Multi-wave-mixing of two-dimensional excitons in semiconductors

The signals due to six-wave mixing (SWM) processes are observed for two-dimensional excitons and biexcitons. The signal intensity of the SWM process strongly depends on the pulse area of the excitation light. The pulse area is estimated by comparing the intensities of four-wave mixing (FWM) signals in two different directions. The photon echo and the transient-grating signals due to the biexcitonic state of GaSe were observed by SWM. When we change the order of the excitation pulses in our SWM experiments, the excitonic phase relaxation process can be observed in the negative delay-time region.