DIRECT OBSERVATION OF FREE-EXCITON THERMALIZATION IN QUANTUM-WELL STRUCTURES

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