We report a blue emission upconversion solid-state laser based on Tm3+:YLF. Under double resonance excitation at 780.8 nm (near-ir) and 648.8 nm (red), the Tm3+ ion is sequentially excited from the 3H6 ground state to the 1D2 excited state through the 3H4 intermediate level. The laser output at 450 and 453 nm corresponds to the 1D2 -> 3F4 transitions of Tm3+ ions in YLF.
Dinh C. Nguyen,
George E. Faulkner,
Michael E. Weber,
"Blue upconversion thulium laser", Proc. SPIE 1223, Solid State Lasers, (1 April 1990); doi: 10.1117/12.18393; https://doi.org/10.1117/12.18393