THz generation under double-pump excitation . (a) Schematic of double-pump excitation. Two optical pumps propagating collinearly with a certain time delay are focused on a water line (top view). is controlled by a delay stage. The THz signal is detected via EOS. (b) THz waveforms generated by pump 1 (black line) and pump 2 (blue line). The bottom red line shows the waveform generated by two pumps when is 3.9 ps. (c) The red line shows the dynamic enhancement of the THz peak field versus . The black line plots the autocorrelation signal of the two pumps, from which the 0 timing is determined. means that the pump 1 ionizes the target first.