Phase evolution of terahertz radiation from femtosecond laser-induced air plasma.

The phase evolution of terahertz (THz) radiation from single-color femotsecond laser-induced air plasma controlled by a DC-bias is investigated experimentally. When the DC-bias is moved from the end to the beginning of the laser plasma filament, the produced THz waveform is advanced temporally, and its carrier-envelope phase is changed. Our phase spectrum analysis suggests that the slope and the intercept of the phase spectrum, respectively, determine the temporal shift and the carrier-envelope phase of the THz waveform. Therefore, the observed THz waveform evolution is mainly due to the THz propagation effect in plasma filament and the Gouy phase shift associated with the detection scheme. This Letter also illustrates explicitly the temporal order of THz radiation from different parts of a filament.