Post-Ionization Medium Response to a Femtosecond Laser Pulse*

When an intense, femtosecond laser pulse ionizes an atmospheric-pressure gas, the optical response of the medium differs drastically from that of a regular weakly-ionized plasma. The initial charge distribution resulting from ionization is microscopically inhomogeneous, with the average electron density of 10$^{15}$-10$^{16}$ cm$^{-3}$. We considered the oscillations of virtually isolated and expanding electron clouds forced by the laser electric field. A simple analytical model predicts the amplitude of these forced oscillations as a function of time. This amplitude, as well as the related polarization of the medium, undergoes considerable enhancement when the system evolves through the transient resonance with the laser carrier frequency. The results impact the currently accepted picture of laser filamentation dynamics and call for modifications in existing theoretical models.