An Optically Transparent Near-Field Focusing Metasurface

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.