Microscopic model for the higher-order nonlinearity in optical filaments

TL;DR

This paper presents a solvable atomic model to explain high-order nonlinear effects in optical filaments, linking virtual transitions and continuum states to observed intensity-dependent focusing behaviors.

Contribution

It introduces a theoretical model that connects high-order nonlinearity in optical filaments with continuum state transitions, explaining experimental phenomena.

Findings

Qualitative agreement with experimental cross-over from self-focusing to de-focusing

High intensities induce significant ionization effects

Nonlinear refraction exhibits strong memory effects

Abstract

Using an exactly soluble one-dimensional atomic model we explore the idea that the recently observed high-order nonlinearity in optical filaments is due to virtual transitions involving the continuum states. We show that the model's behavior is qualitatively comparable with the experimentally observed cross-over from self-focusing to de-focusing at high intensities, and only occurs at intensities which result in significant ionization. Based on these observations, we conjecture that this continuum electron nonlinear refraction exhibits strong memory effects, and most importantly, the change of its sign is effectively masked by the de-focusing due to free electrons.