Theory of transient excitonic optical nonlinearities in semiconductor quantum-well structures

We present theoretical results for the effects of an exciton gas and an electron-hole plasma on the excitonic optical absorption in a two-dimensional semiconductor and compare these with recent experimental results on absorption saturation in single- and multiple-quantum-well structures. A simple theoretical description of the nonlinear optical properties of these microstructures is developed for the case of low-density optical excitation near and above the band edge. We argue that the effects of Coulomb screening of excitons by the plasma are relatively weak in these structures but that the consequences of phase-space filling and exchange are significant in each case. We are able to explain the recent unexpected experimental result that ``cold'' excitons are more effective than ``hot'' carriers in saturating the excitonic absorption. Good agreement with the experimental data is obtained without adjustable parameters.