Growth and characterization of rare earth doped sesquioxides crystals

This work focuses on the growth, spectroscopic, and laser characterizations of rare-earth sesquioxides, which are in high demand as host materials for high-power lasers. However, their high melting points present considerable challenges for achieving high-quality crystal fabrication. Rare-earth-doped Lu2O3 crystals were grown using the optical floating zone method, though the strong thermal gradients during growth introduced significant challenges. To mitigate these issues, detailed thermal investigations of the ternary system Lu2O3-Sc2O3-Y2O3 were conducted, revealing a broad compositional range with significantly reduced melting points. For the first time, Er3+- and Yb3+-doped mixed sesquioxides were successfully grown using optical floating zone, Czochralski, and micro-pulling-down methods. The resulting Er3+ and Yb3+- doped (YzSc(1-z))2O3 crystals (hereafter also referred to as YScO3) were successfully grown and systematically characterized with respect to their crystallographic properties, as well as their composition, thermal conductivity, and optical absorption in the 1µm wavelength range. All grown materials were thoroughly investigated for their spectroscopic properties, including absorption and emission cross-sections along with fluorescence lifetimes. Compared to pure sesquioxides, the mixed crystals exhibited broader absorption cross-sections due to inhomogeneous line broadening caused by the disordered crystal structures resulting from the statistical distribution of cations. In addition, the spectroscopic properties of Er3+-doped Y2O3, Lu2O3, and Sc2O3 were revised, and the Judd-Ofelt theory was employed to calculate transition probabilities based on absorption cross-section measurements. Stimulated-emission cross-sections for transitions at ~2.8µm and ~1.6µm were determined, and the temperature-dependent luminescence dynamics of both laser levels were analyzed. Laser operation was demonstrated using an Er3+:YScO3 crystal grown by the Czochralski method. Optically pumped at a wavelength of 970.5nm, the crystal produced a maximum laser output of 0.44W at 2715nm, with a slope efficiency of 19%. In experiments with Yb3+:YScO3 crystals grown by the same method, a laser output of 1.3W at ~1040nm was achieved with a slope efficiency of 87% when pumped at 975.5nm. Comparable performance was observed with a micro-pulling-down-grown Yb3+-doped sample, which delivered a maximum output power of 1.21W at laser wavelengths near 1040nm with a slope efficiency of 89% and an optical-to-optical efficiency of 85%. These results demonstrate a good laser performance of Yb3+- and Er3+-doped mixed sesquioxides in the 1µm and 3µm wavelength ranges, respectively. Notably, Yb3+-doped (YzSc(1-z))2O3 demonstrates significant potential for efficient ultrafast 1µm lasers, achieving performance comparable to other state-of-the-art Yb-doped sesquioxides.