Adiabatic Floquet model for the optical response in femtosecond filaments

TL;DR

This paper introduces an improved model for femtosecond filamentation that incorporates multiphoton resonance effects using non-Hermitian Floquet theory, enhancing accuracy over the standard phenomenological approach.

Contribution

It proposes a modified standard model including resonance effects via non-Hermitian Floquet theory, aligning well with quantum simulations in the adiabatic regime.

Findings

Resonance-induced plasma dominates nonlinear index saturation.

The augmented model agrees with quantum simulations.

Easy integration into existing filament simulation codes.

Abstract

The standard model of femtosecond filamentation is based on phenomenological assumptions which suggest that the ionization-induced carriers can be treated as free according to the Drude model, while the nonlinear response of the bound carriers follows the all-optical Kerr effect. Here, we demonstrate that the additional plasma generated at a multiphoton resonance dominates the saturation of the nonlinear refractive index. Since resonances are not captured by the standard model, we propose a modification of the latter in which ionization enhancements can be accounted for by an ionization rate obtained from non-Hermitian Floquet theory. In the adiabatic regime of long pulse envelopes, this augmented standard model is in excellent agreement with direct quantum mechanical simulations. Since our proposal maintains the structure of the standard model, it can be easily incorporated into existing codes of filament simulation.