Probing metamaterials with structured light.

Photonic metamaterials and metasurfaces are nanostructured optical materials engineered to enable properties that have not been found in nature. Optical characterization of these structures is a challenging task. We report a reliable technique that is particularly useful for characterization of phase properties introduced by small and spatially inhomogeneous samples of metamaterials and metasurfaces. The proposed structured light, or vortex based interferometric method is used to directly visualize phase changes introduced by subwavelength-thick nanostructures. In order to demonstrate the efficiency of the proposed technique, we designed and fabricated several metasurface samples consisting of metal nano-antennas introducing different phase shifts and experimentally measured phase shifts of the transmitted light. The experimental results are in good agreement with numerical simulations and with the designed properties of the antenna arrays. Due to the presence of the singularity in the vortex beam, one of the potential applications of the proposed approach based on structured light is step-by-step probing of small fractions of the micro-scale samples or images.