An open chiro-waveguide enabled by anisotropic impedance surfaces

Metasurfaces, which are quasi-two-dimensional (quasi-2D) textured surfaces comprised of a periodic or aperiodic array of sub-wavelength building blocks, provide a fully-planarized platform for controlling the propagation, radiation, and scattering of electromagnetic waves. Compared to their three-dimensional (3D) metamaterial counterparts, 2D metasurfaces offer several advantages including lower losses, more versatility, ease-of-fabrication, and a conformal solution. In recent years, metasurfaces have been exploited to enable a variety of novel electromagnetic devices including new leaky-wave antennas, coatings with abnormal refraction and reflection properties, ultrathin polarizers and wave-plates, low-profile cloaks, various planar microwave and optical lenses, compact wearable antennas, tensor impedance surface waveguides, and so on. In this contribution, we show that a homogeneous anisotropic metasurface, whose surface impedance is characterized by a tensor rather than a scalar, can be designed to wrap around a dielectric rod to create an open-waveguide that exhibits effective power confinement within a sub-wavelength regime. In particular, the dispersion relations for the fundamental HE
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mode are derived for a dielectric rod with one or more anisotropic impedance surface coatings. It is shown that by controlling the anisotropy of the coating, much stronger power confinement can be achieved. By further exploiting the inhomogeneity of the anisotropic impedance surface coating, two advanced functionalities can be achieved. First, a broadband chiro-waveguide, in which the polarization state of a linearly-polarized wave gradually rotates as it propagates along the waveguide, can be realized by a rotational perturbation in the alignment of the unit cells. Secondly, by enforcing momentum matching of the unit cells comprising the anisotropic impedance surface, a right-angle-bend waveguide with nearly the same transmission level compared to its straight counterpart can be realized. The new metasurface-enabled waveguide system demonstrated here provides additional degrees of freedom in manipulating guided electromagnetic waves, which holds great promise for the development of more advanced, multi-functional, and versatile microwave as well as terahertz devices for guidance, radiation, and imaging applications.