Study On Controllable Synthesis Of Graphene Film And Its Optical Modulating Property/Device

Graphene was discovered in 2004 to overturn the prediction of traditional condensed matter physics theory about that quasi-two-dimensional material can not stably exist at room temperature.Because of the special energy structure and excellent optical,electronic,mechanical and thermal properties,graphene has become a hotspot in various fields,especially in the new optoelectronic devices.Optical modulators are essential components for photonics and optoelectronics,which have been used in the fields of optical interconnects,environmental monitoring,biology,medical and safety monitoring.As the development of these emerging technologies,the optical modulators have put forward higher requirements.Compact,efficient,fast and ultra-wideband optical modulators are urgently needed.Graphene has high carrier mobility,carrier concentration,and wide band electromagnetic waves responsivity.Thus,graphene can be used to design various high speed optical modulators.At present,large amount of research work has shown that graphene-based optical modulator possess huge advantages over traditional materials in the near infrared band and terahertz band.In this thesis,a systemic research has been carried out:the preparation of APCVD graphene,the regulation of terahertz conductivity of APCVD graphene,the regulation of terahertz conductivity of rGO-Gel flexible films,the fabrication and characterization of graphene near infrared electro-optic modulator.Contents of the research are as follows:(1)Controllable preparation of large area of graphene films by APCVD.Acetylene(C2H2)or methane(CH4)was used as carbon source.Copper foil was used as catalyst and substrate.Graphene films was synthesized by atmospheric chemical vapor deposition(APCVD).Preparation process and transfer method of graphene are introduced,and the growth parameters of graphene are optimized.The quality and the number of graphene layer prepared by acetylene and methane as carbon sources are compared respectively.The graphene fabricated in this study was then used as basis material for the graphene-related research in our group.Therefore,the application of the graphene film in our group,which was then briefly introduced.(2)Hydrogen kinetics on scalable graphene growth by APCVD with C2H2.Graphene films was synthesized by atmospheric chemical vapor deposition with C2H2.The influence of hydrogen concentration on the sample quality was studied.Our research demonstrated that hydrogen plays a very important role in graphene preparation.High quality bilayer graphene films were synthesized with a small amount of H2.The results also show that hydrogen has a catalytic property to promote the nucleation of graphene,while hydrogen has etched effect,affecting the nucleation morphology,density and graphene size.This chapter has a major significance for understanding the kinetics of hydrogen to the influence of graphene fabrication.(3)Study on regulation of terahertz conductivity of APCVD graphene.In recent years,graphene has shown great potential in terahertz applications.The terahertz conductivity of graphene is an important parameter in evaluating the performance of terahertz devices,such as graphene-based optical modulator.In this chapter,the changes of graphene terahertz conductivity at different growth temperatures or hydrogen concentration,and chemical doping were studied in detail.The results show that terahertz conductivity of graphene can be improved by optimizing the growth temperature or the hydrogen concentration.This is because that the high quality of sample has more carrier scattering time.Meanwhile,the carrier concentration of graphene can be increased by chemical doping.These results provide a better understanding to the graphene growth dynamics for improving its terahertz conductivity by means of in-situ growth or post-treatment.(4)Study on regulation of Terahertz conductivity of rGO-Gel flexible films.In this chapter,different concentration of self-supporting rGO-Gel flexible films were prepared.The photo-induced terahertz time-domain spectroscopy was used as a non-destructive measurement method to study the optical response of rGO-Gel films in the wavelength range of 0.2-2.0 THz.Under the 800 nm laser illumination,rGO-Gel films were observed to exhibit photoconductance,resulting in modulating terahertz transmission.Modulation depth of terahertz transmission at 1.9 THz is around 33.5%.Most importantly,terahertz persistent photoconductive phenomena(PPC)were observed in rGO-Gel films.This is due to photo-induced electron trap in the defects of rGO and the interface between rGO and gelatin,and is enhanced by the strong interaction between rGO and gelatin.This work provides new ideas for fabricating terahertz optoelectronic devices based on rGO-Gel flexible films,such as all-optical modulators,optical switches,photodetectors,optical sensors,and optical memory devices.(5)Fabrication of graphene-based electro-optical modulators.For different purposes,the electro-optical modulator of the graphene flat capacitor was designed and fabricated on two kinds of substrates(quratz and waveguide)respectively.Quratz has small insertion loss in the near infrared band.Therefore,the modulator was used to realize the active Q-switching in 1550 nm fiber laser.In the design of graphene based modulator on waveguide,graphene interact with evanescence field at the surface of waveguide,so the different insulation materials on waveguide surface were simulated.And the better one was found.When HfO2 was as insulating layer,the light intensity of evanescence field is more than 0.5 within the 90 nm from waveguide.The technical problems encountered in micro-nano processing of graphene-based optical modulator on waveguide were solved here.And the modulator was fabricated.