Several semiconductor materials, including CdS, CdSe, GaAs, and GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As multiple quantum wells were studied with use of femtosecond pump-probe spectroscopy. Oscillatory structures are observed in the differential transmission spectra of the probe pulse at the very early times when the probe pulse precedes the pump pulse. In addition to oscillations around the exciton frequency when pumping is either above or below the exciton, oscillatory structures are also observed in the vicinity of the pump frequency when the pump is tuned inside the semiconductor band. A semiclassical theory qualitatively explains the observed data. These oscillations are found to be the precursors of spectral hole burning (oscillations around the pump for pumping into the band), exciton bleaching (oscillations around exciton for pumping into the band) and exciton optical Stark shift (oscillations around exciton for pumping below the band).

• Received 10 May 1988

DOI:https://doi.org/10.1103/PhysRevB.38.7615