ABSTRACT:
An intense femtosecond laser pulse propagating in a gas may collapse into one or multiple “filaments” when its peak power exceeds the critical power (5 – 10 GW in air) for nonlinear self-focusing. In atmosphere, the laser intensity is typically ~ 1013 W/cm2 in the filament, leaving a weakly-ionized plasma channel which can extend meters in length with a diameter of < 100 μm. While it has been generally accepted that laser filamentation is the consequence of self-focusing-induced beam collapse stabilized by plasma generation and de-focusing, neither the field-induced nonlinearity nor the plasma generation had been directly measured. This uncertainty has given rise to recent controversy about whether plasma generation does indeed counteract the positive nonlinearity, as an alternate theory suggests that the stabilization mechanism is contributed by saturation of optical nonlinearity.
For a basic understanding of femtosecond filamentation and for applications, the focusing and defocusing mechanisms – nonlinear self-focusing and ionization – must be understood. By employing a single-shot, time-resolved technique based on spectral interferometry to study the constituents of air, it is found that the rotational responses in O2 and N2 are the dominant nonlinear effect in filamentary propagation when the laser pulse duration is longer than ~ 100 fs. Furthermore, we find that the instantaneous nonlinearity scales linearly up to the ionization threshold, suggesting that an ionization-free, negative stabilization of filamentation does not exist. This is confirmed by space-resolved plasma density measurements in meter-long filaments using optical interferometry with a grazing-incidence probe laser pulse.