Fig. 1. Femtosecond time-resolved pump-probe shadow imaging experimental device. (a) Optical path of the pump-probe shawdow graphy; (b) Plasma self-luminescence signal directly collected when there is only pump light; (c) Plasma shadow signal collected in the background of probe light

Fig. 2. Plasma shadow image at 100 fs moment with focal autofluorescence imaging. (a) Shadow image with a time delay of 100 fs; (b) Corresponding background image; (c) Background image with ambient light only; (d) Color iso-intensity distribution of the lateral profile of the focal autofluorescence after focusing the microscope objective of the femtosecond laser taken at atmospheric pressure

Fig. 3. Femtosecond time-resolved air plasma shadow change image at 20 times of microscopic objective and NA=0.4

Fig. 4. Evolution of plasma shadowing process. (a) Curve of the position of the plasma front with time; (b) Curve of the length of the plasma shadow with time; (c) Velocity of the dissociated wave front of the air plasma at different time periods

Fig. 5. Time-dependent evolution of the peak and average values of the transient electron density in a femtosecond laser-induced air ionization plasma

Fig. 6. Distribution of relative electron density positions on the axis at different moments. (a) Density distribution of electron density on the axis at 12 fs, 54 fs, 82 fs moments; (b) Density distribution of electron density on the axis at 122 fs, 155 fs, 221 fs moments

Fig. 7. Electron density variation versus normalized laser pulse time for 20× versus 40× focused ionization at single pulse energies of 160 μJ, 320 μJ, and 640 μJ. (a) Electron density variation of ionization under 20× focusing conditions; (b) Electron density variation of ionization under 40× focusing conditions

Fig. 8. Evolution of the free electron density at the center of the focal point with time and the normalized femtosecond laser pulse intensity distribution when a laser pulse with a single pulse energy of 160 μJ is focused into the air by 20× and 40× objective lenses. (a) Relationship between electron number density and normalized pulse intensity under 20× focusing condition; (b) Relationship between electron number density and normalized pulse intensity under 40× focusing condition

Fig. 9. The diffusion process of free electron density with time at the center of the focus when a laser pulse with a single pulse energy of 160 μJ is focused into the air by 20× and 40× objectives. (a) The diffusion process of free electron density with time under 20× focusing condition; (b) The diffusion process of free electron density with time under 40× focusing condition