Simple examples of perfectly invisible and trapped modes in waveguides

TL;DR

This paper demonstrates how specific symmetric waveguide geometries can achieve perfect invisibility and support trapped modes by tuning the waveguide's branch length, revealing new insights into wave propagation control.

Contribution

It introduces simple waveguide geometries where tuning the branch length L achieves perfect invisibility and trapped modes, expanding understanding of waveguide mode manipulation.

Findings

Existence of geometries with transmission coefficient equal to one

Support for trapped modes with finite energy solutions

Ability to tune waveguide parameters for desired wave behavior

Abstract

We consider the propagation of waves in a waveguide with Neumann boundary conditions. We work at low wavenumber with only one propagating mode in the leads, all the other modes being evanescent. We assume that the waveguide is symmetric with respect to an axis orthogonal to the longitudinal direction and is endowed with a branch of height $L$ whose width coincides with the wavelength of the propagating modes. In this setting, tuning the parameter $L$, we prove the existence of simple geometries where the transmission coefficient is equal to one (perfect invisibility). We also show that these geometries, for possibly different values of $L$, support so called trapped modes (non zero solutions of finite energy of the homogeneous problem) associated with eigenvalues embedded in the continuous spectrum.