Coherent secondary emission from resonantly excited two-exciton states: A novel intrinsic phenomenon

The coherent interaction of light and the electronic states of semiconductors near the fundamental bandgap has been a very active topic of research since the advent of ultrafast lasers. While many of the ultrafast nonlinear properties of semiconductors have been well explained within mean field theories as e.g. the semiconductor Bloch equations, recent experimental and theoretical developments have demonstrated contributions to the third order nonlinear susceptibility from exciton-exciton correlations beyond the mean field approximation. Ultrafast transient four-wave mixing (TFWM)has been the experimental technique of choice to investigate these effects, and comparison of experiment and theory have so far been based on calculated TFWM line shapes. In this presentation a new experimental approach has been used to investigate directly the contribution from exciton-exciton correlations to the nonlinear susceptibility. The method exploits that emission from two-exciton coherences can occur in non-specular directions, with the recoil momentum taken up by an exciton left behind in the sample. Using ultrafast spectral interferometry we demonstrate the presence of this new coherent component of the secondary emission from quantum wells following ultrafast resonant excitation and find that it provides information on not only the bound biexcitons but also the biexciton continuum. Due to the heterodyne nature of the experimental technique we obtain both amplitude and phase of the coherent emission. This allow us to present our data using the Wiegner-function formalism and compare directly with recent theoretical results for the contribution to the nonlinear susceptibility from exciton-exciton correlation in semiconductor quantum wells.