Intense ultraviolet upconversion luminescence from hexagonal NaYF4: Yb3+/Tm3+ microcrystals

Under 980 nm excitation, unusual P-3(2) -> H-3(6) (similar to 264 nm) and P-3(2) -> F-3(4) (similar to 309 nm) emissions from Tm3+ ions were observed in hexagonal NaYF4: Yb3+(20%)/Tm3+(1.5%) microcrystals. In comparison with the strong emissions from D-1(2) and I-1(6), the emissions from (1)G(4) and H-3(4) almost vanished due to the efficient cross-relaxation of (1)G(4) + H-3(4) -> F-3(4) + D-1(2) (Tm3+). Double logarithmic plots of the upconversion emission intensity versus the excitation power are neither straight lines nor typical saturation curves. Theoretical analysis indicated that the complicated dependent relationships were mainly caused by phonon-assisted energy transfers and nonradiative relaxation. (C) 2008 Optical Society of America.