We propose a novel optically transparent reflection-type metasurface based on indium tin oxide (ITO) material for simultaneously achieving high transmission of visible light and near-field focusing (NNF) of microwave, demonstrating its potential for wireless power transfer (WPT) and harvesting applications. By achieving high impedance of the metasurface, this work overcomes the main challenge in designing metasurface with lossy metal materials, i.e., optimizing the tradeoff between phase shift characteristics and efficiency loss. We propose a new element with two degrees of freedom to ensure that the phase shift range can reach 350° while keeping $\vert S_{11}\vert $ less than −2.5 dB. In addition, we adopt the grid ground (GND) instead of the complete GND plane to further improve the light transmittance. Based on the above considerations, we design two types of metasurfaces for deployments in ambient wireless energy harvesting (plane-wave feeding) and WPT (horn feeding), respectively. Its NNF transfer efficiency can reach more than 60% of the metasurface based on good conductor materials. The relative bandwidth with 50% transfer efficiency can reach 34.5% (4.9–6.9 GHz). We fabricate an ITO-based prototype of the metasurface with the dimension of $342 \times 342 \times 4.4$ mm ³ ( $6.6 \times 6.6 \times 0.08\lambda _{0}^{3}$ ) with the sheet impedance of $1~\Omega $ /sq and a light transmittance of 60%. We also perform near-field scanning measurements to verify that the focusing position is accurate. Finally, through WPT and harvesting tests, we achieve a WPT and receiving efficiency (from power source to receiving antenna) of 12.6% and a rectification efficiency of 55%, confirming the practicability and effectiveness of the proposed work.

中文翻译：

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光学透明的近场聚焦超表面

我们提出了一种基于铟锡氧化物（ITO）材料的新型光学透明反射型超颖表面，用于同时实现可见光的高透射率和微波的近场聚焦（NNF），证明了其在无线电力传输（WPT）和收获方面的潜力应用程序。通过实现超表面的高阻抗，这项工作克服了用有损金属材料设计超表面的主要挑战，即优化了相移特性和效率损失之间的权衡。我们提出了一种具有两个自由度的新元件，以确保相移范围可以达到350°，同时保持 $ \ vert S_ {11} \ vert $ 小于-2.5 dB。此外，我们采用栅格接地（GND）代替整个GND平面，以进一步提高透光率。基于以上考虑，我们分别设计了两种类型的超颖表面，分别用于环境无线能量收集（平面波馈电）和WPT（喇叭馈电）中。基于良好的导体材料，其NNF传输效率可达到超表面的60％以上。传输效率为50％的相对带宽可以达到34.5％（4.9–6.9 GHz）。我们制作了一个基于ITO的超颖表面原型，其尺寸为 $ 342 \次342 \次4.4 $ 毫米 ³（ $ 6.6 \ times 6.6 \ times 0.08 \ lambda _ {0} ^ {3} $ ）的薄层阻抗为 $ 1〜\ Omega $ / sq和60％的透光率。我们还执行近场扫描测量，以验证聚焦位置是否正确。最后，通过WPT和收割测试，我们实现了WPT和接收效率（从电源到接收天线）为12.6％，整流效率为55％，证实了拟议工作的实用性和有效性。