Mechanisms for Ce3+ excitation at energies below the zero-phonon line in YAG crystals and nanocrystals

► YAG:Ce luminescence is excited with photon energies below the Ce3+ zero-phonon line ► We studied fluorescence dependencies on temperature, excitation wavelength and time ► The effect is explained by nonequilibrium phonon-assisted nonradiative energy transfer ► The observed process is interesting from the point of view of laser cooling of solids