Study Of Silicon-based Photonic Devices By Nonlinear Optical Effects Of Graphene

Silicon is common semiconductor materials.The fabrication process of silicon-based opto-electronic devices is compatible with process of complementary-metal-oxidesemiconductor,which lays a foundation for mass production.In telecommunication bands,because of high linear refractive index and low linear absorptive index of silicon,the integrated opto-electronic devices made of silicon-on-insulator are provided with strong light field confinement and low transmission loss.These make silicon be a good platform for high-density opto-electronic integration.Silicon has abundant nonlinear optical effects,which can be used to realize optical modulation,optical pulse broadening and compression,optical wavelength conversion and so on.Materials with excellent optical and electrical properties heterogeneously integrated on silicon opto-electronic devices can further improve their performance.Graphene is a two-dimensional material of monolayer carbon atom sheet.It has a cone-shape energy band structure and strong covalent bonding,which induce remarkable optical,electrical,and thermal properties.Compared with silicon,graphene has stronger nonlinear optical effects,higher electrical conductivity and thermal conductivity,and faster carrier relaxation.By chemical doping and electrical gating,the tunable Fermi energy of graphene is able to change the above properties.Graphene properly integrated onto chips can improve performance of opto-electonic devices.In this dissertation,the nonlinear optical effects of graphene and corresponding applications on silicon-based photonic devices are investigated.The major contents of dissertation are listed as follows:(1)A silicon all-pass microring resonator(APMRR)assisted by graphene is fabricated to realize photo-induced thermo-optical refraction switching.The optimized cladding length of low-doped graphene on an overcoupled silicon APMRR realizes nearly critical coupling with an extinction ratio of 21 d B.For an input power of 250 μW,this optical switch has a tuning efficiency of 216 pm/m W,which is 3.6 times as large as the value of pure silicon optical switch as a reference.Meanwhile,a static extinction ratio of10 d B is realized.Furthermore,a 0.2-μs rise time and a 1.5-μs fall time following the10%–90% rule are obtained.(2)Theoretically,the dependence of Kerr effect and two-photon absorption on the Fermi energy of monolayer graphene is investigated by perturbation theory.Due to thermal fluctuations at finite temperature,the incident photon with energy below twice the Fermi energy of graphene could also induce nonlinear optical effects.For the photon with an energy of 0.8 e V,when the Fermi energy is about 0.42 e V,giant nonlinear refractive index coefficient(~10-13 m2/W)and negligible nonlinear absorption could be obtained.(3)Theoretically,the pulse evolution dynamics in two-stage temporal compression are studied by an improved nonlinear Schr?dinger optical transmission equation.In the temporal compressor,a graphene-silicon hybrid waveguide serves as a phase modulator.Graphene exhibits obvious saturable absorption but negligible two-photon absorption when the optical intensity in graphene cladding is below the threshold intensity of two-photon absorption of graphene.By the utilization of saturable absorption and the repression of two-photon absorption,the temporally compressed pulse is improved.The temporal shape of pulse can be achieved to be pedestal-free,and the temporal acceleration of pulse can be weakened to be negligible.Meanwhile,the enhancement of Kerr effect from graphene can contribute to the decrement of the length of device and the input power of pulse.Compared with pure silicon devices,the graphene-assist silicon device can make the product of the length of device and the input power of pulse decreased to 1/6 of the value with the pure silicon device realizing equivalent temporal compression ratio of pulse.Finally,the full width at half maximum of a pulse with a peak power of 1 W and a temporal half width of 5 ps is compressed by 4.48 times through the graphene-assisted device with a length of 1.91 mm.