Kramers-Kronig relations and the analogy between electromagnetic and mechanical waves

TL;DR

This paper explores the Kramers-Kronig relations linking dissipation and dispersion in media, demonstrates mathematical proofs in electromagnetism, extends the analogy to mechanics, and introduces a numerical algorithm using FFT.

Contribution

It provides two new demonstrations of the Kramers-Kronig relations, extends the relations to elastic media via analogy, and offers a novel FFT-based numerical computation method.

Findings

Dissipative media imply dispersion due to KK relations.

Electromagnetic and mechanical wave analogies are established.

A new FFT-based algorithm for numerical KK relation computation is introduced.

Abstract

The important consequence of the Kramers-Kronig relations (KKrs) is that dissipative behavior in material media inevitably implies the existence of dispersion, i.e., a frequency dependence in the constitutive equations. Basically, the relations are the frequency-domain expression of causality and correspond mathematically to pairs of Hilbert transforms. The relations have many forms and can be obtained with diverse mathematical tools. Here, two different demonstrations are given in the electromagnetic case, illustrating the eclectic mathematical apparatus available for this purpose. Then, we apply the acoustic (mechanical)-electromagnetic analogy to obtain the elastic versions. Finally, we discuss the concepts of stability and passivity and provide a novel algorithm to compute the relations numerically by using the fast Fourier transform.