Ultrafast imaging the light-speed propagation of a focused femtosecond laser pulse in air and its ionized electron dynamics and plasma-induced pulse reshaping

The light-speed propagation of a focused femtosecond (fs) laser pulse in air was recorded by a pump–probe shadowgraph imaging technique with femtosecond time resolution. The ultrafast dynamics of the laser-ionized electrons were studied, which revealed a strong reshaping of the laser field due to laser–air nonlinear interaction. The influence of laser fluence and focusing conditions on the pulse reshaping was studied, and it was found that: (1) double foci are formed due to the refocusing effect when the laser fluence is higher than 500 J/cm² and the focusing numeric aperture (NA) is higher than 0.30; and (2) a higher NA focusing lens can better inhibit the prefocusing effect and nonlinear distortion in the Gaussian beam waist.