Fig. 1. Schematic of the experimental setup. The QWP is a zero-order rotating quarter-wave plate. Lens 1–Lens 4 are UV-grade fused silica lenses with focal lengths of 30 (50), 20, 10, and 8 cm, respectively.

Fig. 2. (a) Measured light ellipticity for an initial LP pulse of 3.94 mJ and 4.69 mJ as a function of the rotating angle of the QWP. The red line shows the theoretical ellipticity evolution of an LP pulse after passing through a QWP. (b) Real-color images of a laser filament in air under different ellipticities. The filamenting pulse energy was 4.58 mJ and the focal length of the plano-convex lens was 50 cm. The vertical green line showed the onset position of the filament with a linear polarization pulse and the horizontal green lines were the top and bottom of the rectangular area of images that was integrated to calculate the fluorescence intensity distribution along the plasma channel.

Fig. 3. (a) Onset positions of filamentation and (b) the length of filaments as a function of the QWP rotation angle. (c) Onset positions of the filamentation and (d) the length of filaments as a function of the pulse energy at three different polarization states. The effective length of the filaments was defined as the full width of the fluorescence intensity curve at 3σ, where σ is the standard deviation of the noise. The filament onset position was fixed at the position where the fluorescence intensity reached 3σ at the leading edge of the filaments. When the pulse polarization is LP, the filament onset position is defined as “0”. The other positions after the defined “0” position are positive, and the opposite positions before the defined “0” position are negative, which means the corresponding filament onset positions are in front of the defined “0” position and closer to the focusing lens.

Fig. 4. Energy deposition ratio as a function of the rotation angle of the QWP under different filamenting pulse energies. The focal length of the focusing lens is 50 cm and the laser pulse duration is 30 fs.

Fig. 5. (a) Side fluorescence spectra in the range of 330–430 nm emitted by air filaments pumped by LP, elliptical polarization, and CP pulses, respectively. (b) 337 nm and 391 nm peak signals as a function of the rotation angle of the QWP together with the side-scattered peak intensity of the filamenting pulse centered at 800 nm. The focal length for filamentation is 30 cm. The laser pulse duration is 30 fs and the pulse energy is 3.50 mJ.

Fig. 6. Ratios of single-ionization rates in air for an elliptically polarized pulse over an LP pulse as a function of the ellipticity under different laser intensities.

Table 1. Ionization Energy and the Cl Coefficients for Diatomic Molecules