Real-to-virtual transition in bulk GaAs: the thickness dependence

Abstract

A femtosecond degenerate four-wave mixing (FWM) experiment has been performed on bulk GaAs as a continuous function of the sample thickness. The FWM signals exhibit a transition from the real, excitonic region to the virtual region as the thickness increases from 3 mum to 17.5 pm. The results for a negative time delay show an extraordinary signal where the energy extends well above the band-edge in the FWM transmittance although the thickness is an order of magnitude larger than the penetration depth. These above-the-band-gap signals are mostly confined to the negative time-delay region and shift further into the negative time-delay region as the detection energy increases. These unusual phenomena can be understood by using the third-order frequency mixing (2 omega (2) - omega (1); omega (2) > omega (1)) between positively chirped spectral components