Upconverted luminescence from nonequilibrium vibronic states of Cr ions in forsterite

Nonequilibrium excited vibronic states of photoexcited ${\mathrm{Cr}}^{4+}$ ions in a forsterite crystal were studied using picosecond laser excitation and ultrasensitive photon counting detection. The excitation involves the absorption of two photons per photoexcited ion by means of excited-state absorption. Discrete peaks are observed in the hot upconverted luminescence spectrum and are attributed to the population of nonequilibrium vibronic levels during the deexcitation of the ions by phonon emission. This work reveals some of the phonon modes participating in the nonradiative relaxation of the photoexcited ions. The shape of the luminescence spectral envelope suggests a bottleneck in the nonradiative decay at about 2.1 eV from coupling between the ${}_{}{}^1{}_{}{}^{}$E and ${}_{}{}^3{}_{}{}^{}$ ${\mathit{T}}_1$ states.