Bandwidth and size limits of achromatic printed-circuit metasurfaces
Bandwidth and size limits of achromatic printed-circuit metasurfaces
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DOI：
10.1364/oe.26.029440
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发表时间：
2018-10-29
期刊：
影响因子：
3.8
通讯作者：
Powell, David A.
中科院分区：
文献类型：
--
作者：
Fathnan, Ashif A.;Powell, David A.
超表面可以利用亚波长层实现各种各样的波操纵功能。它们通常由谐振元件产生，因此它们的折射特性强烈依赖于频率。由此产生的色差对于大多数应用来说是不期望的，这激发了最近在开发消色差超颖表面方面的努力。然而，目前还不清楚是否有任何物理限制的可实现的工作带宽的消色差超颖表面。在这里，我们解决这个问题，考虑一个共同的微波超颖表面几何形状的基础上的三个金属层，介电基板分离。由于这些金属层中的每一个都被建模为阻抗，我们应用福斯特的电抗定理来确定使用无源、因果和无损结构在物理上可实现的带宽。我们推导出的带宽和总尺寸的元表面的限制，这两个参数之间的权衡。对应于透镜的较大数值孔径的较高折射角进一步限制了可实现的带宽。我们认为惠更斯和欧米茄双各向异性超颖表面类型，并表明，限制是更严重的双各向异性超颖表面，使他们不太适合宽带消色差设计。(C)2018年美国光学学会根据OSA开放获取出版协议的条款
Metasurfaces can implement a wide variety of wave-manipulation functions with sub-wavelength layers. They are typically created from resonant elements, thus their refraction properties depend strongly on frequency. The resulting chromatic aberration is undesirable for most applications, motivating recent efforts in the development of achromatic metasurfaces. However, it remains unclear whether there are any physical limits on the achievable operating bandwidth of achromatic metasurfaces. Here we address this question, considering a common microwave metasurface geometry based on three metallic layers, separated by dielectric substrates. Since each of these metallic layers is modeled as an impedance, we apply Foster's reactance theorem to determine the bandwidth over which they are physically realizable using passive, causal and lossless structures. We derive limits for the bandwidth and total size of the metasurface, showing that there is a trade-off between these two parameters. A higher angle of refraction, corresponding to a larger numerical aperture for a lens, further limits the realizable bandwidth. We consider both Huygens' and Omega-bianisotropic metasurface types, and show that the limit is more severe for bianisotropic metasurfaces, making them less suitable for broadband achromatic designs. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement