Spectrally resolved nonlinear optical response of weakly confined excitons under femtosecond laser pulse excitation in GaAs thin films

We have investigated the dynamical properties of the weakly confined excitons in GaAs thin films by a transient grating method (three-beam four-wave mixing) with the use of femtosecond-laser pulses. The time-domain signals under the exciton-excitation conditions show a nonlinear optical response with a rise time of 4 ps and a long decay time of 120 ps. Moreover, in the case of that the probe pulse is parallel polarized to the two excitation pulses, the ultrafast optical response comparable to the pulse width appears. These characteristics of the nonlinear optical response is different from the experimental results by the picosecond-laser-pulse excitation. The spectra of the transient grating signals at the various time delays show several peaks at the energies, which agree with the energies of excitons confined in the thin films. This fact demonstrates that the characteristics of the nonlinear optical response of the weakly confined excitons are drastically changed by the spectral width of the excitation laser-pulse.