Optical dephasing of homogeneously broadened two-dimensional exciton transitions in GaAs quantum wells

We report optical dephasing of two-dimensional excitons in $\mathrm{GaAs}-{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ single quantum wells by means of time-resolved degenerate-four-wave mixing and transmission experiments in the temperature range below 80 K. The observed absorption linewidths correspond directly to the excitonic phase coherence times, confirming the homogeneous broadening of the excitonic absorption. A linear temperature dependence of the homogeneous linewidth indicates acoustic-phonon scattering as the broadening mechanism.