Bound electron nonlinearity beyond the ionization threshold

TL;DR

This paper reports the first absolute measurements of bound electron nonlinear polarizability in gases under high-intensity laser fields, revealing a linear relationship beyond the ionization threshold, which is vital for understanding ultrashort pulse propagation.

Contribution

It provides the first absolute, space- and time-resolved measurements of nonlinear polarizability in gases at ionization levels, extending understanding beyond the ionization threshold.

Findings

Bound electron nonlinearity is approximately linear with intensity beyond ionization.

Measurements cover argon, krypton, xenon, nitrogen, and oxygen gases.

Results enable better modeling of high-intensity laser-matter interactions.

Abstract

Although high field laser-induced ionization is a fundamental process underlying many applications, there have been no absolute measurements of the nonlinear polarizability of atoms and molecules in the presence of ionization. Such information is crucial, for example, for understanding the propagation of high intensity ultrashort pulses in matter. Here, we present absolute space- and time-resolved measurements of the ultrafast laser-driven nonlinear polarizability in argon, krypton, xenon, nitrogen, and oxygen up to an ionization fraction of a few percent. These measurements enable determination of the non-perturbative bound electron nonlinearity well beyond the ionization threshold, where it is found to be approximately linear in intensity.