Exceptional Points of Degeneracy with Indirect Bandgap Induced By Mixing Forward and Backward Propagating Waves

TL;DR

This paper reveals that exceptional points of degeneracy can occur in coupled waveguides without gain or loss, by coupling forward and backward waves, leading to zero group velocity and enabling photonic indirect bandgaps.

Contribution

It introduces a novel mechanism for achieving EPDs through mode coupling of forward and backward waves, without the need for gain/loss, and provides a design framework.

Findings

EPDs occur in coupled waveguides with forward and backward waves

Group velocity vanishes at the EPDs

Photonic indirect bandgaps can be realized using this mechanism

Abstract

We demonstrate that exceptional points of degeneracy (EPDs) are obtained in two coupled waveguides without resorting to gain and loss. We show the general concept that modes resulting from a proper coupling of forward and backward waves exhibit EPDs of order two and that there the group velocity vanishes. We verify our insight by using coupled mode theory and also by fullwave numerical simulations of light in a dielectric slab coupled to a grating, when one supports a forward wave whereas the other (the grating) supports a backward wave. We also demonstrate how to realize photonic indirect bandgaps in guiding systems supporting a backward and a forward wave, show its relations to the occurrence of EPDs, and offer a design procedure.