Upconversion luminescence of YAl3(BO3)4:(Yb3+,Tm3+) crystals

Single crystals of yttrium aluminum borate co-doped with 5% thulium and 10–20% ytterbium demonstrate upconversion luminescence both in the red and in the blue. All spectral components of the upconversion luminescence originate from 1G4 thulium ion state. A cooperative upconversion mechanism with a nearly quadratic slope of the power dependence dominates in the studied pump power density range. The excitation spectra are asymmetric due to different contributions from the crystal field split components of ytterbium 7F7/2–7F5/2 transition. Optimal ytterbium doping for efficient upconversion is below 20%.