Toward Low‐Power Laser Refrigeration ‐ Cooling a Resonant WGM Crystalline Cavity

ABSTRACT We demonstrate laser refrigeration of a millimeter‐scale crystalline whispering‐gallery‐mode (WGM) resonator fabricated from 10% :YLF, operating at atmospheric pressure. Under resonant pumping at 1031 nm, the resonator exhibits a measurable temperature reduction of K below room temperature with an input power of only 100 mW, corresponding to a temperature drop rate of about 4.8 K/W. Although the present undercoupled configuration yields an effective cavity enhancement factor close to unity, these results represent the experimental demonstration of optical refrigeration in a crystalline WGM resonator. This work establishes crystalline WGMs as a viable platform for thermally stable rare‐earth doped photonic devices, and with optimized coupling conditions, significantly deeper cooling and improved performance are expected for applications in precision photonics and quantum technologies.