Distributed feedback gratings for surface-plasmon polaritons based on metal nano-groove/ridge arrays

The existence of gap states within the first stop band is theoretically investigated for surface-plasmon polaritons propagating along metal surfaces structured with finite arrays of nano-grooves/ridges with imperfections. By means of rigorous calculations of all scattering channels on the basis of the reduced Rayleigh equation, it is shown that a mismatch of the grating period, rather than simple vacant/defect sites, favors the opening of gap states (located at midgap for half-period shifts) as a phase step does in distributed feedback gratings. Quality factors of the resulting surface-plasmon polaritons' resonances are limited by radiative leakage and ohmic losses.