Surface-plasmon-polariton (SPP) resonators consisting of metal strips in free space, and gap plasmon polariton
resonators consisting of a metal strip close to either a block of metal or a metal surface, are studied as optical
resonators. The analysis is performed using the Green's function surface integral equation method. For strips
in free space, we show how the scattering resonances can be understood, by thinking of the strips as optical
resonators for short-range SPPs. The two gap resonator configurations, strip-block and strip-surface, have
different structure terminations as the width of the strip and the block are identical whereas the surface is
infinite. In the strip-surface configuration, the scattering resonances are broader and red-shifted, compared to
the strip-block configuration. This is explained as a consequence of the effective length of the resonator being
larger in the strip-surface configuration. By varying the gap size, we study the transition from a SPP resonator
to a gap plasmon polariton resonator. In the strip-surface configuration, light can be scattered into both out-of-plane propagating waves and into SPPs that propagate along the surface. For small gaps of a few tens of
nanometers, a large enhancement in the scattering cross section is seen due to strong scattering into SPPs.
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