Controllable Preparation Of Graphene On Quartz Substrate And Its Characteristics For Terahertz Emissison And Modulation

Since mechanical exfoliation in 2004,graphene due to its zero band gap structure,high mobility and linear energy dispersion can be widely applied in optical modulators,solar cells,transistors,photodetectors etc.It is necessary to transfer the graphene film from the metal substrate to the dielectric substrate through etching technology for graphene-based optoelectronic devices.During the transfer process,impurities and defects will inevitably be introduced to reduce the quality of graphene.Therefore,researchers tried to grow graphene films directly on dielectric substrates by metal-free method.Among dielectric substrate materials,quartz has excellent optical properties in broadband and low thermal expansion coefficient,so it has become one of the common substrates to maintain high transmittance and been widely used in the graphene-based photoelectric applications.Therefore,it is of great significance to study the controllable preparation of graphene on quartz substrates.Recently,terahertz technology has shown excellent applications in high-speed optical communication,biomedicine,time-domain spectroscopy,chemical sensing etc.One of the main factors which surpressing the further development of terahertz technology is the lack of functional devices.Due to its high carrier mobility and relatively low momentum relaxation rate,graphene can be used to generate terahertz waves,which offers potential applications in the field of new terahertz radiation sources.More importantly,due to the absorption characteristics of graphene in terahertz band follow Drude conductance model and have the property of tunable Fermi level,the terahertz absorption characteristics of graphene can be actively modulated through electrical doping,all-optical doping,and magneto-optical effect etc,which can be enhanced by introducing stack graphene layer,cavity,and metastructure as well.Therefore,according to these research background,this dissertation first studied the controllable preparation process of graphene on quartz substrate,and then the terahertz emission characteristics of reduced graphene oxide were studied.Moreover,the mechanism of terahertz emission from reduced graphene oxide was analyzed.Finally,the fabrication and variable angle modulation of terahertz optical modulator based on graphene micro-ribbon and hybrid graphene metamaterial are studied.The main work and innovations are listed as followed:?1?The preparation of graphene films on quartz substrate by chemical vapor deposition.In order to avoid the impurities introduced in graphene transfer process and the damage to the quality of graphene,the graphene film was synthesized on the quartz substrate with acetylene?C₂H₂?as the precursor by atmospheric chemical precipitation.The influence of growth time and hydrogen concentration on its growth quality was studied,and a large area of continuous graphene with AB-stacked layer was successfully prepared.At the same time,a surface-assisted catalytic growth mechanism was obtained,and the hydrogen kinetics analysis showed that there was a certain competitive effect between hydrogen catalysis and etching during the growth of graphene.The former was dominant when hydrogen partial pressure ratio was less than 0.33,while the latter is dominant when hydrogen partial pressure is higher.In addition,by assembling a graphene/quartz based optical modulator,the maximum modulation depth measured at wavelength of 1100 nm with voltage of 3.0 V can be up to 3.4%.This work provides a controllable preparation method for directly grown graphene,which provides a better development platform for its application in optoelectronic devices.?2?Terahertz emission from reduced graphene oxide.Reduced graphene oxide?RGO?films with different thickness and reduction degree were prepared on quartz substrate by soluable method.A femtosecond laser pulse was then used to excite the RGO film to produce terahertz radiation,which shows that the stimulated THz radiation can be enhanced by improving the reduction degree and reducing the thickness of the film,and the maximum value of the generated THz radiation is around 0.3 THz.Further results show that the amplitude of THz radiation increases linearly with the laser power,implying a second-order nonlinear process.Considering the symmetry of RGO film,we fitted the dependence between THz electric field component and polarization of pump beam through theoretical calculation,and the fitting results were in good agreement with the experimental results,which also verified that the terahertz generation mechanism in RGO was controlled by photon traction effect.In this work,the terahertz radiation characteristics of RGO thin films with different thickness and reduction degree were systematically studied,and the THz generation mechanism was clarified,which provided a research basis for ultrafast terahertz photonics applications based on graphene-based materials.?3?Study on fabrication technology of micro-nano processing.By learning from fabrication techniques,which includes electron beam lithography,optical lithography,electron beam evaporation,reactive ion etching,and atomic layer deposition etc,the influence of preparation parameters on the performance of devices were studied in detail.The quality of the materials and devices can also be confirmed by using scanning electron microscopy,atomic force microscopy,Raman spectroscopy,etc.At the same time,wire-bonding technology was employed to study the performance combined with electrical test platform.?4?The variable angle-dependent modulation with graphene micro-ribbon and hybrid graphene metastructure in terahertz region.By using CST microwave studio software,the processing parameters were determined and the electro-optical modulator was made successfully.By utilizing graphene as an actively tunable conductivity employing ion-gel serves for electrical gating,the near-field interaction and the THz transmission can be modified.The graphene micron band structure has resonance absorption at 0.60 THz due to electrical mode coupling,and its modulation depth increases with the increase of applied voltage.Meanwhile,due to the magnetic mode coupling,the graphene composite metal metamaterial structure has resonance absorption at 0.64 THz,and the optical modulation depth is significantly enhanced with the increase of incident Angle.The modulation depth of the gated grahene/metastructure device can obtain?26.5%at 0.64 THz with gate voltage of-3 V.This provides a useful reference for the study of Angular dependent modulation in terahertz?THz?region by coupling graphene with a metastructure.This section is being written.