Study On The Propagating Properties Of Graphene Surface Plasmonic Optical Waveguide

The surface plasmon polaritons?SPPs?occurs as a form of collective charge oscillation at the interface of metal and dielectric,is a kind of elec-tromagnetic wave due to the coupling between the input light wave and the free electrons on the metal surface.The SPPs can break through the diffrac-tion limit and confine the electromagnetic wave in subwavelength scale.In recent years,SPPs has been used to transmit and control light signals.How-ever,to support the propagation of SPPs,noble metals generally have larger propagation losses as well as heat losses.In addition,it is hard to change the shape after beenmade,and which will limit its application range and prospect.Recently,people have found that the use of graphene instead of metal materi-als can also support the transmission and controlling of SPPs.Compared with the traditional noble metal materials,graphene has some major advantages.First,graphene surface plasmons?GSPs?can be found in the mid-infrared and THz band.Second,GSPs have strong mode confinement and small propagation loss.Third,the dielectric constant of graphene can be adjusted by chemical doping and external bias voltage.In recent years,many achieve-ments have been made in the transmission and controlling of GSPs,and great breakthroughs has been made in both experimental and theoretical aspects.On the basis of these research results,several waveguide devices based on GSPs are designed and studied by numerical simulation in this paper.The main contents are as follows:?1?A type of a surface plasmon waveguide composed of dielectric nanowire coated with two layers of graphene was proposed.The dependences of the mode effective refractive index,propagation length,mode area and the figure of merit on the geometrical parameters and electromagnetic parameters are discussed.The simulated results show that the proposed waveguide has advantages of simultaneously achieving lower propagation loss and stronger mode confinement with extremely small normalized mode area which is helpful for high density photonic integrating.?2?To further obtain more compact waveguide structure with more op-timal propagating properties,a novel plasmonic waveguide composed of graphene coated dielectric bow-tie type nanowire pair is designed.Simulation result shows that the propagating properties of the proposed waveguide structure can be effectively modulated by changing the geometry parameters and the electromagnetic parameters of graphene and surrounding dielectric medium.In addition,it shows that the proposed waveguide can strongly en-hance the optical field in the gap region of the proposed waveguide structure.The minimum normalized mode area can obtain to an order of 10^-7 of magni-tude.Moreover,it has a good tolerance to practical fabrication errors.More-over,a novel type of plasmonic waveguide consists of graphene-coated nan-owire integrated with a triangle dielectric nanowire is designed,which can also achieve the extremely compact light confinement and low propagation loss owing to the strong coupling between the GSPs and the triangle dielec-tric nanowire.The fundamental electromagnetic field mode properties of this proposed structure can also be effectively modulated by changing the geo-metric parameters and electromagnetic parameters.?3?According to the understanding of the propagating properties of graphene-coated nanowire waveguide structure,a type of graphene-based waveguide supporting the property of plasmon induced transparency?PIT?effect is designed and investigated.The waveguide is composed of a gra-phene bus waveguide side-coupled with a graphene strip resonator and a graphene ring resonator.The research results show that the PIT propagating effects can be adjusted by changing the length of graphene strip resonator,the radius of graphene ring resonator,the coupling distance and the chemical po-tential of the graphene.Meanwhile,the proposed waveguide structure can also work as a nanoscale refractive index sensor with high sensitivity.The research result can help to deepen the understanding of PIT-like effect and the application of nanoscale refractive index sensor,and is also beneficial for the designes and the applications of nanoscale graphene-based optical devic-es,such as optical modulator,optical switch and optical sensor.