We present numerical simulations of the near-field focusing capabilities of a dynamically reconfigurable holographic metasurface aperture. The aperture consists of a parallel-plate waveguide in which the upper plate is patterned with a number of metamaterial irises that can be dynamically switched between radiating (ON) and non-radiating (OFF) states. A cylindrically symmetric waveguide mode, excited by a coaxial probe in the center of the lower plate, serves to excite the radiating irises, forming a focused spot in the radiating near-field (or Fresnel zone). The layout of the metamaterial elements and their tuning states is determined using holographic design principles, in which the interference pattern of the waveguide (or reference) mode and the desired radiated field pattern leads to the required phase distribution over the surface of the aperture. We also develop an analytical model of the aperture to confirm the numerical simulations, and to illustrate the advantage of the guided-mode as the reference wave versus a plane-wave. We further leverage this analytical model to analyze the diffracted order characteristics of the holographic metasurface aperture, showing high-fidelity focusing patterns even for difficult focusing scenarios across the entire investigated field-of-view.
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