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

Ultraviolet (UV) upconversion (UC) emissions of Gd³ ion were investigated in Y_1.838-xGd_xYb_0.16Ho _0.002O₃ (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 ⁶P_5/2→ ⁸S_7/2 (Gd) and ⁶P_7/2→ ⁸S_7/2 (Gd). The ⁶P_J levels of Gd³ ions are populated by an energy transfer (ET) process from ⁸S_7/2 (Gd)(³P₁, ³L ₈, ³M₁₀) (Ho)→⁶P_J (Gd)⁵I₈ (Ho). A four-photon ET UC process was confirmed by the dependence of the ⁶P_7/2 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.