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Metasurfaces are ultra-thin patterned photonic structures that emerged recently as planar metadevices capable of reshaping and controlling incident light. They are composed of resonant subwavelength elements distributed across a flat surface. Due to the resonant scattering, each element can alter the phase, amplitude and polarization of the incoming light. Many designs and functionalities of metasurfaces suggested so far are based on plasmonic planar structures, however most of these metasurfaces demonstrate low efficiencies in transmission due to losses in their metallic components. In contrast, all-dielectric resonant nanophotonic structures avoid absorption losses, and can drastically enhance the overall efficiency, especially in the transmission regime. Here we utilize this platform to create flat optical elements such as vector beam q-plates, holograms and quantum polarization tomography devices. Holograms, in particular, showcase a potential of the metasurface platform as they rely on a complex wavefront engineering. Metasurface platform enables a new way to create highly efficient holograms with single-step patterning. Here, we design and realize experimentally greyscale meta-holograms with superior transmission properties. Another promising area for implementation of all-dielectric metasurfaces is quantum optics. We suggest and develop experimentally a new concept of quantum-polarization measurements with a single all-dielectric resonant metasurface. A metasurface enables full reconstruction of the state of entangled photon pairs based on the photon correlations with single-photon detectors. The subwavelength thin structure provides an ultimate miniaturization, scalability to a larger number of entangled photons, and gives the possibility to study the dynamics of quantum states in real-time.
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