Understanding and controlling on-axis and off-axis THz emission patterns from 2-color femtosecond laser filaments

Gas-plasma strings from 2-color femtosecond laser filaments constitute one of the most convenient and promising sources of pulsed terahertz (THz) radiation. It is one of the approaches that allow one to achieve at the same time ultrabroadband THz spectra and very high amplitudes of THz electric fields. Such sources present major advantages for applications of sensing, spectroscopy, imaging, and nonlinear optics. In view of the numerous applications an understanding of the spatial distribution of the terahertz radiation, from this source, is crucial. One of the first works towards this direction [1], revealed a highly directional THz radiation while a more recent publication [2] proposes a donut-shape spatial pattern. The most recent results reported by Kim et.al. [3], propose an off-axis phase matched emission from the filament due to interference effect between the different point sources across the filament, resulting thus in a donut-shape profile. A simple theoretical model was also proposed in [3], which is though insufficient to explain all observations.