Propagation of terahertz wave inside femtosecond laser filament in air

Terahertz (THz) generation in femtosecond laser filament has recently been found to greatly expand THz applications in atmospheric remote sensing. In this article, by investigating the THz waveform emitted from different length of filaments, the refractive index of the THz wave has been found to be smaller than the unity within the filament region. It indicates that the THz pulse may propagate inside the filament though the diameter of the filament is much smaller than the wavelength of the THz wave. The hypothesis is supported by further numerical simulation, which considers the radially non-uniform plasma density distribution of the filament.