General properties of retardation-based resonances involving slow surface plasmon-polariton (SPP) modes supported by metal nanostructures are considered. Explicit relations for the dispersion of SPP modes propagating along thin metal strips embedded in dielectric and in narrow gaps between metal surfaces are obtained. Strip and gap subwavelength resonant structures are compared with respect to the achievable scattering and local-field enhancements lending thereby their distinction as nano-antennas and nano-resonators, respectively. It is shown that, in the limit of extremely thin strips and narrow gaps, both structures exhibit the same Q factor of the resonance which is primarily determined by the complex dielectric function of metal.
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