Coherent phonon-plasmon modes in GaAs:AlxGa1-xAs heterostructures

Time-resolved transient Kerr reflectivity was used to study coherent phonon-plasmon excitations in GaAs:AlxGa1-xAs heterostructures containing a two-dimensional electron gas. Ultrafast laser pulses of 65 fs duration were used in a balanced optical bridge to observe the oscillatory component of the reflectivity changes directly. This was found to be strongly dependent on the excitation photon energy, showing a crossover in the spectral amplitude of the longitudinal and transverse-optic-phonon frequencies as the excitation energy was varied. The energy dependence reflects variations in the phonon-plasmon coupling with depth, and hence local electron density, which is in very good agreement with recent predictions.