Manifestation of exciton-amplitude fluctuations in the transient polarization state of four-wave-mixing signals

The polarization state of the diffracted beam in degenerate four-wave mixing is experimentally and theoretically investigated, resolved in the time as well as in the frequency domain. The use of thin ZnSe quantum wells having large (bi)exciton binding energies makes it possible to restrict the optical excitation by broadband 110 fs pulses to quasi-two-dimensional heavy-hole excitons and biexcitons only. Despite these most simple experimental conditions, an extremely complex dynamics of the polarization state is observed when one circularly and one linearly polarized beam are used for excitation. The detailed behavior sensitively depends on the excitation density, showing the importance of nonlinearities of higher than third order even at exciton densities less than 0.01 times the Mott density. A comparison of the experimental data with theoretical results obtained by a microscopic density-matrix approach yields very close agreement. It turns out that a dynamical object describing fluctuations of the exciton amplitude has to be included in the theory in addition to biexcitonic contributions and correlated parts of the exciton-exciton scattering continuum.