High-intensity laser pulses self-focus as they travel in air. But instead of completely collapsing, it ionizes the air to eventually form a self-guided beam called a filament, followed by a plasma channel in its trail. The ionized plasma channel characteristics are determined by the initial conditions, such as the laser energy and external focusing. These are crucial to many filamentation applications such as guiding microwaves for telecommunication. Many table-top experiments are limited in revealing the plasma properties in the tight— or linear — focusing regime, which could be different from the plasma produced in the loosely focused — or nonlinear — regime that could lead to the long-range, real-life applications.

Reyes et al. conducted a systematic study of the laser filament plasma dynamics in the two focusing regimes. They revealed the influence of input energy and external focusing on the laser-produced plasma density, diameter and lifetime in different regimes for a single filament formation.

Interferometry allowed the authors to determine changes in refractive index, indirectly obtaining the spatial and temporal measurements of the electron density in a single filament. They investigated the interplay between the initial laser conditions and the external focusing optics that trigger filamentation.

The experimental results confirmed that the loose focusing induced a plasma channel with a “clamped” electron density that remained unchanged as the laser power increased. The tight focusing, meanwhile, produced a larger electron density that increased with the input energy. Understanding the differences of the plasma characteristics between the two focusing regimes could help advance the long-range, real-life applications of laser filaments.

Source: “Transition from linear- to nonlinear-focusing regime of laser filament plasma dynamics,” by Danielle Reyes, Matthieu Baudelet, Martin Richardson, and Shermineh Rostami Fairchild, Journal of Applied Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5027573.