Tracing Air-Breakdown Plasma Characteristics from Single-Color Filament Terahertz Spectra

In this paper, we measure broadband terahertz spectra from a single 744-nm filament produced in tight, medium, and loose geometrical focusing conditions. Terahertz spectra are measured using interferometer coupled to a helium-cooled bolometer avoiding any cutoff frequencies. Based on THz spectrum maximum position and spectral width, we estimate electron-heavy particle (neutrals and ions) collision rate. Numerical simulations are performed using the state-of-the-art model of unidirectional femtosecond pulse propagation and convergence/divergence of optical radiation at large angles.