  The amplification of evanescent waves by flat superlens requires a
near-resonance coupling which has been linked to resonant surface plasmons. A
subtle interplay has been proposed to exist between the excitation of
well-defined resonant surface plasmons and the focusing capability of a
superlens. To gain insights into these resonant modes and their contributions
to the amplification and recovery of evanescent waves, we performed simple but
robust full-wave FDTD simulations on causal negative index Lorentz models. We
found that well-defined pair of resonant surface plasmons is excited whenever a
coupling of a diverging transmitted beam and a converging refracted beam occurs
at the second interface. The resonant coupling of these modes at the interface
led to the excitation of a single interface resonance predicted by the theory.
The physical consequence of this resonance is that incident wave energy is
pumped into the negative-index material medium and beyond it. These phenomena
contribute substantially to the amplification and recovery of the near-fields
in the image plane. It is noteworthy that, for evanescent wave refocusing to be
achieved in a flat superlens, its thickness, and the source-to-superlens
distance should be optimized. This optimization is critical in that an optimal
thickness alongside optimal source-to-superlens distance will allow evanescent
wave refocusing to dominate material lost. The FDTD simulated result showed an
image of the point source inside the superlens and beyond it when its thickness
was optimized. The resolution of the image beyond the superlens was
$\sim.58\lambda$ and this superlens behaves like a near-perfect lens. Overall,
these numerically simulated results could serve as useful approximations to the
degree of resonant amplification and refocusing of near-fields that can be
achieved by flat superlens in near-fields experimental imaging setups.
