Research On Equivalent Circuit Of Graphene And Its Application In THz Modulator

THz wireless communication has a lot of advantages such as high speedtransmission rate, rich spectrum resources, uneasy to detect and capture, broadbandwidth, high anti-jamming capacity, and so on. THz modulator has been a hotarea of research in THz wireless communication due to THz modulator is one of thekeys for THz wireless communication. The performance of modulator mainlydepends on the selection of materials. Graphene not only has the excellent physicalproperties such as optically transparent, stable, high-strength, high conductivity, highthermal conductivity, but also has the unique band structure for zero band gap andlinear band structure near the Dirac point, the conductivity of graphene can bechanged with the chemical potential. The remarkable properties possessed bygraphene, which make it an ideal material for THz modulator.Based on the analysis of the equivalent circuit of monolayer graphene andmultilayer graphene, a through-beam modulator, a reflect-beam modulator and anreflect-beam modulator based on bilayer graphene are designed and analyzed in thispaper. The primarily work and achievements of the paper are as follows:By using the equivalent circuit of the free space graphene and the equivalenttransmission lines of the medium wave guide, the equivalent circuit of the graphenewhich on top of a SiO2/Sisubstrate and the equivalent circuit of the multilayergraphene are established, and the above-mentioned circuits are simplified.Then, by using the above-mentioned equivalent circuit, three kinds of THzmodulator is designed, which are the through-beam modulator, the reflect-beammodulator and the reflect-beam modulator based on bilayer graphene.By using the equivalent circuit of the graphene which is on top of a SiO2/Sisubstrate, the through-beam modulator and the reflect-beam modulator is designedand analyzed. The through-beam modulator is made up of quartz substrate, singlelayer graphene and SiO2/Si. The modulation depth of the modulator is calculated by itsequivalent circuit, the result is consistent with the result calculated by analytical scattering matrix. The theoretical analysis indicates that the modulation depth is24.12%and the3dB modulation bandwidth55.5KHz when the carrier is600GHz and the biasvoltage change from0-50V. For the further optimization of the modulator by using2layers of graphene, the modulation depth is rising to40.13%and the3dB modulationbandwidth is rising to98.41KHz.The reflect-beam modulator is made up of metal cladding, quartz substrate, singlelayer graphene and SiO2/Si. Due to the electromagnetic wave is reflected by metalcladding, the electromagnetic wave will go through the praphene twice, themodulation depth will higher than the same specification through-beam modulator. Bythe same analytical method, the modulation depth and the3dB modulation bandwidthis calculated as68.8%and55.5KHz when the carrier is600GHz and the bias voltagechange from0-50V.Physically separated2layers of graphene by aluminium oxide, and apply thiskind of structure within the design of the reflect-beam modulator, a reflect-beammodulator based on2layers of graphene is designed. This kind of structure avoids thelimitation of the scale of the multilayer graphene using, and can provide greatermodulation depth. The equivalent circuit of the modulator is built based on theequivalent circuit of the multilayer graphene. The modulation depth of the modulatoris calculated as94.67%by its equivalent circuit when the carrier is600GHz and thebias voltage change from0-50V. The resistance of the device is increased due to theseries connection of the two layers of graphene, so the RC time is longer, and the3dBmodulation bandwidth is reduced to15.355KHz.