Femtosecond frequency mixing in thick bulk GaAs

Femtosecond degenerate four-wave mixing (FWM) experiments have been performed on bulk GaAs as a continuous function of the sample thickness. The FWM signals exhibit the transition from a real, excitonic regime to the virtual regime as the thickness increases from 3 to $17.5\mu \mathrm{m}.$ The results at the negative time delay show an extraordinary signal: even when the thickness is an order of magnitude larger than the penetration depth, there still exists a signal well above the band gap. These above-the-band-gap signals are mostly confined to the negative time delay region and shift further into the negative time delay as the detection energy increases. These unusual phenomena can be understood by the third-order frequency mixing $(2\mathrm{}{\omega }_2-{\omega }_1;{\omega }_2>{\omega }_1)$ between positively chirped spectral components.