Frequency upconversion and its new mechanism in Tm-doped fluoroaluminate glasses

Upconversion fluorescences have been observed in the UV (363 nm) and the blue region (451 and 476 nm) in Tm³⁺-doped fluoroaluminate glass by pumping with a DCM dye laser. The two-photon absorption process of upconversion fluorescences was confirmed by the quadratic dependence of the emission intensity on incident pumping power. The excitation spectra for upconversion fluorescences show that the mechanism of upconversion is a simple ESA (excited state absorption) process. In addition to ESA, it was found that the energy transfer by cross-relaxation (³H₄, ³H₆ → ³F₄, ³F₄) is also involved in the mechanism of the 476 nm blue upconversion fluorescence which differs from that of the UV and the 451 nm blue upconversion fluorescence. The difference was confirmed by measuring both the radiative and the non-radiative decay rate of the ³H₄ level.