Resonances in metal nanostructures supporting propagation of slow surface plasmon-polariton (SPP) modes are considered. Resonant enhancements in scattering and built-up electric fields of nm thin submicron-wide metal strips are investigated using a surface integral equation method and ascribed to the excitation of strip-cavity modes formed by short-range SPPs. A properly positioned narrow gap $\left(5\mathrm{nm}\right)$ in the strip is found to lead, by virtue of boundary conditions, to significant field enhancement ($\sim 30$, inside the gap), as well as to a noticeable $(\sim 10\%)$ blueshift in the resonance wavelength.

• Received 7 December 2006

DOI:https://doi.org/10.1103/PhysRevB.75.073402