Research On Metasurface For Harvesting Electromagnetic Energy

With the development of the Internet of things technology,wireless sensor networks have been widely spread.Harvesting the ambient electro-magnetic(EM)energy for charging the sensors has attracted people’s attention.The development of metamaterial brings the new direction and opportunity for solving this problem recently.In this thesis,the metasurface for harvesting EM energy is investigated based on the effective medium theory.The following innovative research works focus on the EM energy harvesting metasurfaces with the characteristics of polarizationinsensitivity,wide-incident-angle and the combination with rectifier.First,two wide-incident-angle and polarization-insensitive metasurfaces with single via are proposed.The unit cells of two metasurfaces are both central symmetrical structures.A single metal via is placed near the center to collect energy,which simplifies the subsequent energy convergence network.The first metasurface has a wide range of incident angles,and the unit cell is a developed from a cross-shaped patch.The simulation results show that the maximum harvesting efficiency is 88% at 5.8 GHz for both TE and TM waves under normal incidence.At the incidence angle of 75°,the maximum efficiency remains higher than 77%.The measured efficiencies of 5×5 array are in good agreement with simulated ones.The second metasurface operates in four bands and the unit cell evolves from a cross structure.By bending the branches,the number of current loop is increased and multiple resonance frequencies are generated.The simulation results show that the maximum harvesting efficiencies are 68%,41%,81%,and 58% at 5.8/9.2/11.7/13.5 GHz,respectively,for both TE and TM waves under normal incidence.When the incident angle is 45°,this harvesting efficiencies maintain 74%,49%,44% and 62% in four bands,respectively.Second,the frequency stability of polarization-insensitive metasurfaces with a wide range of angles is studied.By introducing the metal via square ring around the fan-shaped unit cell,the mutual coupling between the units is reduced.Thus,the metasurface can keep the maximum efficiency at the fixed operation frequency over wide incident angles.The simulation results show that the maximum harvesting efficiency is 91% at 5.8 GHz for both TE and TM polarizations under normal incidence.When the incident angle increases to 75°,the maximum efficiency is achieved at 5.788 GHz(0.19% shift)with the maximum efficiency of 68%.A 5×5 array is fabricated and the measured results are in good agreement with the simulated ones.Thirdly,a broadband polarization-insensitive metasurface integrated with rectifying diode is proposed.The unit cell of metasurface evolves from a diamond-shaped structure by turning the four sides into arcs to expand the bandwidth.Fan-shaped patches are added around the unit to generate a new resonant frequency for extending the operation band.The rectifying diode is placed at a specific position of the unit cell to achieve the conjugate matching of its complex impedance with that of the metasurface.Two unit cells are connected by an inductance,which act as not only energy convergence but also a direct pass filter.The simulation results show that the metasurface operates in 4.5-5.6 GHz band and the harvesting efficiency is more than 90% within the bandwidth.The efficiency of the rectifying diode is higher than 58% within 4.5-5.6 GHz with the maximum efficiency of 62%.