Analysis of metallized-grating coupled twin-waveguide structures

In this paper, we present an analysis of grating coupled twin-waveguide structures consisting of a semitransparent metal-low-index waveguide on top of a corrugated high-index laser waveguide. A rigorous Floquet analysis of TE modes is performed for trapezoid shaped gratings. Starting from on-resonant Floquet solutions, we establish a relation to simple coupled-mode equations yielding results, that show excellent agreement with the rigorous Floquet solutions. Grating coupled radiation plays a crucial role in the waveguide structures under consideration: when phase matching of the guided modes occurs, narrow-band resonant suppression of radiation loss with linewidths in the subnanometer regime can be achieved. The grating geometry has an important impact on these resonances, which is made evident by various numerical examples demonstrated in this paper