Electro-Optic Modulation Characteristic Of Graphene And Its Applications In Optical Modulators

With the rapid development of informationization process,the amount of information that needs to be transmitted and processed is explosively growing,which leads great challenges for fiber-based optical communication systems and networks.Optical modulator is the key device for modulating electrical signal onto optical carrier,playing critical role in communications system.Therefore,the rapid development of fiber-based optical communication systems and networks demand for better performances of optical modualtor,such as higher modulation speed,smaller footprint,lower consumption,and integratable.However,the modulators based on traditional electro-optic materials can't fulfill those requirements.Graphene is an atomically thin monolayer of benzene-ring packed carbon atoms.Possessing ultra high carrier mobility under room temperature,having excellent electrooptic properities,allying with compatibility with CMOS processing,graphene is considered to be an ideal material for realizing broadband,smaller footprint,and lower consumption optical modulator.Therefore,the graphene-based optical modulator has become a rising and hot research topic.In this thesis,the electro-optic modulation characteristic of graphene and its applications in optical modulators are studied.The numerical model based on graphene's electro-optic properties has built.Integrating graphene with silicon waveguide,seveval novel structures of graphene-based optical modulator are proposed.The nano-fabrication processes of graphene-based modulators and related photomask design are given.The preparation and transfer techniques of graphene have been investigated.The fabrications of lumped modulator and traveling-wave modulator are completed.The details are listed below.1.The methods of graphene's synthesis,techniques of graphene's characterization,and methods for design and analysis of optical waveguide are reviewed.The main methods of graphene's synthesis include mechanical peeling,oxidation-reduction and chemical vapor deposition.The optical microscopy,electron microscopy,scanning tunneling microscopy and Raman spectroscopy are the four main techniques that for characterization of graphene.Four numerical methods for analyzing optical modes in optical waveguides are introduced,including effective index method,beam propagation method,finite difference time domain method and finite element method.2.The electro-optic modulation characteristic of graphene is studied,and numerical model based on graphene's electro-optic properties is built.The optical absorption of graphene varies with its chemical potenial.The chemical potenial of graphene can be controlled by chemical doping or external bias voltages.Therefore,the optical absorption of graphene can be electrical controlled.The relationships between graphene's chemical potenial and biases,graphene's condactivity,graphene's permittivity are investigated,which reveal the mechanism of electro-optic modulation of graphene.3.Three structures of graphene-based lumped modulator are proposed,include dual-graphene-on-graphene electro-absorption modulator,graphene-silicon waveguide(GSW)based phase modulator and polarization independent modulator.Simulations results show that extinction ratio(ER)of 30 dB,bandwidth of 100 GHz and consumption of 5.64 f J/bit can be achieved by only 5 ?m-long modulation length for dual-graphene-on-graphene electro-absorption modulator.For the proposed phase modulator,a ? phase shift can be achieved by a 75.6 ?m-long GSW.Calculations show that the 3dB modulation bandwidth of the optical phase modulator can be as high as 119.5 GHz with a low consumption of 0.452 pJ/bit.In term of the polarization independent modulator,for a 30 ?m–long GSW,throughout the 1.45 ?m~1.60 ?m wavelength ranges,polarization independent modulation can be obtained and ER higher than 20 dB.Calculations show that the 3 dB modulation bandwidth higher than 69.8 GHz.4.Two structures of traveling-wave(TW)modulator are proposed,include graphene-based microstrip line(MSL)TW modulator and graphene-based coplanar waveguide(CP)TW modulator.In the TW modulator design,graphene act as both a microwave transmission line and light absorption medium.When the microwave impedance matching condition is satisfied,the appied microwave signal can be transmitted through the modulator effectively.The simulation results show that the modulation depths of the TW modulators are 0.0792 dB/?m.The 3 d B bandwidth of both the MSL TW modulator and CP TW modulator can be higher than 200 GHz.5.The nano-fabrication processes of graphene-based modulators are investigated.The fabrication related photomask designs are given by using the L-edit software.The preparation and transfer techniques of graphene have been investigated.The reduction of the resistance of the device by two cycles of rapid thermal annealing(RTA)processing has been experimental demonstrated.The results show that RTA processing is a robust and convenient way to reduce the resistance of the device.The detail process recipes are given and the fabrications of lumped modulator and traveling-wave modulators are completed.Some initial tests to the fabricated devices have conducted.For the popped up issues,the suggestions and schemes for improving the devices' s design and fabrications are proposed.