Using picosecond Raman scattering, hot-phonon occupation numbers (N) of GaAs and GaAs-like LO phonons have been studied over a wide range of structural parameters in more than 30 GaAs/Al Gal As superlattices. In addition, simultaneous measurements of these LO phonon modes in bulk GaAs and Al Gal "As alloys are made for comparison. N values of both GaAs and GaAs-like modes of the superlattices are comparable to or larger than those of bulk GaAs or Al Gal As alloys for x &0.2. On the other hand, N values of GaAs or GaAs-like LO phonons are much smaller for 0

Extreme high order nonlinear diffracted signals are found in ZnCdSe/ZnSe quantum wells in two beam, self-diffraction geometry. Diffracted signals of all observed orders are shown to originate predominantly from excitonic resonances. Using four-wave mixing, an indication of hole-longitudinal optical phonon scattering between the heavy and the light hole states is found, and exciton-phonon interaction is studied.

Femtosecond degenerate four-wave-mixing experiments are performed in the two-beam self-diffracted geometry in two inhomogeneously broadened ZnSe/Zn"Cd& "Semultiple-quantum-well samples at the excitonic resonances at 10 K. A large change in the dephasing time across the linewidth of one sample is observed, whereas it is nearly constant in the other, implying a difFerent degree of localization in these two samples. Exciton-exciton interaction rates are determined across the linewidth, and it is shown that the exciton-exciton interaction rate is much less for excitons that are strongly localized compared to those that are extended.When the excitonic resonance of a quantum well (QW) is optically excited with an ultrafast laser pulse, the exci-

No abstract

The initial generation of hot LO phonons by photoexcited hot carriers is studied with picosecond Raman spectroscopy in GaAs and a series of Al"Ga& "As samples with 0&x &0.4. A rapid decrease in the occupation numbers of the GaAs-like and A1As-like LO-phonon modes is observed as x is increased. This decrease cannot be explained if the electrons excited from the lightand heavy-hole bands were the primary source of generating the hot phonons. It is shown that most Raman-active hot LO phonons are initially generated by the photoexcited electrons originating from the split-oF band, when photon energies of 2.33 eU and pulse durations of 1.5 ps are used. We have used a model assuming the instantaneous thermalization of electrons in the I valley which are photoexcited from the split-off hole band. Our experimental results are in good agreement with this calculation.