Phonon-assisted anti-Stokes excitation: Mechanism for the unusual temperature dependence of the Ce³⁺ luminescence in yttrium aluminum garnet

Luminescence emission spectra and decay kinetics of Ce³⁺ in YAG were investigated as a function of temperature (300-800 K) under excitation in the long wavelength tail of the lowest 4f-5d absorption spectrum. Both the anti-Stokes and Stokes emission intensity were found to exhibit an unusual non-monotonic temperature dependence compared to those commonly excited in the absorption band, where luminescence intensity decreases monotonically with increasing temperature. This phenomenon is well explained by a phonon-assisted anti-Stokes excitation model with strong vibronic interaction. It reveals that the intensity enhancement of Ce³⁺ anti-Stokes and Stokes emission is caused by the anti-Stokes excitation efficiency. Excitation efficiency is enhanced with increasing temperature, and thermal ionization of the 5d electron into the conduction band remains as the quenching mechanism of Ce³⁺ anti-Stokes and Stokes luminescence at high temperatures.