Waveform, frequency and polarization evolution of terahertz radiation along two-color filament in air

The evolution of THz waveform, frequency and polarization generated from the different two-color air filament length was experimentally investigated by moving an iris along the filament. By scanning the iris from the beginning to the end of filament, THz waveform with clear phase and amplitude changes were observed, which leads to the polarization trajectory changing from larger amplitude, smaller ellipticity to smaller amplitude, larger ellipticity. Through THz domain spectroscopy, an electron density peak around geometrical focus was observed. The experimental observations are in agreement with numerical simulation results which is attributed to the relative phase between the two-color laser pulses while propagating in the plasma filament and the coherent interference of all local sources along the effective filament.