… We present direct time- and space-resolved measurements of the electron density of
femtosecond laser pulse-induced plasma filaments. The dominant nonlinearity responsible
for extended atmospheric filaments is shown to be field-induced rotation of air molecules. …
In conclusion, we have presented the first direct space and time-resolved electron density
measurements for a nonrelativistic femtosecond laser pulse nonlinearly filamenting in gas,
here the atmosphere. This has allowed a detailed elucidation of the nonlinear physics …