Responsibility of plasma current for the generation of the highest frequency part of ultrabroadband coherent infrared pulses with 200-THz bandwidth

We reveal that, in the generation process of ultrabroadband coherent infrared pulses from two-color pulse excited air plasma, plasma current is essential not only for the generation of low frequency part but also for that of highest frequency part of 150-200 THz by both experiment and simulation, examining the dependence of the generation efficiency on the thickness of nonlinear crystals. The contribution of four wave mixing is relatively minor in the frequency range.