crystals have been synthesized through a safe and scalable polyethylene glycol (PEG)-assisted hydrothermal method. A combination of X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and photoluminescence (PL) measurements were used to characterize the obtained materials. The influence of reaction temperature, time, fluoride source, and precursor amount on the shape and size of the Yb:LLF crystals are also discussed. Calibrated PL spectra of Yb3+ ions show laser cooling to more than 15 K below room temperature in air and 5 K in deionized water under 1020 nm diode laser excitation measured at a laser power of 50 mW. 

Abstract Herein, the role that point defects have played over the last two decades in realizing solid‐state laser refrigeration is discussed. A brief introduction to the field of solid‐state laser refrigeration is given with an emphasis on the fundamental physical phenomena and quantized electronic transitions that have made solid‐state laser‐cooling possible. Lanthanide‐based point defects, such as trivalent ytterbium ions (Yb³⁺), have played a central role in the first demonstrations and subsequent development of advanced materials for solid‐state laser refrigeration. Significant discussion is devoted to the quantum mechanical description of optical transitions in lanthanide ions, and their influence on laser cooling. Transition‐metal point defects have been shown to generate substantial background absorption in ceramic materials, decreasing the overall efficiency of a particular laser refrigeration material. Other potential color centers based on fluoride vacancies with multiple potential charge states are also considered. In conclusion, novel materials for solid‐state laser refrigeration, including color centers in diamond that have recently been proposed to realize the solid‐state laser refrigeration of semiconducting materials, are discussed.