MODULATED PHOTOABSORPTION IN STRAINED GA1-XINXAS/GAAS MULTIPLE QUANTUM-WELLS

Modulated photoabsorption measurements in strained Ga1-xInxAs/GaAs multiple quantum wells are presented. The modulating intensities vary from a few to about 10(5) W/cm2. The absorption near the first heavy-hole exciton is probed with a tunable Ti:sapphire laser. The modulating beam is either from the same Ti:sapphire laser as the test beam or from an Ar+-ion laser whose photon energy is much larger than the first heavy-hole exciton transition energy. A dramatic difference is observed in the modulated transmission spectra for the two modulating wavelengths. This difference in behavior can be explained as arising from screening of the residual surface electric field by Ar+-ion-laser excitation but not by Ti:sapphire laser excitation. The Ar+-ion laser creates high-energy carriers that are initially free to drift in the surface field before they are captured in the quantum wells. Carriers excited by the low-photon-energy Ti:sapphire laser are created in the quantum wells and therefore cannot effectively screen the surface field. We present a model based on surface-field screening and exciton saturation for Ar+-ion-laser modulation and exciton saturation alone for Ti:sapphire laser modulation that describes the observed results.