Excited-State Dynamics of Er³⁺ in Gd₂O₃ Nanocrystals

The temperature-dependent lifetime of the green emission from Er³⁺ ions in 40−50 nm Gd₂O₃ nanocrystals has been measured and analyzed. The luminescence decay from the ⁴S_3/2 state of Er³⁺ shows nonexponential characteristics when excited directly to the thermally coupled ²H_11/2 from 10 to 300 K, indicating energy transfer to defects in the nanocrystals. The intrinsic lifetime of the ⁴S_3/2 state was determined by utilizing the efficient energy transfer from Gd³⁺ to Er³⁺. It is shown that, on the basis of a simplified thermalization model, multiphonon nonradiative relaxation from ⁴S_3/2 to ⁴F_9/2 plays a leading role in the observed temperature dependence of the intrinsic lifetime. Furthermore, a reciprocity method has been successfully extended to the calculation of the room-temperature emission spectrum of the ²H_11/2−⁴I_15/2 transition according to the measured excitation spectrum. Unusual phonon side bands in the laser excitation spectrum are observed in the hypersensitive transitions of Er³⁺. The origin of the phonon side bands is ascribed to the M process that involves the infrared-active lattice modes.