Development of Yb³⁺-doped oxyfluoride glass-ceramics with low OH⁻content containing CaF₂nanocrystals for optical refrigeration

Oxyfluoride glasses and glass-ceramics (GCs) have some niche advantages over other oxide and fluoride glasses, as they possess combined properties. This paper reports the structural, thermal, and photoluminescence (PL) properties of $Yb3+$-doped $SiO2−Al2O3−CaO−CaF2$ oxyfluoride glasses and transparent GCs containing $CaF2$ nanocrystals. Special efforts were undertaken to minimize the hydroxyl ($OH−$) content in the prepared samples to improve their optical features. Differential scanning calorimetry analyses were performed to determine the characteristic temperatures of the base glasses. X-ray diffractometry studies have confirmed the fluorite $CaF2$ nanocrystals to be 10 nm in size. Reduced transparency in the ultraviolet (UV)–visible to the near-infrared (NIR) regions was observed for the GCs compared to the base glass with increasing thermal treatment temperature. A higher PL intensity upon 920-nm excitation was obtained in the GCs compared to that of the base glass. The absolute photoluminescence quantum yield upon 920-nm laser excitation was evaluated using an integrating sphere and an optical spectrum analyzer. It was observed that the lifetime of the $F5/22$ level of the $Yb3+$ ions decreases with increasing ceramization temperature. The potential advantages of using such oxyfluoride GCs over commonly studied single crystals for laser cooling applications are discussed.