Accurate metasurface synthesis incorporating near-field coupling effects

One of the most promising metasurface architectures for the microwave and terahertz frequency ranges consists of three patterned metallic layers separated by dielectrics. Such metasurfaces are well suited to planar fabrication techniques and their synthesis is facilitated by modelling them as impedance sheets separated by transmission lines. We show that this model can be significantly inaccurate in some cases, due to near-field coupling between metallic layers. This problem is particularly severe for higher frequency designs, where fabrication tolerances prevent the patterns from being highly-subwavelength in size. Since the near-field coupling is difficult to describe analytically, correcting for it in a design typically requires numerical optimization. We propose an extension of the widely used equivalent-circuit model to incorporate near-field coupling and show that the extended model can predict the scattering parameters of a metasurface accurately. Based on our extended model, we introduce an improved metasurface synthesis algorithm that gives physical insight to the problem and efficiently compensates for the perturbations induced by near-field coupling. Using the proposed algorithm, a Huygens metasurface for beam refraction is synthesized showing a performance close to the theoretical efficiency limit despite the presence of strong near-field coupling

Substrate-induced bianisotropy in metamaterials

We demonstrate that the presence of a supporting substrate can break the symmetry of a metamaterial structure, changing the symmetry of its effective parameters, and giving rise to bianisotropy. This indicates that magneto-electric coupling will occur in all metamaterials fabricated on a substrate, including those with symmetric designs

Time-varying Huygens' meta-devices for parametric waves

Huygens' metasurfaces have demonstrated almost arbitrary control over the shape of a scattered beam, however, its spatial profile is typically fixed at fabrication time. Dynamic reconfiguration of this beam profile with tunable elements remains challenging, due to the need to maintain the Huygens' condition across the tuning range. In this work, we experimentally demonstrate that a time-varying metadevice which performs frequency conversion can steer transmitted or reflected beams in an almost arbitrary manner, with fully dynamic control. Our time-varying Huygens' metadevice is made of both electric and magnetic meta-atoms with independently controlled modulation, and the phase of this modulation is imprinted on the scattered parametric waves, controlling their shapes and directions. We develop a theory which shows how the scattering directionality, phase and conversion efficiency of sidebands can be manipulated almost arbitrarily. We demonstrate novel effects including all-angle beam steering and frequency-multiplexed functionalities at microwave frequencies around 4 GHz, using varactor diodes as tunable elements. We believe that the concept can be extended to other frequency bands, enabling metasurfaces with arbitrary phase pattern that can be dynamically tuned over the complete 2\pi range

Public Attitudes on State Election Administration, Goals, and Reforms*

While few would disagree that elections serve a fundamental role in democracy, there is considerable debate regarding the rules by which elections should be conducted. State and local officials responsible for carrying out elections face difficult challenges, and often must work to achieve what many view are two competing aims: increasing voter turnout and minimizing voter fraud

Scattering of electromagnetic waves in metamaterial superlattices

The authors study experimentally both transmission and reflection of microwave radiation from metamaterialsuperlattices created by layers of periodically arranged wires and split-ring resonators. The authors measure the dependence of the metamaterial resonance on the spatial period of the superlattice and demonstrate resonance broadening and splitting for the binary metamaterial structures.The authors acknowledge support from the Australian Research Council and thank Ekmel Ozbay for providing additional details of the experimental results published earlier by his group