Coherently enhanced THz-to-microwave ultrashort field waveforms from mid-infrared-driven laser plasmas

With the wavelength of a short-pulse laser driver shifted to the midinfrared range, the low-frequency output of laser-induced plasmas can be drastically enhanced, as our experiments show, providing a source of ultrabroadband radiation with a spectrum spanning across the entire terahertz, millimeter-wave, and microwave bands. At low gas pressures, such ultrabroadband field waveforms are shown to rapidly build up their coherence, developing a well-resolved emission cone, dominated by a radial radiation energy flux. As counterintuitive as it may seem, this behavior of the intensity, coherence, and polarization of the low-frequency plasma output is shown to be consistent with a physical scenario of Cherenkov-type radiation emission by ponderomotively driven plasma currents.