Coherence of four-particle correlations in semiconductors

We report direct investigations of the coherence of four-particle (exciton- exciton) correlations by ultrafast four-wave mixing experiments. We determi ne the sensitivity of the coherence of the four-particle correlations to te mperature and excitation density, comparing it to the coherence of the two- particle correlations. We discuss our results in terms of numerical solutio ns of the general kinetic equations, treating the four-particle correlation s in the second order Born approximation. We introduce a simple model deduc ed from these kinetic equations that provides an intuitive interpretation o f our results.