We have examined the expansion of the photoexcited electron-hole plasma in GaAs quantum wells by using spatial and time-resolved Raman spectroscopy on a picosecond time scale and with approximately 2-μm spatial resolution. The intersubband Raman signals associated with spin-density as well as charge-density fluctuations are employed to measure directly the carrier density as a function of the time delay and the distance from the excitation spot. For an injected plasma density of n≃1.5×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$, the experimental results have shown that the in-plane ambipolar transport of the carriers in GaAs quantum wells can be very well described by a diffusion model and the thickness of the GaAs quantum well does not significantly influence lateral expansion of the plasma.

• Received 5 October 1988

DOI:https://doi.org/10.1103/PhysRevB.39.6276