Excitonic approach to the ultrafast optical response of semiconductors

We present a theoretical approach to treating the coherent dynamics of optically generated charge carriers in semiconductors using an excitonic basis. In contrast to the semiconductor Bloch equations, our approach treats intraband correlations without factorization. It also includes phase space filling effects that have generally been omitted in previous excitonic treatments of coherent dynamics. We show that, in the coherent limit, where the intraband dephasing time and population decay time are both equal to half of the interband dephasing time, our excitonic approach agrees with the semiconductor Bloch equations to at least third order in the optical field, but that it differs significantly in more general situations. Our excitonic equations are shown to be particularly applicable in systems, such as biased semiconductor superlattices, where bound excitons dominate the optical response and where intraband correlations play a central role. Using a simple model of a nanoring, we show how the spectral shifts in the interband response can be explained in terms of phase-space-filling-induced excitonic population dynamics.