Enhancing the optical performance of oxyfluoride glass ceramics by optimizing the oxide: Fluoride ratio and crystallinity for optical refrigeration

The optimization of the oxide and fluoride content, crystallinity and rare earth ion concentration in oxyfluoride glass ceramics (GCs) are of great importance in obtaining high photoluminescence quantum yield (PLQY) for optical refrigeration applications. Presented herein are the important advancements in the development of a novel oxyfluoride GCS of the composition (SiO₂-Al₂O₃)_(100-a) (YLiF₄)_b: (YbF₃)_b (a = 35 and 40; in mol %, b = 1 and 2 mol%) with the corresponding parent glasses with an in-depth investigation on enhancing the optical performance for laser cooling. Depending on the oxide/fluoride (O/F) ratio and ytterbium content the internal quantum yield (iQY) varied between 70 and 99% in glass ceramics at several excitation wavelengths. The optical properties of GCs containing YLiF₄ and YF₃ nanocrystals obtained from the same initial composition (modulated by time and fusion temperature) were compared to find the optimal composition for optical refrigeration. Low fluorine content led to the generation of YLiF₄ as a major phase after ceramization and high fluorine content helped in the generation of the YF₃ phase. An increase in the radiative lifetime of YF₃ GCs compared to YLiF₄ GCs has been found to coincide with the enhancement of the PLQY, which is beneficial for laser cooling. The temperature change (ΔT) change measured using a fiber Bragg grating (FBG) in the glass and glass-ceramic samples with different pump wavelengths showed significant heat mitigation near ∼1030 nm. The observed enhanced PL intensity, iQY and lifetime after purification of YLiF₄ glasses imply that the purity of the material plays a paramount role in lowering the background absorption and enhancing the quantum yield. Looking ahead, we see a bright future for oxyfluoride GCs in applications requiring the ultimate levels of thermal, mechanical and optical performance, especially for the development of cryocooler devices, which are still technologically challenging and expensive. The usage of GCs will open up new possibilities in optical cooling technology, enabling cooling devices of any size and shape.