  Metasurfaces based on gap surface-plasmon resonators allow one to arbitrarily
control the phase, amplitude and polarization of reflected light with high
efficiency. However, the performance of densely-packed metasurfaces is reduced,
often quite significantly, in comparison with simple analytical predictions. We
argue that this reduction is mainly because of the near-field coupling between
metasurface elements, which results in response from each element being
different from the one anticipated by design simulations, which are commonly
conducted for each individual element being placed in an artificial periodic
arrangement. In order to study the influence of near-field coupling, we
fabricate meta-elements of varying sizes arranged in quasi-periodic arrays so
that the immediate environment of same size elements is different for those
located in the middle and at the border of the arrays. We study the near-field
using a phase-resolved scattering-type scanning near-field optical microscopy
(s-SNOM) and conducting numerical simulations. By comparing the near-field maps
from elements of the same size but different placements we evaluate the
near-field coupling strength, which is found to be significant for large and
densely packed elements. This technique is quite generic and can be used
practically for any metasurface type in order to precisely measure the
near-field response from each individual element and identify malfunctioning
ones, providing feedback to their design and fabrication, thereby allowing one
to improve the efficiency of the whole metasurface.
