  An optical metasurface consists of a dense and usually non-uniform layer of
scattering nanostructures behaving as a continuous and extremely thin optical
component, with predefined phase and intensity transmission/reflection
profiles. To date, various sorts of metasurfaces (metallic, dielectric,
Huygens-like, Pancharatman-Berry, etc.) have been introduced to design
ultrathin lenses, beam deflectors, holograms, or polarizing interfaces. Their
actual efficiencies depend on the ability to predict their optical properties
and to fabricate non-uniform assemblies of billions of nanoscale structures on
macroscopic surfaces. To further help improve the design of metasurfaces,
precise and versatile post-characterization techniques need to be developed.
Today, most of the techniques used to characterize metasurfaces rely on light
intensity measurements. Here, we demonstrate how quadriwave lateral shearing
interferometry (QLSI), a quantitative phase microscopy technique, can easily
achieve full optical characterization of metasurfaces of any kind, as it can
probe the local phase imparted by a metasurface with high sensitivity and
spatial resolution. As a means to illustrate the versatility of this technique,
we present measurements on two types of metasurfaces, namely Pancharatnam-Berry
and effective-refractive-index metasurfaces, and present results on uniform
metasurfaces, metalenses and deflectors.
