Nonperturbative Coulomb correlations generated by simultaneous excitation of excitonic and band-to-band continuum transitions

We investigate theoretically the nonlinear optical response of a semiconductor for excitation conditions where simultaneously all kinds of correlated two-pair transitions contribute to the dynamics. This includes transitions to biexcitons, exciton-exciton scattering states, two free electron-hole pairs as well as two-pair states involving an exciton and a free electron-hole pair. For certain excitation conditions two-pair correlations give rise to a complex line shape of four-wave-mixing spectra with an emission spread over the whole range between the exciton line and the band-edge. Even a strong suppression of the signal at the exciton line can be achieved while the emission is still concentrated between the exciton and the band-edge. The dependence of these features on the excitation conditions and on the strength of the Coulomb interaction is discussed. Comparing a nonperturbative treatment of the Coulomb interaction with the Born approximation and the mean-field theory clearly reveals the importance of nonperturbative Coulomb correlations even for excitations involving the continuum of free electron-hole pairs.