Ultra broadband phase measurements on nanostructured metasurfaces

We report on an interferometric method developed for ultra broadband (from λ=0.65μm to λ=1.7μm) phase measurements on metasurfaces in transmission and reflection. Due to a unique performance of our method in terms of the accessible spectral range, accuracy (±0.02 rad), and flexibility with respect to the sample arrangement, this technique can be broadly used as a versatile tool for the comprehensive characterization of a broad class of dispersive optical materials. We compare our experimental technique with an indirect approach and based on the Kramers-Kronig transformation analysis, establish a rule for the use of the indirect method.