The femtosecond laser ionized air can form a filament of a length of centimeter, and broadband THz wave can be radiated from the filament. The plasma density at both ends of the filament is different, and the phase of the radiated terahertz will be different.

So, which factor can change the phase of the THz waveform? What is the physical mechanism behind the change of the phase of the THz waveform?