Waveform and frequency evolution of spatially resolved terahertz radiation along two-color filament in air

The evolution of THz waveform and frequency generated from the two-color air filament was experimentally investigated by moving an iris along the filament. With the increase of the filament length, the peak-to-peak amplitude of THz signal existed an oscillating behavior which is attributed to the relative phase between the two-color laser pulses and the coherent interference of all local sources along the effective filament. By taking the differentiation of the measured THz waveforms, the spatially resolved THz waves were obtained. Two longitudinal positions along the filament were found where the local THz pulse undergoes a remarkable phase shift. Theoretical deduction indicates that the phase shift are caused by the dispersion and Gouy phase in the filament. Through longitudinal resolved THz time domain spectroscopy along two-color laser filament, an electron density peak around geometrical focus was observed.