Excitonic approach to the ultrafast optical response of semiconductor quantum wells

We use a basis of bound and unbound excitons to study the coherent dynamics of optically excited excitons in a semiconductor quantum well. We derive a set of excitonic dynamic equations for quantum wells that includes the influence of phase-space filling and the exchange interaction. We calculate the nonlinear absorption for excitation by a short pulse resonant on the 1s exciton, and show that the 1s excitonic peak is reduced and blueshifted as the exciton density increases. By examining the dynamics of the populations in the different excitonic states, we show that at moderate densities (n=1.3x10(10) cm(-2)) the absorption near the 1s peak is well described using only the 1s excitonic state but that at higher densities (n=5.0x10(10) cm(-2)) the other excitons-both optically active and optically inactive-play a significant role. For moderate densities, we derive analytical expressions to describe the density-dependent blueshift and bleaching of the 1s excitonic resonance. Finally, we discuss the potential advantages of this formalism for the investigation of both interband and intraband dynamics in quantum wells.