Near vacuum ultraviolet luminescence of Gd³⁺ and Er³⁺ ions generated by super saturation upconversion processes

Near vacuum ultraviolet (UV) upconversion (UC) emissions with a spectral resolution of 1 nm, from the ⁶G_J, ⁶D _J, ⁶I_J, ⁶P_J levels of Gd³⁺ and the ²L_17/2, ⁴D _7/2, ²H(2)_9/2, ²D_5/2, ⁴G_7/2, ²K_13/2, ²P _3/2 levels of Er³⁺, were observed under 974 nm laser excitation. Mechanism analyses illustrate that successive energy transfers (ETs) from Yb³⁺ to Er³⁺ generate UV UC radiations in Er ³⁺, while two resonant ETs from Er³⁺ to Gd³⁺ lead to UV UC radiations in Gd³⁺. Power dependence analyses indicate that the expected inefficient four- and five-photon processes have been switched into efficient two-photon processes due to a super saturation UC phenomenon that employs consecutive saturations at the intermediate states.