Nonlinear dynamics of free-carriers in direct bandgap semiconductors at terahertz (THz) frequencies is studied using
intense few-cycle pulses. Techniques as Z-scan, THz-pump / THz-probe, and optical-pump/ THz-probe are employed to
explore nonlinear interactions in both n-doped and photoexcited systems. The physical mechanism that gives rise to such
interactions is found to be intervalley scattering.