Femtosecond degenerate four-wave mixing in 500 μm undoped GaAs and 350 μm undoped InP far below band gap

Rather strong third- and fifth-order four-wave mixing (FWM) signals from a 500 μm thick undoped GaAs and a 350 μm thick undoped InP in detuning ranges of 80–170 meV below the band gap are observed. The spectrally resolved (SR) FWM signals for both samples far below the band gap are blue-shifted significantly with respect to the spectrum of the excitation laser pulse for negative time delays, whereas they are red-shifted for positive time delays. The temporal and spectral shapes of the FWM signals for both samples far below the band gap are nearly consistent with those of the excitation laser pulse but are remarkably different from the excitonic properties. The shifts of SR-FWM signals far below the band gap are interpreted as due to the instantaneously created virtual states by the temporal overlap of the excitation laser pulses.