Fine Structure of a Laser-Plasma Filament in Air

The ability to select and stabilize a single filament during propagation of an ultrashort high-intensity laser pulse in air makes it possible to examine the longitudinal structure of the plasma channel left in its wake. We present detailed measurements of plasma density variations along laser propagation. Over the length of the filament, electron density variations of 3 orders of magnitude are measured. They display evidence of a meter-long postionization range, along which a self-guided structure is observed coupled with a low plasma density, corresponding to $\sim 3$ orders of magnitude decrease from the peak density level.

Figure 1

Schematic of experimental setup.

Figure 2

Breakdown voltage ratio calibration. The right scale shows the intensity dependence of the ratio parameter $h$. The left scale shows the variation in electron density, using $K=8$.

Figure 3

Variation in electron density of a controlled filament formed by a 266 GW laser pulse. The $y$ axis is normalized to the peak electron density along the filament. The trend line is added to guide the eye.