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 mixin g 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 optic al excitation by broadband 110 fs pulses to quasi-two-dimensional heavy-hol e excitons and biexcitons only. Despite these most simple experimental cond itions, an extremely complex dynamics of the polarization state is observed when one circularly and one linearly polarized beam are used for excitatio n, The detailed behavior sensitively depends on the excitation density, sho wing the importance of nonlinearities of higher than third order even at ex citon densities less than 0.01 times the Mott density. A comparison of the experimental data with theoretical results obtained by a microscopic densit y-matrix approach yields very close agreement. It turns out that a dynamica l object describing fluctuations of the exciton amplitude has to be include d in the theory in addition to biexcitonic contributions and correlated par ts of the exciton-exciton scattering continuum. [S0163-1829(99)50112-4].