Perfect gradient metasurfaces are not the most efficient

Metasurfaces rely on the resonances of nanostructures with dimensions in the nanometer range for devices working in the visible regime. The corresponding fabrication technology requires E-beam exposure and can be quite challenging. It is prone to defects associated with the many subtle process steps during fabrication, resulting in missing, distorted, or displaced nanostructures within the metasurface. In this work, we analyze the robustness of gradient metasurfaces to a variety of such defects using both simulations and experiments on purposely miss-fabricated samples. Surprisingly, we observe that a "perfect" structure, without any defect, is not that which exhibits the best performance in terms of efficiency and angular response. We demonstrate that specific, wellcontrolled, defects can actually significantly enhance the performance of such a metasurface and explain this behavior using the resonance and near-field properties of the corresponding nanostructures. This work sheds very new light on the design strategies for metasurfaces and significantly alleviates some constraints on their fabrication accuracy.