Coherent optical nonlinearities and phase relaxation of quasi-three-dimensional and quasi-two-dimensional excitons in ${\mathrm{ZnS}}_x{\mathrm{Se}}_{1-x}/\mathrm{ZnSe}$ structures

We investigate the dephasing of heavy-hole excitons in different free-standing ${\mathrm{ZnS}}_x{\mathrm{Se}}_{1-x}/\mathrm{ZnSe}$ layer structures by spectrally resolved transient four-wave mixing. ZnSe layers of 80, 8, and 4 nm thickness with ternary barriers are studied, representing the crossover from quasi-three-dimensional to quasi-two-dimensional excitons. A common feature of the four-wave-mixing signals is the appearance of two components, a prompt signature and a delayed photon echo, which are identified by their different polarization dependencies and decay times. For crosslinear polarized fields, the rapidly decaying signal is attributed to the response of spin-coupled exciton states, with a decay time given by the inhomogeneous broadening. The photon echo is due to a distribution of localized, noninteracting excitons. We determine the exciton-exciton and exciton-phonon scattering cross sections for different dimensionalities by the intensity and temperature dependencies of the exciton dephasing.