Study On Terahertz Adjustable Metasurface Structure Based On Vanadium Dioxide Thin Film

Terahertz wave with a special frequency range on electromagnetic spectrum known as“THz gap”makes it possess many unique properties and have attracted more and more attention in wireless communications,non-destructive testing and sensing.High-performance modulation devices which could be necessary to preferably manipulate terahertz wave are necessary for the implementation of these various applications.Although many natural materials do not have an effective influence on terahertz radiation,the metamaterials'emergence provides a new solution by combining with tunable materials to achieve the dynamic modulation of terahertz wave.However most current metamaterials based terahertz modulators exhibit band-stop characteristics at the resonant frequency and a relatively high insertion loss resulting in these devices is difficult for installation and adjustment.VO₂ is a kind of tunable materials whose electrical conductivity can change by 5 orders of magnitude under external excitation.Therefore,in this thesis VO₂ film was used to design an electrically controlled normally-open tunable metasurface device to solve this problem.The main contributions are included as follows.Firstly,the CST Microwave Studio software was used to optimize the size of the common electrical resonance unit.By adopting the buried gate electrode as the coplanar feedin,two kinds of normally-open metasurface devices based on VO₂ film were designed.The designed modulators could simultaneously control the amplitude and phase of terahertz wave,and the modulation mechanism of these two devices was obtained by analyzing the electric field at the corresponding resonance frequency.Then the normally-open metasurface device was fabricated by using semiconductor fabrication process technology.In order to characterize its modulation performance,the THz time-domain spectroscopy system was used to test the transmittance of the prepared device.Compared with the simulated curve,a scheme to improve the performance of the device was proposed and implemented.Finally,the modulation speed of the device was tested by the transmitted terahertz continuous wave test system,and the influence of the addition of SiO₂ film on the modulation speed of the device was also analyzed.The simulated results show that the insertion losses of these two proposed devices are not larger than 2 dB,the maximum modulation depth is above 90%,and the maximum phase difference of the patterned VO₂ film modulation device is 116°.It can be seen from the experimental test results that the tested curve and the simulated curve show a same trend,which demonstrates the validation of the simulation design.And by eliminating the influence of the silicon substrate,the transmittance of the device is improved.Introducing a new SiO₂ film can improve the electrical properties of VO₂ film,and thus the modulation speed of the device is improved.All these findings verify the proposed structure could provide a new idea for the development of terahertz devices with better modulation performance.