Completeness and divergence-free behavior of the quasi-normal modes using causality principle

TL;DR

This paper develops a causality-based method to ensure the completeness and divergence-free behavior of quasi-normal modes in open resonators, validated through a Lorentz model resonator example.

Contribution

It introduces a causality principle-based derivation that guarantees the completeness of QNMs expansion and prevents divergence in their field profiles.

Findings

QNM expansion matches exact field distribution in a Lorentz model resonator

The method ensures divergence-free QNM fields

Completeness of QNMs is theoretically validated

Abstract

A fundamental feature of the quasi-normal modes (QNMs), which describe light interaction with open (leaky) systems like nanoparticles, lies in the question of the completeness of the QNMs representation and in the divergence of their field profile due to their leaky behavior and complex eigenfrequency. In this article, the QNMs expansion is obtained by taking into consideration the frequency dispersion and the causality principle. The derivation based on the complex analysis ensures the completeness of the QNMs expansion and prevents from any divergence of the field profile. The general derivation is tested in the case of a one-dimensional open resonator made of a homogeneous absorptive medium with frequency dispersion given by the Lorentz model. For a harmonic excitation, the result of the QNMs expansion perfectly matches the exact formula for the field distribution outside as well as inside the resonator.