Ultraviolet emissions from Gd3+ ions excited by energy transfer from Ho3+ ions

Ultraviolet (UV) upconversion (UC) emissions of Gd3+ ion were investigated in Y1.838−xGdxYb0.16Ho0.002O3 (x=0, 0.16, 0.4, 1, 1.4) bulk ceramics under 976 nm laser diode (LD) excitation. The UC emissions centered at 309 and 315 nm are assigned to the transition of 6P5/2→8S7/2 (Gd) and 6P7/2→8S7/2 (Gd). The 6PJ levels of Gd3+ ions are populated by an energy transfer (ET) process from 8S7/2 (Gd)+(3P1, 3L8, 3M10) (Ho)→6PJ (Gd)+5I8 (Ho). A four-photon ET UC process was confirmed by the dependence of the 6P7/2 level emission intensity on the pumping power. We found that the intensity of the UC emissions increased with Gd3+ ion concentration and peaked at 8 mol%, then starts to decrease until the Gd3+ ion concentration reached 70 mol%. The variation in the UV emission intensity is the result of the competition between the ET process and concentration quenching effect. Theoretical calculations based on steady-state equations validated the proposed UC mechanisms.