Research On Properties And Devices Of Vanadium Dixoide In The Terahertz Regime

The reversible insulator-metal transition(IMT)of VO₂ (vanadium dioxide),which presents dynamic modulation effect in the terahertz(THz)regime,has attracted plenty attention for its potential applications in actively tunable THz devices.In recent years,the investigation on the growth mechanism of VO₂ films expanded the preparation methods of VO₂ films with high modulation performance,which reduced the integration difficulties of VO₂ films into the tunable THz devices.Motivated by these opportunities,this thesis provides a comprehensive survey ranging from the mechanism of the THz modulation phenomenon to the influence of metal dopant,until the applications in the THz devices.The thesis starts with the percolation transition mechanism of VO₂ thin film,with special emphasis on the coexistence of the metal and insulator domains during the phase transi-tion as well as their influence on percolation threshold.Subsequently,the effect of cobalt dopant on the THz modulation performance of VO₂ thin films is introduced.Finally,the dynamic modulation ability of the VO₂ -based THz metamaterial is discussed.However,the study found that the inherent physical properties of VO₂ limit the dynamic range of THz devices.In order to break this limit,artificially designed reconfigurable structure was proposed to couple with the VO₂ -based THz metamaterial.The results turned out that the newly-formed thermal-mechanical dually tunable THz metamaterial combines the advan-tages of mechanical reconfiguration and phase-change material,extensively broadening the dynamic range of the THz metamaterial,which may become a new development di-rection of VO₂ -based tunable THz device.The main contents of this thesis are as follows:1.By investigating the evolution of metal and insulator phase fractions during the THz conductivity transition in epitaxial VO₂ thin film,a new viewpoint regarding the car-rier transport mechanism was proposed.It is proved that the IMT process is not only a percolation transition process,but also highly influenced by the weak carrier confinement effect from the metal domain boundaries in the disordered stage of the phase transition.The confinement effect suppresses the diffusion of free charge carriers and decreases the effective THz conductivity of the whole film.As a result,only approaching the percolation threshold(50%volume fraction),i.e.,forming long-range carrier transport pathways,is not enough to achieve an effective modulation effect of VO₂ thin films in the THz regime.In order to reduce the confining effect of the phase boundaries,the volume fraction of the metal domains needs to reach a larger value,which explains the fact that the THz conduc-tivity transition observed during the thermal-induced phase transition lags far behind the structural phase transition measured by Raman spectroscopy.2.The dynamic modulation ability of VO₂ -hybrid THz devices is based on the phys-ical properties of VO₂ thin films.In this study,it was found that a modified VO₂ film with lower critical temperature,larger modulation depth and narrower transition region can be obtained by using cobalt dopants.In the experiment,Co-doped VO₂ thin film with thickness of 120 nm was deposited on M-Al₂O₃substrate by polymer-assisted deposition method.When the doping concentration reaches 4.0 at.%,the modulation depth of VO₂ thin film could reach 77%over a very narrow(3?)transition temperature window.In ad-dition,X-ray diffraction analysis showed that too much Co(>4.0 at.%)dopants will result in two different phases in the VO₂ thin film,which lead to a decrease of the THz modula-tion effect.Based on the analysis of the effective medium theory,it can be concluded that the newly-formed Co-rich phase does not have IMT behavior and maintains high trans-mittance to the THz wave in the measured temperature region.This phase separation may be responsible for the complex correlation between the Co doping concentration and the THz modulation properties.Based on the phenomenon of phase separation,this study provides a new insight into the modulation mechanism of transition metal doping,which will be beneficial to expand the application of doped VO₂ thin films in the THz regime.3.By combining VO₂ films with metal metamaterial,tunable THz devices with greater modulation depth and frequency selectivity can be obtained.In the experiment,the split ring resonator array was fabricated on VO₂ polycrystalline thin film,the as-prepared device acquire a higher modulation depth at the resonant frequency compared with the pure VO₂ thin film.For pure VO₂ thin film,the modulation phenomenon is broad-band with a mean value of?43%,while the modulation depth of VO₂ -hybrid metamaterials can reach 60%at 0.5 THz and 0.9 THz.The electromagnetic simulation software was used to analyze the dynamic transmission characteristics introduced by VO₂ thin film.It can be found that the THz conductivity and dielectric constant change of VO₂ thin film induced by phase transition plays the essential role in the dynamic response of the VO₂ -hybrid metamaterial.4.By combining the flexible wavy structure with VO₂ -hybrid metamaterial,a thermal-mechanical dual-mode THZ absorber with ultra-broadband tunable resonant frequency can be obtained.The tunability of VO₂ -hybrid metamaterial is limited by the intrinsic physical properties of VO₂ .In this way,in order to realize THz devices with versatile functions,it is essential to combine VO₂ -hybrid metamaterial with reconfigurable and deformable structure design to introduce additional functions.In this study,the flexi-ble wavy structure was combined with VO₂ -hybrid metamaterial,which is equivalent to folding the metamaterial along the direction of THz electric field.Such design can ef-fectively rearrange the meta-atoms spatially and remain the modulation ability of the VO₂ film meanwhile.The experimental results show that the phase transition based on VO₂ thin film can only reduce the resonant frequency from 1.7 THz to 1.4 THz,while stretch-ing the wavy structure can increase the resonant frequency from 1.7 THz to 2.1 THz.It is proved that the dynamic modulation of THz devices can be realized by phase change ma-terials and structural control simultaneously,which can greatly expand the functionality of tunable metamaterials.