Interaction between negative and positive index medium waveguides

TL;DR

This paper theoretically investigates the coupling phenomena between negative and positive index medium waveguides, revealing unique backward guiding modes and a new exponentially varying coupled mode solution with high coupling efficiency.

Contribution

It develops and verifies a coupled mode theory for negative-positive index waveguides, discovering new solutions and phenomena not previously reported.

Findings

Coupling can achieve 100% efficiency with long enough coupling length.

Backward power output occurs when propagation constants differ greatly.

New exponential coupled mode solution identified in nearly identical propagation constants.

Abstract

The coupling between negative and positive index medium waveguides is investigated theoretically in this paper. A coupled mode theory is developed for such a waveguide system and its validity is verified. Interesting phenomena in the coupled waveguides are demonstrated, which occur in the case when the negative index medium waveguide in isolation guides its mode backward. A new type of coupled mode solution that varies exponentially with the coupling length is found in the special case when the propagation constants of two individual waveguides are nearly the same. A coupler operating in this case is insensitive to the coupling length, and its coupling efficiency can reach 100% as long as the coupling length is long enough. However, when the propagation constants of the two individual waveguides differ greatly, the coupled mode solution is still a periodic function of the coupling length, but the coupled power is output backward. In addition, the modes in the composite waveguide system are also studied using the coupled mode theory, and their fundamental properties are revealed.