Red and green upconversion luminescence in Eu³⁺ and Tb³⁺ doped high-silica glass via multiphoton absorption of 800 nm femtosecond laser irradiation

We report red and green upconversion luminescence in high-silica luminous glass under irradiation by an infrared femtosecond laser. The luminous glasses were fabricated from sintered porous glass impregnated with the rare earth ion Eu³⁺ or Tb³⁺. The luminescent spectra under femtosecond laser excitation reveal that the red and green upconversion luminescence results mainly from the transitions ⁵D₀ → ⁷F₂ in Eu³⁺ and ⁵D₄ → ⁷F₅ in Tb³⁺. The dependence of the fluorescence intensity on the pump power indicates that the conversion of infrared radiation to visible emission is dominated by two- and three-photon excitation processes, respectively. It is suggested that the upconversion mechanism is simultaneous absorption of two or three infrared photons, which shifts the Eu³⁺ and Tb³⁺ ions to their excited states; they quickly cascade down to the lowest excited states. Thereafter, the Eu³⁺ and Tb³⁺ ions relax radiatively to their ground states, creating bright red and green fluorescence. (paper)