Azimuthal Modulational Instability of Vortices in the Nonlinear Schr\"odinger Equation

TL;DR

This paper analyzes the azimuthal modulational instability of vortices in the 2D focusing nonlinear Schrödinger equation, combining analytical predictions with numerical simulations to understand vortex stability and potential stabilization methods.

Contribution

It introduces a novel stability analysis method based on a quasi-one-dimensional azimuthal equation and provides analytical and numerical insights into vortex stability in the NLS.

Findings

Vortices are unstable below a critical azimuthal wave number.

The stability analysis accurately predicts growth rates of unstable modes.

Numerical simulations confirm the analytical stability predictions.

Abstract

We study the azimuthal modulational instability of vortices with different topological charges, in the focusing two-dimensional nonlinear Schr{\"o}dinger (NLS) equation. The method of studying the stability relies on freezing the radial direction in the Lagrangian functional of the NLS in order to form a quasi-one-dimensional azimuthal equation of motion, and then applying a stability analysis in Fourier space of the azimuthal modes. We formulate predictions of growth rates of individual modes and find that vortices are unstable below a critical azimuthal wave number. Steady state vortex solutions are found by first using a variational approach to obtain an asymptotic analytical ansatz, and then using it as an initial condition to a numerical optimization routine. The stability analysis predictions are corroborated by direct numerical simulations of the NLS. We briefly show how to extend the method to encompass nonlocal nonlinearities that tend to stabilize solutions.