Amplification of Transition-Cherenkov Terahertz Radiation of Femtosecond Filament in Air

The transition-Cherenkov terahertz (THz) radiation from a femtosecond laser filament in air is enhanced by 3 orders of magnitude in the presence of a longitudinal static electric field, while the radiation pattern and the polarization remain the same. An amplified longitudinal electron current inside the filament is responsible for this amplified THz emission. A plasma filament formed in air by femtosecond laser pulse is a promising source for electromagnetic radiation in the THz domain , due to the simplicity of alignment, availability of the effective medium, and also due to the ability to position the THz emitter in the proximity of a remote sample far away from the laser system 5, . The femtosecond laser filament arises from the dynamic competition between Kerr beam self-focusing and beam defocusing by the produced air ionization. The filament characteristics such as length and position can be easily controlled by manipulating the laser pulse or the focusing conditions . The forward THz radiation from a single filament was found to be radially polarized and exhibit a hollowcone structure where the radiation intensity on the laser propagation axis is zero and the cone opening angle ∗ Corresponding author : [email protected] ha l-0 04 58 42 9, v er si on 1 5 M ar 2 01 0 Author manuscript, published in "Applied Physics Letters 93, 5 (2008) 051108" DOI : 10.1063/1.2965612