Energy transfer and up-conversion in Bi2TeO5 crystals co-doped with Yb3+ and Tm3+

Yb + Tm co-doped Bi2TeO5 single crystals were grown by the Czochralski method. Optical absorption and luminescence spectra of the crystals were determined and interpreted. The dynamics of the Tm and Yb excited states were followed by luminescence decay curves. Up-conversion processes were detected after 980 nm diode laser excitation, that resulted in an 800 nm and a weaker 475 nm emission corresponding to the 3H4 → 3H6 and 1G4 → 3H6 transitions of Tm3+. The excitation power dependence of the integrated intensity of the up-converted emission implied two-step excitation process including Yb–Tm energy transfer for the 800 nm emission, and three-photon excitation process for the 475 nm up-converted emission. Examination of absorption and emission spectra of Yb and Tm ions revealed that the consecutive energy transfer steps did not fulfill the resonance conditions, and they have to be assisted by phonon emission.