Research On Tunable EIT Metamaterials And Polarization Converters Based On VO₂

Terahertz(THz)technology has potential applications in many fields,such as the future of 6G communication technology,non-destructive testing and radar imaging.At present,there is still a lack of THz devices,and the research on THz functional devices can better promote the development of THz technology.Metamaterials are artificially synthesized electromagnetic materials that can achieve different functions by using different microstructural units,materials and arrangements.Designing THz devices with metamaterials is an effective way.In this paper,two types of tunable THz metamaterial devices based on vanadium dioxide(VO₂)are studied in detail.The specific research contents are as follows:1.Two tunable Electromagnetically Induced Transparency(EIT)metamaterials based on VO₂ are designed,which are EIT metamaterial with double grating stripe structure and EIT metamaterial with double C-shaped structure.The former consists of two grating strips with different lengths and an ultra-thin quartz(20?m)substrate,in which VO₂ is embedded at both ends of the short grating strip.VO₂ is a phase-change material that undergoes a metal-insulator phase transition when stimulated by heat(or electricity),accompanied by a change in electrical conductivity.When VO₂ is in the insulating phase,the EIT metamaterial has a slow light effect.The surface electric/magnetic field distribution and surface current distribution are used to analyze the physical mechanism of the EIT effect,and the influence of the dimensional variation of the device structure on the performance is discussed in detail.In addition,considering the experimental preparation,a quartz substrate with a thickness of 254?m is used,and the top layer structure is optimized as a double C-shaped structure with oppositely placed and different sizes,which can achieve an EIT window at 0.39 THz;VO₂ is embedded at the ends of the two arms of the small left C-shaped structure,and the modulation of the EIT window is achieved by adjusting the electrical conductivity of VO₂.Finally,the EIT effect of the metamaterial and the tunable properties of VO₂on the EIT window were verified by terahertz time-domain spectrometer tests.2.A transmission mode tunable polarization converter based on VO₂ is proposed,which realizes multi-band cross-polarization conversion and linear-circular polarization conversion in reflection mode,and realizes single-frequency cross-polarization conversion in transmission mode.The device consists of a split-cardioid metal ring on the top layer,a metal grating on the bottom layer and an intermediate dielectric layer,and VO₂ fills the gaps between the metal grating bars.When VO₂ is in the insulating phase,the polarization converter realizes cross-polarization conversion at a single frequency point of 2.25 THz in transmission mode,and the physical mechanism is explained by the surface current distribution;When VO₂ is in the metal phase and in reflection mode,the polarization converter realizes cross-polarization conversion in the frequency range of 1.76-1.88THz and 3.05-3.32THz.At the same time,the linear-circular polarization conversion is realized in the three frequency ranges of 1.46-1.60THz,2.08-2.76THz and 3.64-4.00THz.It is insensitive to the polarization of incident electromagnetic waves.The principle of polarization conversion is explained by the electric field vector decomposition and optical polarization theory analysis.The effect of its structural parameter changes on performance is also explored.In both modes,the polarization converter can maintain its performance in the range of 30°incidence angle.Finally,the polarization converter is extended to realize broadband line-circular polarization conversion in reflection mode.The proposed structure has potential applications in tunable THz integrated optics.