Coupling configurations between extended surface electromagnetic waves and localized surface plasmons for ultrahigh field enhancement

Local enhancement of electromagnetic (EM) fields near dielectric and metallic surfaces is usually associated with the existence of a confined EM wave at least in one direction. This phenomenon finds applications in enhancing optical spectroscopic signals, optical emission, nonlinear optical processes, biosensing, imaging contrast and superresolution, photovoltaics response, local heating, photocatalysis, and enhanced efficiency of optoelectronic devices. A well-known example is when the surface electromagnetic wave (SEW) is excited at the interface of two media, the field gets enhanced normally to that interface. This article reviews the different configurations revealing enhanced EM fields, particularly those giving ultrahigh enhancement, such as when a localized SEW is excited not from free space but via an extended SEW. Of particular interest are surface plasmon waves (SPWs) excited at the surface of metal-dielectric and particularly when exciting localized SPWs using extended ones. The latter case so far gave the highest local field enhancement; however, configurations involving Bloch SEWs, guided mode resonances, and cavity resonances have also been shown to give significant enhancement when used to excite localized surface plasmons. With this strategy, field enhancement by more than an order of magnitude can be attained. Using this ultrahigh enhancement, the strong coupling experiments between molecules and the intense optical field will be possible and new devices may emerge from those new methodologies for ultrahigh sensitive sensing for environmental and medical applications, as well as for improved optoelectronic devices.