Sensitized holmium upconversion emission in LiNbO3 triply doped with Ho3+, Yb3+, and Nd3+

Observed is 15 times enhancement of Ho3+ green upconversion (UC) emission in Ho3+(0.75 mol %)/Nd3+(0.75 mol %) doped lithium niobate crystal after incorporating with 4 mol % of Yb3+ under 800 nm ferntosecond laser excitation. Yb3+ acting as bridging ion increases the energy transfer efficiency from Nd3+ F-4(3/2) level to Ho3+ in these triply doped systems. The nonradiative energy transfer efficiency from Nd3+ F-4(3/2) to Yd(3+) F-2(5/2) level can reach to 56%. The UC emission mechanisms are proposed based on spectral, kinetic, and pump power dependence analyses. The green UC emission is accomplished through a multi-ion interaction involving ground state absorption by the Nd3+ (I-4(9/2) -> H-2(9/2), F-4(5/2)) followed by three successive energy transfer processes involving one Nd3+-Yb3+ (F-4(3/2) + F-2(7/2) -> I-4(9/2) + F-2(5/2)) and two Yb3+-Ho3+ pairs (F-2(5/2) + I-5(8) -> F-2(7/2) +I-5(6) and F-2(5/2)+ I-5(6) -> F-2(7/2) + S-5(2), F-5(4)); whereas the red is dominated by excited state absorption of the Ho3+ (I-5(7) -> F-5(5)) following two successive energy transfer processes involving the Nd3+-Yb3+ (F-4(3/2) + F-2(7/2) -> I-4(9/2) + F-2(5/2)) and Yb3+-Ho3+ pairs (F-2(5/2) + I-5(8) -> F-2(7/2) + I-5(6)) followed by the Ho3+ nonradiative relaxation to I-5(7). (C) 2008 American Institute of Physics. [DOI:.10.1063/1.2982396]