Ultrabroadband coherent electric field from far infrared to 200 THz using air plasma induced by 10 fs pulses

We generated an ultrabroadband infrared pulse ranging from far infrared to 200 THz through a plasma by focusing a hollow-fiber compressed intense 10-fs pulse and its second harmonic in air. We coherently detected the signal up to 100 THz with electro-optic sampling and clarified its drastic dependence on the orientation of the second harmonic crystal in a range of 100–200 THz with an HgCdTe detector. From these, we confirmed the whole frequency components originated from the AC biased plasma and were phase locked. This result opens the possibility of a pump-probe spectroscopy which covers the whole infrared range.