Transmissive-type metasurfaces represent an ultrathin alternative to traditional optical elements, e.g., lenses and waveplates. However, transmissive-type plasmonic metasurfaces (PMs) have significantly low efficiency compared to dielectric metasurfaces and reflective type PMs particularly in the visible range. For example, the state-of-the-art geometric PMs transmission efficiency is ≤10% with extinction ratios ~ 0 dB. The low transmission efficiency is mainly due to three loss channels (i) absorption losses in metals, (ii) diffraction to undesired high-orders, and most importantly (iii) symmetric forward-backward scattering which puts a 25% theoretical limit on cross-polarization conversion for ultrathin metasurfaces. We use tunable, multipole-interference-based meta-atoms to address all loss channels simultaneously. The experimentally demonstrated transmission efficiency and extinction ratio of our geometric PM are 42.3% and 7.8dB, respectively. As for dielectric metasurfaces, we demonstrate a new class of metasurfaces where the meta-atoms consist of a simple anti-reflective coating (ARC). ARCs enable the control over the entire 2 pi phase range by varying the dielectric films thicknesses while realizing ~ 99% transmission efficiency even in the visible range. The metasurface consists of patches of ARC meta-atoms with dielectric optical thicknesses much lower than that required in Fresnel optics to control the entire phase range.
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