Ultrafast switching of SHG and THG. (a) Normalized cross-polarized signals with and without optical pumping; the inset shows the corresponding time-domain second harmonic signal. (b) Normalized SHG amplitude versus the relative time delay of the optical pump and fundamental THz pulse. (c) Normalized co-polarized signals with and without optical pumping; the inset shows the corresponding time-domain third harmonic signal. (d) Normalized THG amplitude versus the relative time delay of the optical pump and fundamental THz pulse.