| Metasurface has become a hot research topic worldwide because of many characteristics,such as small thickness,low loss and easy machining.Due to the materials and structures of design,traditional metasurface devices have many limitations.The fixed structure can only be used in the sole frequency and device,which performance cannot meet the flexible requirements in various reality.To expand the application scenarios of metasurface devices in the terahertz band,improve device efficiency,and reduce the difficulty of processing.In this work,by introducing phase change materials in the structure,we proposed several VO2 film-based multifunctional devices.Besides,the tunability of multifunctional devices based on the unpatterned VO2 are processed and verified experimentally.The main work and innnovations are as follows:(1)Based on the unpatterned VO2 film,an N-shaped structure combining electromagnetically induced transparency and multi-band absorption is proposed.The unpatterned VO2 could reduce the processing difficulty and improve the flexibility of control.When VO2 is an insulating state,the spectrum generates transparent windows at 0.836 THz and 0.979 THz,and its principle is well explained by temporal coupled-mode theory.When VO2 is the metallic state,the structure achieves narrowband absorption at 0.47 THz,0.713 THz and 0.946 THz for TE polarization,with the maximum absorption rates of 83.5%,60%and 97%,respectively.At 0.263 THz,the structure could absorb EM waves with an amplitude of 92%for TM polarization.Additionally,it achieved broadband absorption around the center frequency 0.97 THz.The absorption effect can be explained by the impedance matching theory.(2)Based on the unpatterned VO2 film,an E-shaped structure combining polarization-controlled electromagnetically induced transparency and broadband absorption is designed.When VO2 is an insulator,transparent peaks could be obtained at 0.428 THz and 0.19 THz,respectively,by changing TE and TM polarization of the electromagnetic wave.When VO2 is the metal,the system can achieve broadband absorption for TE and TM polarization,and the relative bandwidths with absorption rates greater than 60%are 119.8%and 68.8%,respectively.The absorption effect is not sensitive to the polarization angle.(3)Based on the phase transition characteristics of the VO2 and Pancharatnam-Berry(PB)phase,a double L-shaped multifunctional metasurface is proposed.When VO2 is an insulating state,dual-band transparency effect at 0.4465 THz and 0.639 THz appear in the spectrum,which could be verified by the traditional two-particle model.In the metallic state of VO2,the structure is worked as a reflected linear-circular polarization converter in the range of 0.3-0.42 THz and 0.72-0.92 THz.By combining PB phase to align the metasurface elements,it is found that it can be used as the terahertz beam steering and splitter under different temperature conditions,and the beam deflection angle is related to the arrangement of elements.(4)A 2-bit(four-state)coding metasurface with only two elements is proposed,and the reflected beam can be adjusted in one dimension.In addition,a visualized metasurface array antenna system is designed,which can reversely calculate the coding sequence by presetting the beam reflection angle.In the design process,deep neural network is introduced to optimize the antenna array arrangement,which is verified by numerical calculation and CST simulation.(5)A variety of metasurface based on patternless VO2 was fabricated on polyimide and quartz substrates respectively and verified experimentally.The experimental results show that the processing method and heating conditions make the polyimide in the experiment couldn’t support the phase transition of VO2.However,the measured results of structure based on quartz are basically consistent with the simulation results,which can confirm the phase change properties of VO2 to some extent and further verifies the feasibility of the designed multifunctional tunable metasurface. |
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