Exciton Dephasing in Strained GaAs Films Studied with Heterodyne Four-Wave Mixing

We studied the coherent exciton dynamics in a 470-nm thick GaAs film on sapphire substrate using 3-beam heterodyne four-wave mixing (HFWM) in a photon echo configuration at 14 Kelvin. The GaAs film reveals different biaxial tensile strain levels at different film locations. This property enabled us to study the impact of differing strain on the dephasing time of strain-split heavy- (hh) and light-hole (lh) excitons. The strain in the film also causes a splitting and shift in the energy levels of acceptor-bound excitons prolonging the dephasing time of the light-hole exciton when their energies align. The HFWM measurements evidenced a homogeneously broadened exciton polarization in the high strain region near the center of the film while a small degree of inhomogeneous broadening is present in low strain regions at the border of the GaAs sample. The heterodyne FWM experiments further reveal different hh-lh exciton quantum beat structures for co- and cross-linear and -circular polarized excitations. The experimental studies are accompanied by numerical calculations based on the optical Bloch equation which assisted in our interpretation of the data.