YAl3(BO3)4:Yb&Tm a nonlinear crystal: Up- and down-conversion phenomena and excited state relaxations

Excitation and relaxation phenomena within the Yb3+–Tm3+ system in the YAl3(BO3)4 (YAB) crystal were investigated upon continuous wave and short pulse excitation at 975 and 660 nm. With the 975 nm excitation, an unusually intense luminescence from 3H4, and a considerably weaker one from 1G4 of Tm3+ have been observed at 800 (IR) and 480 nm (VIS), respectively. The intensity ratio of the IR to VIS up-converted emissions is strongly affected by dopant concentrations. The dependence of the anti-Stokes emission intensities on the excitation power implies a two-step excitation mechanism of the 3H4 luminescence and a three-step excitation of the 1G4 one. Down-converted emission from the 2F5/2 ytterbium level has been observed following the 660 nm excitation of the 3F2 thulium state. The optical and kinetics results provided that energy transfer processes are responsibly for the up- and down-conversion phenomena. Even though the Yb3+–Tm3+ energetic system is far from resonance, the energy transfer processes are surprisingly efficient in the YAB host. A high intensity of the up-converted emission at 800 nm may offer a laser potential.