Fig. 1. Diagram of the setup for femtosecond laser filamentation and nitrogen fluorescence detection. QWP, quarter-wave plate; Mirror 1 and Mirror 2, broadband dielectric mirrors; Lens, UV-fused silica plano-convex lens (AR coated: 245–400 nm); PMT, photomultiplier tube.

Fig. 2. False-color maps and fluorescence intensity curves illustrating the variation in fluorescence intensity of neutral nitrogen molecules with propagation distance and the quarter-wave plate (QWP) rotation angle. The QWP angles of 0° (and 180°), 45°, 90°, and 135° correspond to horizontal linear polarization, left-handed circular polarization, vertical linear polarization, and right-handed circular polarization, respectively. (a) The QWP rotation angle varies from 0° to 90°. (b) The QWP rotation angle varies from 90° to 180°. Variation in the intensity of nitrogen fluorescence signals excited by the filament at a laser propagation distance of (c) 30 m, (d) 30.1 m, and (e) 30.2 m as a function of the polarization state (QWP rotation angle: 0°–180°).

Fig. 3. Theoretical calculation of polarization-dependent characteristics. (a) Under the intensity clamping condition (I=5×1013  W/cm2), the tunneling ionization rate of nitrogen molecules varies with the polarization state. (b) The self-focusing threshold and filament length in air as functions of the polarization state (angle of the quarter-wave plate, QWP).

Fig. 4. (a) Extracting filament starting point and length information from ultrasonic signal images. The red line (3σ) represents 3 times the standard deviation of the blank sample. (b) Variation of the filamentation starting point under different polarization states (QWP rotation angle: 0°–180°). (c) Variation of the filament length in different polarization states (QWP rotation angle: 0°–180°). (d) Comparison of theoretical and experimental results for the variation in the total integrated fluorescence intensity with the polarization state.