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

Excitation of YAG:Ce3+ crystals and nanocrystals was performed at λexc=473–584 nm over a wide temperature range. It was observed that the luminescence of both nano- and single-crystal YAG:Ce samples is efficiently excited with photon energies well below the Ce3+ absorption band and at least 1650 cm−1 below the ZPL of the 4f1(2F5/2)−4f05d1 transition, located at 489 nm. The studies of Ce3+ fluorescence spectra as a function of temperature and excitation wavelength and of their temporal evolution point to the role of phonon-assisted nonradiative energy transfer between different groups of Ce3+ centers in the excitation mechanism.