Optical metasurfaces consisting of designed nanoresonators arranged in a planar fashion were successfully demonstrated to allow for the realization of a large variety of flat optical components . While most metasurfaces realized so far focused on the isolated scattering properties of the metasurface itself, the opportunities offered by tailoring the substrate properties are often neglected. Here we consider a silicon nanocylinder metasurface exhibiting electric and magnetic dipolar Mie-type resonances at near-infrared frequencies, which is situated on a gold mirror. The metasurface and the mirror are separated by a dielectric spacer layer with a gradually varying thickness. We analytically, numerically and experimentally investigate how systematic changes in the spacer layer thickness influence the optical reflection spectra of the metasurface. For experimental realization, we covered the pre-fabricated silicon nanocylinder metasurface with a wedge-shaped dielectric layer. Afterwards, the dielectric layer was coated with a gold layer, which acted as the mirror. For optical characterization, we measured the reflection spectra of the sample from the metasurface side at different positions on the sample, where each position corresponds to a different spacer thickness. Our measurements show that a transition from perfect reflection to perfect absorption occurs when the spacer thickness changes by only few tens of nanometers. To understand the physical origin of the observed features, we numerically calculated the metasurface reflection spectra as a function of the spacer layer thickness and furthermore employed a semi-analytical S-matrix based theory to our system , revealing that the observed feature originates from an interference effect of different types of modes supported by the structure. Our results offer interesting new opportunities for tunable and switchable functional flat optical devices.
 N. Yu et al., Nat. Mater. 13, 139–150 (2014)
 C. Menzel et al., Phys. Rev. A 93, 063832 (2016)