Silicon-Graphene Hybrid Integrated Devices For Photodetection And Modulation

Silicon photonics has shown great potentials in the applications of optical communication,optical interconnection and optical sensing,and has been vigorously developed in the recent years.However,as an indirect band gap material,silicon needs to be integrated with traditional semiconductor materials or use ion implantation to achieve some active functions,such as photodetection and modulation,which will bring issues on huge fabrication complexity and lattice mismatch.As a novel two-dimensional material,graphene has several extraordinary properties,such as broadband spectral absorption,high carrier mobility,tunable Fermi level and high thermal conductivity.By using wet-transfer method or imprint method,graphene can be easily transferred on to silicon waveguide.Moreover,there is no lattice mismatch or mode mismatch issue.Though graphene has only a single atomic layer thickness,the light-graphene interaction length can be extended by utilizing silicon waveguide,and thus enhance the light-graphene interaction strength.Therefore,silicon-graphene hybrid integrated devices have drawn extensive attention in the recent years.This thesis first introduces the development of the integrated optics,the advantage of silicon-on-insulator platforms and the typical applications of silicon photonics devices in the fields of optical communication and optical sensing.After that,this thesis introduces the basic properties of graphene in electrical,optical and thermal,outlines some applications of silicon-graphene hybrid waveguide devices,and briefly introduces the simulation methods of hybrid devices.In this thesis,a high-performance near-infrared/mid-infrared graphene photodetector based on a plasmonic waveguide is demonstrated,and high-speed photodetection with graphene in the 2 ?m band is achieved for the first time.We have designed and optimized the structure of plasmonic waveguide,which reduces the metal absorption and improves the graphene absorption rate at the same time.On the other hand,we introduced gate electrode to tune the graphene doping.By scanning the gate voltage and bias voltage,we realized photodetection mechanism switching among photo-conductive,photothermal-electric and bolometric effect,and gave a theoretical analysis.In the 2 ?m wavelength-band,the device responsivity is about 70 mA/W at-0.3 V and measured 3 dB bandwidth is above 20 GHz(limited by the setup);In the 1.55 ?m wavelength-band,the device responsivity is about 0.4 A/W at-0.3 V and measured 3 dB bandwidth is above 40 GHz(limited by the setup),and a 30 Gbps eye diagram test was performed.Secondly,this thesis studies graphene waveguide photodetectors based on a reflective structure,which can shorten the length of graphene in order to increase the RC-limit 3 dB bandwidth,reduce dark current,improve SNR and reduce power consumption.First,we designed and fabricated graphene photodetector based on an ultra-thin silicon waveguide with a loop mirror reflector for near-infrared wavelength-band.Based on this structure,the graphene absorption rate for TE fundamental mode is increased by about 5 times.With 20 ?m long graphene length,80%optical field energy can be absorbed in the active region.In the 1.55 ?m wavelength-band,the photodetector shows a responsivity of 25 mA/W at 0.3 V bias and the measured 3 dB bandwidth is about 17 GHz.In addition,we designed and simulated a graphene photodetector based on a slot waveguide with a Bragg grating reflector for mid-infrared wavelength-band.The absorption rate of graphene can be enhanced to 0.225 dB/?m.The double gate voltage control can be used to build a p-n junction to enhance the PTE effect.Thirdly,this thesis studies the applications of graphene-silicon waveguide based devices in light modulation.First,we designed a graphene electro-optic modulator based on a subwavelength grating structure in the near-infrared/mid-infrared wavelength-band,and then tested the performance of the device in the near-infrared band.The static modulation results show that the extinction ratio of the device is about 0.075 dB/?m.By measuring S11 parameter and building the equivalent circuit model,the 3 dB bandwidth of the device is estimated to be 137 MHz.Second,we designed and simulated a graphene thermo-optic modulator based on a symmetric MZI structure.With bent directional coupler,we realized a thermo-optic device with low insertion loss,low power consumption,high extinction ratio and large optical bandwidth.