Direct observation of free-exciton thermalization in quantum-well structures

We report on a direct observation of free-exciton thermalization in quantum-well structures. A narrow energy distribution of free $1s$ excitons is created in ZnSe-based quantum wells by emission of one LO phonon after optical excitation of the continuum states with picosecond laser pulses. The subsequent relaxation dynamics within the $1s$-exciton dispersion is directly monitored by time-resolved studies of the phonon-assisted photoluminescence. It is demonstrated that the free-exciton distribution remains nonthermal for some 100 ps. The observed dynamics is in reasonable agreement with numerical results of a rate-equation model which accounts for the relevant exciton-phonon coupling mechanisms.