Localized Lattice Resonance Near Surface Pits or Bumps

Abstract

Small regions in the neighborhood of pits or bumps on the surface of an ionic crystal will resonate under the influence of infrared radiation at frequencies intermediate between the frequencies of long wavelength transverse optic and associated longitudinal optic modes. Absorption due to similar intermediate frequency resonance was first intentionally observed in transmission through thin grating shaped films by Hass.(1) The effect is due to distortion of electric displacement fields caused by the presence of curved boundaries. Maxima in resonant polarization occur at points on the curved surfaces of the pits or bumps. We have computed the electric polarization associated with hemispherical pits and bumps on an otherwise plane surface. From this we have deduced the approximate effect of a random array of such bumps or pits on the reststrahl reflectance of an otherwise flat surface on an isotropic medium. With normally incident radiation, either bumps or pits will produce a dip in the top of an ordinary reststrahl reflectance band, together with a rather sharp peak of enhanced reflectance just below the frequency of the longitudinal optic mode. Anomalous band shape similar to this is observed in nearly all ionic crystals.