Prospects of UV and blue laser emission from the 5D3 level in Tb3+:LiYF4

Abstract

We present room-temperature spectroscopy of Tb³⁺:YLF crystals with different doping concentrations between 0.2 % and 15 % evaluating the potential for laser operation at 415 nm or 435 nm starting from the ⁵D₃ level of Tb³⁺. Evaluation of the fluorescence decay curves by the Inokuti-Hirayama model yields a radiative lifetime of the ⁵D₃ level of 2.53 ms. The shorter effective lifetime of a Tb³⁺(0.2 %):YLF sample of 2.1 ms indicates quenching of the ⁵D₃ level by cross relaxation even at such low Tb³⁺ doping concentrations. The ratio of the ⁵D₃ and ⁵D₄ fluorescence yields a branching ratio of the ⁵D₃ → ⁵D₄ transition of 0.45 in Tb³⁺:YLF, which allows to calculate the stimulated emission cross sections for the potential UV and blue laser transitions ⁵D₃ → ⁷F_J to be on the order of 10⁻²¹ cm² for the first time. In pump-probe measurements, we observe gain around 415 nm and 435 nm for an Tb³⁺(0.8 %):YLF sample, but no detrimental excited state absorption. However, room-temperature lasing is not achieved for up to 0.87 W of incident cw UV pump power. Our calculations find a Tb³⁺ doping concentration of 1.7 % to yield an optimized product of doping ion density and quantum efficiency, which maximizes the possible single-pass gain to 1.4 % assuming typical laser parameters. This study thus paves the way for future UV to blue lasing of Tb³⁺ starting from the ⁵D₃ level with further optimized conditions.