Subdiffraction-limited imaging based on longitudinal modes in a   spatially dispersive slab
  It was proposed that a flat silver layer could be used to form a sub-diffraction limited image when illuminated near its surface plasmon resonance frequency [J. B. Pendry, Phys. Rev. Lett. 86, 3966 (2000)]. In this paper, we study the possibility of obtaining sub diffraction resolution using a different mechanism, with no surface plasmons involved. Instead, by taking into account the non-local response of a thin silver slab, we show that longitudinal modes contribute to the formation of a sub-diffraction limited image in a frequency regime above the plasma frequency. The differences between these two distinct mechanisms are studied and explained. 
Avner Yanai and Uriel Levy
physics.optics