Broad periodic waveguides: Slow light and its reconciliation with cavity modes

A “slow” high-order mode in a photonic crystal waveguides can be exploited using conversion from a “fast” fundamental mode to achieve demultiplexing. A further interesting slow-light regime of more general broad periodically corrugated waveguides with many modes will be shown: it arises from higher-order modes coupled among themselves and hinges on the familiar Littrow diffraction geometry. We will show some of its less obvious features, notably the “critical coupling” regime for maximal slow down. This “critical coupling” regime induces modes whose transmission closely follows that of multiple Fabry-Perot resonators. Experiments on broad Silicon-on-Insulator waveguides evidencing this regime are shown. We argue that this system reconciles cavity vs. band-edge enhancement and is dual to the popular "CROW" introduced by Yariv in 1999.