(1) The time-integrated (TI) FWM signal always peaks at negative time delay (Fig. 1b).
(2) The signal decreases rapidly as the detuning increases, approximately as (detuning)-6 (Fig. 2a).
(3) There exists very fast pump-probe signal even when the laser is tuned completely below the band gap (Fig. 2b).
To understand our results, we performed first principle calculations. It is shown that virtual excitation is responsible for the observed signal.