Enhancement of violet and ultraviolet upconversion emissions in Yb3+/Er3+-codoped YF3 nanocrystals

Pumped with a 980-nm diode laser, violet/ultraviolet upconversion fluorescence was presented in Y0.83Yb0.15Er0.02F3 nanocrystals. Observed emissions at 318 nm and 379 nm were affirmed coming from a four-photon excitation process. In comparison with a bulk sample having the same chemical compositions, the nanocrystals had a markedly enhanced ability of emitting violet/ultraviolet upconversion fluorescence. By employing Tm3+ ions as structural probes in the samples, we found that the enhancement could be attributed to the decrease of Judd–Ofelt parameter Ω2. A model for revealing the four-photon excitation process was proposed based on spectral analysis.