Coherent LO phonons are excited at the surface of bulk GaAs with femtosecond laser pulses. They are observed for the first time through electro-optic modulations of the transient reflectivity. The corresponding signal oscillations are superimposed upon an additional longitudinal polarization feature which decays exponentially and whose rise time is sufficiently short to act as a driving force for the coherent phonons. The components of the coherent phonon states dephase relative to each other in the order of picoseconds depending on the density of optically excited electron-hole pairs.
Transient pump-probe signals of a GaAs/Al(x)Ga(1-x)As quantum well are investigated both theoretically and experimentally for different excitation conditions. The calculations are based on a microscopic density-matrix approach taking higher-order correlations into account. We present spectrally resolved transients which provide insights into the nature of the differential transmission change. Especially the complex phase signature of heavy-hole-light-hole quantum beats in dependence of the detection energy allows for an identification of different microscopic processes in the pump-probe signal. The theoretical results are verified experimentally by comparison with the coherent and incoherent parts of the time-resolved data. We find a dominant role of the Coulomb-induced correlations up to the chi(exp3)relevant four-point level in the coherent signature in comparison to the Pauli blocking contributions
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