Evidence of six-particle Coulomb correlations in six-wave-mixing signals from a semiconductor quantum well

Six-wave-mixing signals from a ZnSe quantum well are analyzed experimentally and with a microscopic density-matrix description using the dynamics-controlled-truncation scheme. For each physically distinct combination of polarizations of the exciting pulses, the spectrum of six-wave-mixing emission is measured as a function of time delay. The experimental results are compared with calculations performed at different levels of approximation. Although the leading order contributions to six-wave-mixing signals are of fifth order in the laser field, we show that there are significant signal components that are due to at least ${\chi }^{\mathrm{}\left(7\right)\mathrm{}}$ processes. The sensitivity of six-wave-mixing signals to high-order Coulomb correlations is demonstrated. Six-point density matrices are found to be indispensable for the interpretation of our experiments, while some details seem to indicate the involvement of even higher-order correlation functions. Furthermore, we find a remarkable dynamical decoupling of spectral signatures and the delay-time behavior after excitation with linearly polarized pulses.