MICHIGAN UNIV ANN ARBOR HARRISON M RANDALL LAB OF PHYSICS
Optical resonant excitation of excitons with nearly monochromatic light of energy E leads to an optically induced polarization coherence and a population of excitons within Delta E of E where Delta E approx.-Hbar gamma hYh is the homogeneous linewidth of the exciton. The decay of this excitation must be characterized by the decay rate of the polarization called the dephasing or transverse relaxation rate, gamma sub tgamma h as well as the decay rate of the population about energy E called longitudinal the relaxation rate, gamma sub y. In a simple ideal quantum well, quasi two dimensional excitons are described by a Bloch type wave function and are free to move in the well plane. At low exciton density, decay of the excitation is then expected to be predominantly due to exciton-phonon scattering along with exciton recombination. In practice, however, nonideal growth processes result in interface roughness and corresponding disorder, leading to localization of lowest energy excitons and inhomogeneous broadening of the exciton absorption spectrum. Higher energy excitons remain quasi-delocalized or extended and the transition region, identified as the middle of the absorption resonance, is designated the exciton mobility edge. Extended excitons are expected to dephase quickly approx. ps due to phonon and defect scattering whereas scattering processes for localized excitons are characterized by longer time scales approx. 100ps. Localized excitons are in a local minimum in energy, and at very low temperature, decay of the localized exciton at energy E is expected to be dominated by migration between localization sites. The migration is accompanied by absorption or emission of acoustic phonons to compensate for the energy difference.
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, p41-44.