Stimulated emission and relaxation processes in molecular crystals

Abstract

In the present paper experimental results on the emission of organic molecular crystals with strong optical pumping are analysed. Incoherent processes are studied, namely stimulated emission, vibrational state saturation, nonequilibrium phonon generation, optical transitions stimulated by nonequilibrium phonons, etc. Experimental data on light generation by doped molecular crystals at 300 and 77K are presented. A comparison is made between the parameters of light generation in crystals on the one hand, and liquid and polymer solutions on the other hand. Reabsorption of the emission by the excited molecules preventing light generation is considered. Spectral and kinetic studies of stimulated emission of crystals in the temperature range 1.6–40K are presented. The advantages of doped molecular crystals below 20K over liquid and polymer solutions as Q-switched frequency transformers are emphasized. Much attention is paid to stimulated emission of molecular crystals at low temperatures to study vibrational and phonon relaxation. Relaxation times of vibrational levels of the ground state of the impurity molecule are obtained for a number of doped molecular crystals. Data on the relaxation of vibrational levels obtained by stimulated emission, hot luminescence and hole burning are analysed. Vibrational relaxation times in the ground and excited states of an impurity molecule are compared. The role of host crystal phonons in the evolution of the emission is studied. Generation and propagation of nonequilibrium phonons are discussed. Phonon lifetimes are estimated by kinetic and spectral studies.