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.
THz generation under two-color four-photon interaction of femtosecond TiSa laser radiation in a low-pressure nitrogen at sub-and terawatt power levels has been studied. Method for characterizing the parameters of a femtosecond filament formed in a low-pressure gas based on the average ablation rate and X-ray output from a solid-state target interacting with the filament was developed. In sub-terawatt laser power it was found that under the condition P/Pcr=const, the pressure decrease and the energy increase within 10 times correspond to enhance in the average ablation rate by 2 times and an increase in the fluence and intensity by about 3 times. Under terawatt laser power, energy density in the filament is about 15 J/cm2, the intensity is 2.5*1014W/cm2, and the THz signal reaches a maximum at a nitrogen pressure of 10 mbar (this corresponds to the 3Pcr regime) and is of the order of 1300 nJ.
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