Light Trapping in Finite Arrays of Metallic U-Shaped Cavities for Sensing With High Figures of Merit

We report conditions for light trapping in subwavelength metallic U-shaped cavities. Full and partial light trapping exhibiting optical power flow vortices originate in coupling of surface plasmons into cavity modes. Localized surface plasmons coupled into cavity modes realize full light trapping, whereas propagating surface plasmons coupled into cavity modes result in partial light trapping. For full light trapping, an intense power flow is entirely confined in the cavities, reaching its maximum values on the cavities' surfaces. The full light trapping resonance realizes high sensitivity and figure of merit in structures as short as five adjacent cavities.