Increasing the Fluorescence Efficiency of Ti$${}^{\mathbf{3+}}:\mathbf{Al}_{\mathbf{2}}$$O$${}_{\mathbf{3}}$$ Crystal under Cryogenic Cooling and Diode Pumping at 454 and 507 nm

In this study, the temperature dependence of the fluorescence of a Ti $${}^{3+}:\textrm{Al}_{2}$$ O $${}_{3}$$ crystal excited by radiation with wavelengths of 507 and 454 nm was measured. The fluorescence quantum yield was determined, exceeding 100 $$\%$$ and reaching 113 $$\%$$ in the cryogenic temperature range when the crystal was excited by 454 nm radiation.