ИСТИНА

(Invited talk) An extended femtosecond filament is a nonlinear light structure, which can emit a continuum of frequencies as well as quasi-isolated pulses in certain spectral ranges [1]. Generation of mid-infrared (MIR) ultrashort pulses can be enhanced by seeding the filament with the pulse at the central frequency close to its second harmonics [2]. With a filament the coherent THz radiation can be delivered to the desired position far through the atmosphere, avoiding thereby strong water vapor absorption [3,4]. In this paper we perform a complete study of spatio-temporal and frequency-angular distribution of terahertz and infrared radiation generated in extended femtosecond filaments in gases (Fig.1). We demonstrate that in the case of 800 nm single-color filament the near-infrared radiation forms the quasi-isolated pulse centered in the vicinity of 900-1000 nm. In MIR range the radiation spreads out in a ring while preserving the local on-axis maximum. We have shown an order of magnitude increase in the on-axis few-cycle MIR pulse enegy if a seed pulse is added into a single – color filament. THz radiation diffracts out of the filament forming rings in agreement with the experiment [4,5] Fig.2. We revealed that the two-color 800-400 nm filament initiates the on-axis terahertz radiation due to four-wave mixing and provides us with high conversion efficiency from 800 nm to the terahertz frequency range. The simulations are performed based on a unidirectional pulse propagation equation model [6] with the fine resolution better than 0.25 THz.