Postionization medium evolution in a laser filament: A uniquely nonplasma response.

Theoretical consideration of the optical response of nascent free electrons in the process of laser filamentation reveals that the initial microscopically inhomogeneous charge distribution causes a transient electromagnetic response of the medium that differs drastically from that of a homogeneous plasma with the same degree of ionization. An analytical model, describing the forced oscillations of virtually isolated and expanding electron clouds, predicts considerable enhancement of these oscillations caused by transient resonance with the laser field. The transient resonance processes should play a role in the currently accepted picture of filament formation dynamics.