  Motivated by the recent renewed interest in compact analog computing using
light and metasurfaces (Silva, A. et al., Science 2014, 343, 160-163), we
suggest a practical approach to its realization that involves reflective
metasurfaces consisting of arrayed gold nanobricks atop a subwavelength-thin
dielectric spacer and optically-thick gold film, a configuration that supports
gap-surface plasmon resonances. Using well established numerical routines, we
demonstrate that these metasurfaces enable independent control of the light
phase and amplitude, and design differentiator and integrator metasurfaces
featuring realistic system parameters. Proof-of-principle experiments are
reported along with the successful realization of a high-quality poor-man's
integrator metasurface operating at the wavelength of 800 nm.
