Nonlinear THz optics-ultrafast spectroscopy

Our recent development of a simple and reliable method to generate THz pulses with high electric field amplitudes has paved the way for nonlinear optics in the THz regime. We report on two experiments on n-type GaAs, which are in strong contrast to the predictions of Drude theory. (i) Nonlinear propagation of intense THz pulses through a thin n-type GaAs layer shows a coherent emission at 2 THz with a picosecond decay of the emitted field. This nonlinear response is dominated by the super-radiant decay of optically inverted impurity transitions. (ii) A nonlinear THz-pump-midinfrared-probe experiment shows that ultrafast acceleration of free carriers in n-type GaAs in a strong THz field results in an oscillatory occurrence of midinfrared gain/absorption with the LO phonon frequency.