Fig. 1. (a) Schematic of the experimental setup. Below: Image of the filament fluorescence record by a CCD camera. (b) The x and y components of the generated THz amplitude. (c) The polarization angle and (d) the ellipticity expressed as the ratio b/a versus the phase delay of the two-color pulses. In (c)–(d) each point is an average of four groups of data, and the error bars show the rms error. (e) Measured THz wave components for the relative phase θ0=0 (in black and red dots), and the THz wave fitted by the subcycle pulse expression (in blue and pink lines). Inset, left: temporal evolution of THz field of the experimental data (blue line), and the fitted results (black line); right: the calculated angle-resolved THz power distribution of the measured results.

Fig. 2. (a) Measured THz waveforms as the filament length increased for the initial relative phase θ0=−0.5π, the colorbar shows the amplitude of the THz waves. (b) Modulation of the THz amplitude as the function of the filament length, taken from (a) at the delay time of 0 ps. (c) Polarization angle and (d) ellipticity of the constructed THz field versus the filament length. Inset shows the temporal evolution of the THz field for 6.5 mm-long filament.

Fig. 3. THz waves generated from each section of the filament by differentiating the measured THz waves for the initial relative phases (a) θ0=−0.5π and (b) θ0=0, respectively; the calculated THz waves at the filament positions for the different initial relative phases (c) θ0=−0.3π and (d) θ0=0.2π, respectively. The color bar shows the amplitude of the THz waves.

Fig. 4. (a) Total relative phase shift Δθtot (black solid line), linear phase shift due to the linear refractive index Δθlin (pink dash line), and phase shift response for the Gouy phase shift ΔθGouy (real dash line). (b) Phase shift due to the refractive index changes caused by the Kerr nonlinearity ΔθKerr (blue dash line) and the plasma formation Δθplas (green solid line).