We propose an impedance-matching technique for metasurfaces generating evanescent fields with complex free-space impedance for subwavelength focusing. The proposed technique is mandatory for the subwavelength focusing far beyond the diffraction limit. First, we theoretically show the fact that the wave impedance becomes a complex number due to the generated evanescent fields and give an explicit form of the wave impedance as a ratio of local electric and magnetic fields. Then, we discuss the impedance-matching technique for the distributed complex wave impedance by a metasurface with a position-dependent impedance matrix. To demonstrate the proposed technique, we design a subwavelength focusing 1-D metasurface with the null-to-null beamwidth of one-twentieth of the free-space wavelength at 2.45 GHz, and its performance is verified by wave propagation simulations based on the equivalent circuit model. In order to further verify the validity of the proposed technique, a metasurface is implemented with the trilayer configuration, and the subwavelength focusing is experimentally demonstrated.

中文翻译：

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超表面产生用于亚波长聚焦的渐逝场的阻抗匹配技术

我们提出了一种阻抗匹配技术，用于超表面生成具有复杂自由空间阻抗的渐逝场，用于亚波长聚焦。所提出的技术对于远远超出衍射极限的亚波长聚焦是强制性的。首先，我们从理论上证明了这样一个事实，即由于产生的倏逝场，波阻抗变成了复数，并给出了波阻抗的明确形式，作为局部电场和磁场的比率。然后，我们通过具有位置相关阻抗矩阵的超表面讨论分布式复波阻抗的阻抗匹配技术。为了演示所提出的技术，我们设计了一个亚波长聚焦一维超表面，其零到零光束宽度为 2.45 GHz 自由空间波长的二十分之一，并通过基于等效电路模型的波传播仿真验证了其性能。为了进一步验证所提出技术的有效性，使用三层配置实现了超表面，并通过实验证明了亚波长聚焦。