Metastability of Breather Modes of Time-Dependent Potentials

TL;DR

This paper investigates the metastability and decay of breather modes in time-dependent Schrödinger equations with localized, periodic potentials, combining analytical methods and numerical experiments to understand long-term behavior.

Contribution

It provides a rigorous analysis of the decay and frequency shift of breather modes under small periodic perturbations, extending existing theories to long-time asymptotics.

Findings

Breather modes are long-lived but eventually decay exponentially.

Decay rate is given by an analogue of Fermi's golden rule.

Frequency shifts occur on intermediate time scales.

Abstract

We study the solutions of linear Schroedinger equations in which the potential energy is a periodic function of time and is sufficiently localized in space. We consider the potential to be close to one that is time periodic and yet explicitly solvable. A large family of such potentials has been constructed and the corresponding Schroedinger equation solved by Miller and Akhmediev. Exact bound states, or breather modes, exist in the unperturbed problem and are found to be generically metastable in the presence of small periodic perturbations. Thus, these states are long-lived but eventually decay. On a time scale of order $\epsilon^{-2}$, where $\epsilon$ is a measure of the perturbation size, the decay is exponential, with a rate of decay given by an analogue of Fermi's golden rule. For times of order $\epsilon^{-1}$ the breather modes are frequency shifted. This behavior is derived first by classical multiple-scale expansions, and then in certain circumstances we are able to apply the rigorous theory developed by Soffer and Weinstein and extended by Kirr and Weinstein to justify the expansions and also provide longer-time asymptotics that indicate eventual dispersive decay of the bound states with behavior that is algebraic in time. As an application, we use our techniques to study the frequency dependence of the guidance properties of certain optical waveguides. We supplement our results with numerical experiments.