STRONG FEMTOSECOND DEGENERATE 4-WAVE-MIXING SIGNALS FROM 350 MU-M THICK UNDOPED AND N-TYPE DOPED INP FAR BELOW BAND-GAP

Femtosecond two-pulse self-diffracted four-wave mixing (FWM) experimen ts have been performed at 10 K for 350 mu m thick undoped and n-type d oped InP, with an excitation below the band gap (detuning from near th e band gap down to 90 meV). We have observed rather strong FWM signals from the third- to the seventh-order for a 350 mu m thick undoped InP , whereas we have observed only the third-order FWM signal for a 350 m u m thick n-type doped InP. The third-order time-integrated (TI) FWM s ignals for both samples decrease as detuning increases as (detuning)(- 2). Furthermore, the spectrally resolved (SR) FWM signals for both sam ples far below the band gap are blue-shifted significantly as the time delay moves away from positive to negative. In the thick semiconducto rs the FWM signals far below the band gap exhibit remarkably different behaviors from the excitonic properties. Based on our observations, w e conclude that the FWM signals far below the band gap are due to the virtual states created by the excitation laser pulses.