Fig. 1. Spatial distribution images of filaments obtained by propagating a laser pulse with an energy of 10 mJ in the air after adding three-leaf or five-leaf diaphragm^[65]. (a) Filamentation pattern obtained by three-leaf diaphragm; (b) filamentation pattern obtained by five-leaf diaphragm; (c) light intensity (continuous curves, left axis) at the point where the light intensity of the filament is most vigorous and the corresponding electron density (dashed line, right axis), where delicate curve represents three-leaf membrane and thick curve represents five-leaf membrane

Fig. 2. Evolution of femtosecond laser propagation in air and gaseous lattice media structures^[69]. (a) Three-dimensional isoenergy densities of femtosecond laser pulse propagation in y-z plane for different gaseous lattice lengths [ξ=0 m (no lattice), ξ=1 m, ξ=2 m, and ξ=3 m]; (b) cross-sectional intensity distributions of laser pulse at z=1.5 m under lattice-free (i,i′,i″) and lattice (ii, ii′, ii″) conditions, where figures (i) and (ii) are spatiotemporal intensity distributions of laser pulse, figures (i′) and (ii′) are cross-sectional instantaneous intensity distributions (at 0 fs) of central part of laser pulse, and figures (i″) and (ii″) are transverse section hot spot profile; (c) distribution of "hot spots" at distances z=2.1, 2.7, 3.3, 3.9 m for ξ=3 m

Fig. 3. Experimental setup for capturing filament profiles by a digital camera^[78] (photos have to be taken in two directions due to asymmetric focusing geometry, i.e. horizontal and vertical direction, which are labeled as x and y, respectively)

Fig. 4. Focal distribution of nonlinear propagation with different ellipticities and powers^[81] (intensity unit: W/cm²)

Fig. 5. Linear polarization with changing azimuth (topological charges m=1, 2) induces filamentation behavior under three different initial phase (δ₀=0, π/4, π/2) conditions^[86]

Fig. 6. Measured six consecutive 10-time-averaged CCD images of two azimuthally varying mixed polarization vector field filaments at 2.0 μJ pulse energy^[87]. (a) Topological charge m=1; (b) topological charge m=2

Fig. 7. Cross-sectional view of filament under different conditions^[92]. (a) Cross-sectional view of filament before narrowing; (b) cross-sectional view of filament after narrowing with telescopic system

Fig. 8. Cross-sectional views of filament at 2 cm in front of focusing lens (diaphragm aperture is 3, 6, and 13 mm, respectively, and incident pulse energy is 1.35, 3.76, and 6.07 mJ, respectively)^[96]