One-dimensional mapping of femtosecond laser filaments using coherent microwave scattering

TL;DR

This study demonstrates a novel method using coherent microwave scattering to perform one-dimensional, spatially resolved measurements of electron density in femtosecond laser filaments, revealing invariant electron density and intensity clamping conditions.

Contribution

The paper introduces a new technique for absolute, longitudinally resolved electron density measurements in laser filaments, enhancing understanding of filamentation dynamics.

Findings

Electron density remains constant (~2x10^{15} cm^{-3}) along the filament

Laser intensity in the middle of the filament is nearly constant (30-40 TW/cm^2)

Intensity clamping conditions are confirmed in the experiments

Abstract

This paper reports on the use of coherent microwave scattering (CMS) for spatially resolved electron number density measurements of elongated plasma structures induced at mid-IR femtosecond filamentation in air. The presented studies comprise one-dimensional mapping of laser filaments induced via 3.9 um, 127.3 fs laser pulses at output energies up to 15 mJ. The axial electron number density was measured to be invariant (about 2x10$^{15}$ cm$^{-3}$) along the entire filament length and for all tested laser pulse energies 5-15 mJ, and the corresponding laser intensity in the middle portion of the filament was estimated to be nearly constant for 5-15 mJ pulse energies (about 30-40 TW/cm$^2$). These fundings support that intensity clamping conditions were achieved in the experiments. The proposed approach enables capabilities that are currently unavailable to perform absolute and longitudinally resolved measurements of electron number density in laser filaments and to precisely characterize conditions associated with self-focusing and intensity clamping.