Intense photoluminescence across wavelengths from ultraviolet to NIR-II in Yb3+/Er3+ co-doped cubic HfO2 single crystals with ultra-high melting point excited at 980 nm

Growth of laser crystal with ultra-high melting-point, high laser damage threshold and highly efficient photoluminescence is a formidable challenge. In present work, the transparent Er 3+ /Yb 3+ co-doped cubic HfO 2 single crystals with an ultra-high melting point (∼2900 °C) have been successfully grown by the optical floating zone (OFZ) technique, and the exceptional physical and optical properties of the grown YbSH (ytterbia stabilized hafnia), LuSH (lutecia stabilized hafnia), YbSH:Er 0.5 and LuSH:Er 0.5 crystals have been investigated for the first time. Under 980 nm excitation, the YbSH:Er 0.5 crystal exhibits emission peaks across wavelengths from ultraviolet to NIR-II. Notably, a high-intensity ultraviolet emission peak at 387 nm is observed, attributed to a 3-photon up-conversion process, demonstrating the crystal's potential as an ultraviolet laser material. Additionally, a stronger emission band centered at 1550 nm within the Near-Infrared (NIR-II) window is also observed through a down-conversion process, highlighting the crystal's promise as a promising candidate for NIR-II laser applications in telecommunications. For YbSH:Er 0.5 crystal, the intensities of the emission peaks at 387 and 410 nm increase as the temperature is decreased from 500 to 80 K, which can be attributed to the reduction in phonon energy with temperature. Conversely, the intensities of the emission peaks at 545 and 680 nm increase with rising temperature, likely due to the rise in phonon energy and the thermal assistance optically stimulated luminescence effect. The fluorescence intensity ratio ( I 680 / I 410 ) of Er 3+ in the YbSH:Er 0.5 crystal demonstrates excellent temperature sensing property within a temperature range of 80–500 K.