Visible up-conversion luminescence in Er³⁺:BaTiO₃ nanocrystals

Visible emissions at 548, 528 and 660 nm have been observed from the sol-gel derived nanocrystalline Er³⁺:BaTiO₃ powders under infrared excitations at both 980 and 810 nm. Combined with the energy level structure of Er³⁺ and the decay kinetics of the green emissions, the up-conversion mechanism has been analyzed and explained. The bright green emission at 548 nm has been attributed to the ground state-directed transition from ⁴S_3/2, which is populated through excited state absorption (ESA) for 810 nm excitation, and through both ESA and energy transfer (ET) processes for 80 nm excitation.