Structural Slow Waves: Parallels between Photonic Crystals and Plasmonic Waveguides

The slowdown of propagating electromagnetic waves by engineering waveguide structures is receiving considerable attention in nanophotonics. Two types of structures are independently investigated, namely, diffraction-limited photonic-crystal waveguides and highly confined plasmonic waveguides. Here we propose a unified viewpoint on slow waveguide structures by analyzing both approaches within a single theoretical framework. This allows us to clarify the physical mechanisms underlying slowness—thereby highlighting their inherent differences—and to remove some inaccurate ideas on the topic. We further discuss the strengths and weaknesses of plasmonic and photonic slow waveguides, assess the technical and scientific challenges recently solved, and critically identify those that still stand in the way.