Influence of a gas flow on THz generation from optical air-breaking filament

We perform THz generation by optical breaking in air, with a helium gas flow blown perpendicularly to optically induced filament. The gas flow modifies both the magnitude and polarization of the radiated THz field, due to 1) the different optical/THz properties of air and helium; 2) the weaker optical/THz generation efficiency of helium; 3) the different nonlinear optical properties of both gases. We develop a model that fits the experimental data and permits us to estimate the second order nonlinear dispersion of He relatively to the one of air.