Enhanced ultraviolet upconversion in YF3:Yb3+/Tm3+ nanocrystals

Unusual intense infrared-to-ultraviolet upconversion luminescence was observed in YF3:Yb3+(20%)/Tm3+(1%) nanocrystals under 980 nm excitation. The intense ultraviolet emissions (I-1(6)-> H-3(6), I-1(6)-> F-3(4), and D-1(2)-> H-3(6)) were affirmed arising from the excitation processes of five-photon and four-photon. In comparison with the bulk sample with the same chemical compositions, ultraviolet upconversion luminescence of the nanocrystals was markedly enhanced. Spectral analysis indicated that the enhancement was attributed to the decrease of Judd-Ofelt parameter Omega(2), which precluded the transition rate from F-3(2) to F-3(4), enhanced the energy transfer process and populated the D-1(2) level: F-3(2)-> H-3(6) (Tm3+): H-3(4)-> D-1(2) (Tm3+).