Terahertz radiation from a dense gas optical discharge

Summary form only given. The process of fast ionization of a neutral gas in presence of external (DC, AC or RF) electric fields is accompanied by excitation of eigen-oscillations inside a bounded volume of the created plasma and consequent radiation of the energy stored in these oscillations into a surrounding media. These phenomena may be exploited in a new class of electromagnetic radiation sources of different frequencies including frequency ranges being presently poorly developed. In particular, as it has been shown by our analytical and numerical stimulations, plasma oscillations generated at the front of ionization wave resulted from focusing of an ultrashort laser pulse of terawatt power level, may be an effective source of electromagnetic radiation in the terahertz frequency range. Peculiar feature of the source based on axicon focusing is that the ionization front (and therefore a phase of emitting plasma currents) moves along the focusing axis with the velocity larger than the speed of light, resulting in the radiation of a coherent broadband electromagnetic pulse through a Cherenkov-type mechanism. Amplitude of the plasma oscillations and intensity of the emitted radiation get maximum values under the following conditions: omegaa/c ~ 1, lLta, where omega is the characteristic frequency of the electromagnetic pulse, a and l are the radius and the boundary layer depth of plasma column formed in a vicinity of the axicon caustic, t is the characteristic time of plasma density increase. Such conditions may be realized in an air breakdown at pressure about 1 torr by a laser pulse with duration about 100 fs and focusing angle <10deg. Taking an extreme value of the external pulsed RF electric field be about 1 MV/cm (realized e.g. in accelerator systems of modern electron-positron colliders), one can estimate the GW power level of the generated terahertz radiation, which is essentially greater than the power achieved in "conventional" laser-plasma systems bas- d on alternative (ponderomotive) mechanisms of plasma-oscillation excitation