Sign-Reversal Coupling in Coupled-Resonator Optical Waveguide

TL;DR

This paper experimentally demonstrates that the coupling factor in coupled-resonator optical waveguides can reverse sign for certain resonance modes, leading to two distinct dispersion behaviors within the same frequency range.

Contribution

It extends the original CROW theory by showing the possibility of sign-reversal in the coupling factor for multipole resonance modes through experimental imaging.

Findings

Coupling factor can reverse sign in CROWs for specific modes.

Two different waveguiding dispersion curves are observed in the same frequency range.

Experimental confirmation via Bloch wavevector and frequency matching.

Abstract

Coupled-resonator optical waveguides (CROWs), which play a significant role in modern photonics, achieve waveguiding through near-field coupling between tightly localized resonators. The coupling factor, a critical parameter in CROW theory, determines the coupling strength between two resonators and the waveguiding dispersion of a CROW. However, the original CROW theory proposed by Yariv et al. only demonstrated one value of coupling factor for a multipole resonance mode. Here, by imaging the tight-binding Bloch waves on a CROW consisting of designer-surface-plasmon resonators in the microwave regime, we demonstrate that the coupling factor in the CROW theory can reverse its sign for a multipole resonance mode. This determines two different waveguiding dispersion curves in the same frequency range, experimentally confirmed by matching Bloch wavevectors and frequencies in the CROW. Our study supplements and extends the original CROW theory, and may find novel use in functional photonic systems.