In type-II Alx,Gal-xAs AlAs multiple quantum-wells MQW the lowest lying electronic states in the conduction band are X-related states confined in the AlAs layers. However, the absorption spectrum is governed by the direct optical transitions at higher energies, involving the Gamma valence band and Gamma conduction band states of the AlxGal-xAs. As a consequence of this type-11 band alignment. electrons photocreated in the Gamma-states confined in the AlxGa1-xAs layer scatter to the lower lying X-states located in the adjacent AlAs layer. We demonstrate that the associated change of the dielectric constant leads to a complete recovery of the initially bleached absorption, if the laser photon energy is at the low-energy side of the direct heavy-hole excitonic transition. The response time of this large excitonic nonlinearity is thus given by the Gamma-X transfer time, which is on a picosecond or even subpicosecond time-scale.
This article is from 'Organization of the Optical Society of America Photonic science Topical Meeting Series. Volume 7. Quantum Optoelectronics Held in Salt Lake City, Utah on 11-13 March 1991,' AD-A253 823, p202-205.