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Transmissive and reflective all-dielectric metastructures will be presented that offer tailored polarization con- versions and spectral responses. The metastructures consist of stacked deeply subwavelength, high contrast gratings of different fill factors and rotations. Broadband metastructures that perform a given polarization conversion over a wide continuous bandwidth will be shown, as well as multiband metastructures that perform a common polarization conversion over different bands. Multifunctional metastructures that realize different polarization conversions over different bands will also be reported at the conference. Unlike earlier stacked grating geometries, the transmissive metastructures do not require antireflection layers since impedance matching is incorporated into their design. The subwavelength gratings are modeled as homogeneous anisotropic layers, allowing an overall metastructure to be treated as a stratified dielectric medium. Quasi-static analysis is used to homogenize the subwavelength gratings and represent them with effective dielectric constants. Plane-wave transfer matrix techniques are employed to model the interactions between gratings, allowing for rapid design and optimization. In the metastructures, higher-order Floquet harmonics are bound to the gratings due to their subwavelength periodicities. As a result, only zeroth-order coupling between the gratings needs to be considered. The multifunctional performance and compactness (wavelength-sized thickness) of the proposed devices will find use at millimeter-wave wavelengths and beyond. Two example designs are described in this paper. Measured performance of prototypes will also be reported at the conference and various fabrication approaches reviewed. This work demonstrates the flexibility with which cascaded subwavelength gratings can realize unprecedented polarization control with varied spectral responses.
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