5,004 research outputs found
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
Tunable fishnet metamaterials infiltrated by liquid crystals
We analyze numerically the optical response and effective macroscopic
parameters of fishnet metamaterials infiltrated with a nematic liquid crystal.
We show that even a small amount of liquid crystal can provide tuning of the
structures due to reorientation of the liquid crystal director. This enables
switchable optical metamaterials, where the refractive index can be switched
from positive to negative by an external field. This tuning is primarily
determined by the shift of the cut-off wavelength of the holes, with only a
small influence due to the change in plasmon dispersio
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