PROBING OPTICAL-PHONON PROPAGATION IN GAAS ALXGA1-XAS QUANTUM-WELL SAMPLES VIA THEIR NONEQUILIBRIUM POPULATION/

We develop a theoretical model to analyze a nonequilibrium optical- (L O-) phonon population by Raman scattering in GaAs/AlxGa1-xAs quantum w ebs. With the assumption of bulklike hot-electron relaxation, the effe ct of LO-phonon confinement on a nonequilibrium optical-phonon populat ion (NOP) is isolated. Our analysis shows that the decrease in spatial extent or coherence length of LO phonons is reflected by a decrease i n NOP. This is because the contributions from large q wave vectors wit h small occupation numbers dominate as the spatial extent decreases. O ur method is applied to explain picosecond Raman-scattering experiment s on GaAs/AlxGa1-xAs. The increasing NOP with decreasing x is interpre ted as the result of an increase in the coherence length of LO phonons , since for smaller x, the AlxGa1-xAs barrier is no longer effective i n localizing GaAs LO phonons within the well. Using this model, we als o deduce coherence length of LO phonons as a function of x. Our result s show that for values of x between 0.2 and 0.4, the GaAs LO phonon in GaAs/AlxGa1-xAs quantum wells changes from a bulklike propagating mod e to one localized within the wells.