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.arXiv:1905.06475v1 [physics.optics]
Printed circuit metasurfaces have attracted significant attention in the microwave community for their capability of versatile wavefront manipulation. Despite their promising potential in telecommunications and radar applications, few transmissive metasurfaces have been reported operating at millimeter-wave frequencies. Several secondary effects including fabrication tolerances, interlayer near-field coupling and the roughness of conductors are more severe at such high frequencies and can cause significant performance degradation. Additionally, experimental characterization techniques reported previously are not accurate enough for the verification of such effects. In this work, we present highly efficient refracting metasurfaces operating at 83 GHz. We use a sophisticated synthesis technique that minimizes possible effects of performance degradation such as interlayer near-field coupling and the influence of fabrication tolerances. Additionally, we propose a new experimental technique for the characterization of periodic metasurfaces. Using this technique, we present for the first time an accurate determination of the intensity of propagating Floquet harmonics in a broad frequency range. The proposed method gives deep insight into the beam refraction problem as it accurately quantifies in which direction energy is scattering. Additionally, it verifies our numerical model.
Metasurfaces have emerged as a promising technology for the manipulation of electromagnetic waves within a thin layer. In planar ultrathin metasurfaces, there exist rigorous design methods, based on the equivalent surface impedance of patterned metallic layers on dielectric substrates. In this work, we derive a limit on bandwidth achievable in these metasurfaces, based on constraints that their meta-atoms should be passive, causal and lossless and that they should obey the time-bandwidth product rules of a single resonance structure. The results show that in addition to elementary design parameters involving variation of the surface impedance, the bandwidth is critically limited by the dielectric substrate thickness and permittivity. We then propose a synthesis method for broadband ultrathin metasurfaces, based on an LC resonance fit of the required surface impedance and experimentally verify a broadband dispersive structure at millimeter-wave frequencies. This results in a bandwidth enhancement of over 90%, relative to a reference metasurface created with the narrowband design process.
Switched beam antennas provide an efficient and cost effective alternative to complex phased array and digital beam forming techniques. In this paper, we present the design and characterization of an electrically thin Huygens metasurface lens operating at 83 GHz with high transmission efficiency and we demonstrate its applicability to switched beam antenna applications. We accurately characterize the 3D scattered field distribution and determine the focal length and transmission efficiency of the lens by near-field scanning. A model based on ideal Huygens sources is used to predict the focal performance, allowing the geometrical measurement parameters to be determined in advance. Finally, the ability of the lens to operate as a switched beam antenna is experimentally verified by exciting it with an omnidirectional waveguide antenna placed at different positions. The experimental results are in very good agreement with numerical simulations, showing steering angles up to 12 • while keeping the side lobe level below-15 dB.
We investigate near-infrared photodetectors based on subwavelength Ge nanoparticles. While the photodetector size guarantees a high-bandwidth device, the high quantum efficiency is possible by the localization of the optical energy.
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