Ionization rate and plasma dynamics at 3.9 micron femtosecond photoionization of air

TL;DR

This paper measures ionization rates and plasma dynamics during 3.9 micron femtosecond laser filamentation in air using microwave scattering, providing precise data crucial for understanding mid-IR laser-matter interactions.

Contribution

It introduces a microwave scattering method to accurately measure photoionization rates and plasma parameters in mid-IR femtosecond filamentation, advancing the understanding of laser-induced ionization processes.

Findings

Photoionization rates range from 5.0x10^8 to 6.1x10^9 s^-1.

Ionization occurs at intensities between 1.3x10^13 and 1.9x10^14 W/cm^2.

Method enables precise tabulation of ionization rates for various gases and intensities.

Abstract

The introduction of mid-IR optical parametric chirped pulse amplifiers (OPCPAs) has catalyzed interest in multi-millijoule, infrared femtosecond pulse-based filamentation. As tunneling ionization is a fundamental first stage in these high-intensity laser-matter interactions, characterizing the process is critical to understand derivative topical studies on femtosecond filamentation and self-focusing. Here, we report constructive-elastic microwave scattering-based measurements of total electron count, electron number densities, and photoionization rates generated by 3.9 micron femtosecond mid-infrared tunneling ionization of atmospheric air. Consequently, we determine photoionization rates in the range of 5.0x10$^{8}$-6.1x10$^{9}$ s$^{-1}$ for radiation intensities 1.3x10$^{13}$-1.9x10$^{14}$ W/cm$^{2}$, respectively. The proposed approach paves the wave to precisely tabulate photoionization rates in mid-IR for broad range of intensities and gas types and to study plasma dynamics at mid-IR filamentation.