Coherent longitudinal-optical phonons are generated in semiconductor heterostructures. The coupling of the coherent, longitudinal-optic ͑LO͒ phonons to collective carrier excitations oscillating parallel to the growth direction of GaAs/Al 0.36 Ga 0.64 As quantum wells is investigated with femtosecond time-resolution. This coupling is found to be weak for small well widths and evolves towards the bulk plasmon phonon coupling at increased well widths. We present a theory for the dielectric function in the growth direction of the heterostructure and calculate the frequency response of the system on the pulsed optical excitation. It is shown that the observations are based on the coupling of coherent phonons to intersubband plasmons.

Quantum beats of heavy and light holes in GaAs quantum wells are investigated in femtosecond time-resolved four-wave mixing and transmission experiments as a function of optical excitation energy. Under nonexcitonic excitation conditions, the four-wave mixing signal disappears due to the immediate loss of the interband coherence of continuum states. In the transmission experiment, the quantum beats are observed up to excess energies of 75 meV above the exciton resonances.The experimental data clearly demonstrate the coherence of continuum states in the valence band. Changes of the beat frequency with the excitation energy are due to the dispersion of the valence bands.[ S0031-9007(96) Time-resolved investigations of quantum beats in semiconductor quantum wells have received considerable attention in the last few years, since they provide insight into the scattering and coupling mechanisms between light, excitons, free carriers, and phonons in these systems. Most of this work deals with quantum coherence of excitonic states, which may have dephasing rates up to some picoseconds at low lattice temperatures in GaAs based heterostructures. The most commonly used tool for the time-resolved detection of optically excited coherence in semiconductors is four-wave mixing (FWM) spectroscopy [1]. In self-diffracted degenerate FWM, the third order nonlinear interband polarization is detected, which gives a selective sensitivity for the excitonic coherence based on two facts: (i) The signal is proportional to the eighth order of the transition matrix elements [2], which is significantly enhanced at the exciton energy, and (ii) continuum states lose their interband coherence on a time scale of 100 fs resulting in the dominance of the long-living excitonic contribution [3]. The immediate loss of the interband coherence of continuum states arises from the k-space band dispersion, which is equivalent to an "inhomogeneous" broadening of interband transitions with the energetic width of the exciting laser spectrum. Furthermore, the excitation of continuum states with excess energy above the band minimum provides additional scattering channels for momentum and energy relaxation so that a decreased interband dephasing time is expected. Several different techniques have been applied for the time-resolved investigation of coherent electronic states in semiconductors, e.g., THz emission spectroscopy [4][5][6], time-resolved transmission spectroscopy [2,7], and time-resolved resonant luminescence up-conversion [8]. All these techniques provide distinctly different information on coherent states than FWM. In THz emission spectroscopy, the detected THz radiation arises from the real space oscillation of coherent wave packets and is thus not necessarily restricted to excitonic transitions. Therefore, under appropriate excitation conditions, the detected signal may give information on the intraband coherence of the excited states. These observations have been made for the case of nonexcitonic excitation of Bloch oscillations in superlattice...

The thermalization of optically excited cold holes in a GaAs quantum well is investigated by femtosecond two-color pump-probe measurements. Clear evidence is found for scattering from heavy-holes into the lowest light-hole band due to LO-phonon absorption. We obtain firm data on scattering times which depend strongly on lattice temperature. They vary from 230 fs at room temperature to 900 fs at Tϭ105 K. The experimental data are well reproduced by numerical calculations. © 1996 American Institute of Physics. ͓S0003-6951͑96͒03021-5͔Firm experimental data on ultrafast carrier dynamics in semiconductors and semiconductor heterostructures are of prime importance for the development of novel ultrafast opto-electronic devices. The development of ultrashort-pulse lasers has stimulated a large number of time-resolved studies addressing the picosecond and subpicosecond dynamics of nonequilibrium carriers in GaAs and other III-V semiconductors. 1,2 Since the density of states in the conduction band is generally much lower than that of the valence band, in most studies the experimental signals are dominated by the dynamics of optically excited electrons. 3 Only in a few specific experiments could information on the hole relaxation dynamics be obtained in special sample structures. [4][5][6][7][8] In these studies, holes were excited with a certain excess energy and subsequent cooling processes were observed. In contrast, optical excitation of excitons close to the band edge will create initially cold holes. In this case, the hole dynamics at elevated lattice temperatures is expected to be dominated by the absorption of longitudinal optical ͑LO͒ phonons that leads to a heating of the initial hole distribution. 3,7 In this letter, we investigate the intervalence band thermalization of optically excited holes. Femtosecond timeresolved measurements of the bleaching of the heavy-hole ͑HH͒ and light-hole ͑LH͒ exciton transition in a GaAs multi quantum well ͑MQW͒ sample are performed. The nonlinear absorption of the excitonic transitions is determined by two contributions, namely the reduction of the oscillator strength and the broadening of the exciton line. 9 It has been shown previously that the reduction of the oscillator strength strongly depends on the subband occupation number, while the intrasubband carrier distribution does not play a significant role. 9,10 In the same work it was found that broadening is not affected by changes of the subband occupation. Furthermore, measurements of absorption changes at the lowenergy edge of the E1H1 transition-which are dominated by broadening-under various excitation conditions only show an ''instantaneous'' signal without any further dynamics on a picosecond time scale, indicating that this signal contribution is not sensitive to the exact intrasubband carrier distribution, either. 11 These facts make it possible to measure changes of the hole subband occupation by choosing sample and excitation parameters that prevent any electron intersubband dynamics.Our experiments are performed...