[Tm3+ concentration dependence of upconversion intensity of YLiF4: Er3+, Tm3+, Yb3+].

A series of YLiF4: Er3+ , Tm3+ , Yb3+ samples were synthesized by hydrothermal method. The concentrations of Er+ and Yb3+ were 1 mol% and 1.5 mol%, respectively. Tm3+ concentration changed from 2 to 8 mol%. The absorption of Er3+, Tm3+ and Yb3+ were observed, and the absorption of Tm3+ increased along with the increase in Tm3+ concentration. The color of upconversion luminescence of YLiF4: Er3+, Tm3+ and Yb3+ excited by 980 nm was white when Tm3+ concentration was low. The blue light resulted from the transition of 1G4-->3H6 of Tm3+, the green light from the transition of 4S(3/2)(2H(11/2))-->4I(15/2) of Er3+, and the red light from the transitions 1G4-->3F4 of Tm3+ and 4F(9/2)-->4I(15/2) of Er3+. All the upconversion intensities decreased when Tm3+ concentration increased, but the relative intensity decrease was different due to the interaction between Er3+ and Tm3+.

[Upconversion luminescence properties of YLiF4:Er, Tm, Yb synthesized by hydrothermal method].

YLiF4 codoped with Er3+, Tm3+ and Yb3+ ions was synthesized by hydrothermal method. The absorption spectra and upconversion spectra excited at 980 nm were investigated. Under the excitation of infrared photons, strong white emission can be obtained. According to the value of n obtained from the power law, the 484 nm blue emission, 524 and 552 nm green emission, and 665 nm red emission are all due to two photon process, while the UV emission is attributed to three photon process. The authors also analyzed the upconversion mechanism and process.

[Donor concentration dependence of upconversion luminescence in YLiF4: Er3+, Yb3+].

YLiF4 :Er3+, Yb3+ was synthesized by hydrothermal method. The concentration of Er3+ was 2 mol%, and Yb3+ changed from 0 to 7 mol%. At room temperature the absorbance of YLiF4: Er3+, Yb3+ in all samples was measured from 200 to 1 200 nm. The absorbance around 980 nm increased with increasing Yb3+ concentration. The result shows that the upconversion intensity excited at 980 nm was enhanced. The enhanced intensity is dependent on the Yb3+ concentration. The excitation spectra and photoluminescence spectra were measured in the sample with Yb3+ concentration of 2 mol%. The red and green luminescence came from the transition 4F(9/2) --> 2 I(15/2), and 4 S(3/2) -->2 I(15/2) and 2H(11/2) --> 2 I(15/2), respectively.