Research On The Novel Graphene-based Waveguide And Tunable Micro-nano Optical Device

Graphene,in which carbon atoms arranged in the form of honeycomb lattice,is a two-dimensional monolayer metamaterial.It has attracted much attention of experts and scholars on account of the unique electrical and optical properties and it is regarded as a revolutionarily novel material.Due to the electromagnetic properties of graphene are similar to metallic materials in the infrared band,the surface plasmons(SPs)can be supported by graphene.SPs can break through the inherent diffraction limit to realize the ultra-high resolution imaging and the ultra-high precision lithography.Meanwhile,graphene plasmons(GPs)have lower ohmic loss and longer propagation length compared with SPs supported by the conventional noble metal.Furthermore,graphene has a stronger ability of field confinement than noble metal for the electromagnetic field of the transmission mode.Since the optical properties and plasmon resonance of graphene can be tuned by changing fermi level through electrical gating,the GPs can transcend the geometric constraints of graphene devices.It is possible to adjust the devices using a bias voltage and it will expand the applied scope of graphene-based devices.In this paper,novel graphene-based optical devices are proposed.The characteristics of graphene-based optical devices are researched and analyzed theoretically by geometric modeling and numerical simulation.The main research results and innovation points are summarized as follows:1.A graphene multilayer nanowire waveguide with ultrahigh refractive indices is proposed.The multilayer nanowire waveguide consists of graphene layers and dielectric layers wrapped around each other.It has a strong ability of field confinement and the GPs can be perfectly controlled to propagate in the waveguide.The graphene multilayer nanowire waveguide can support many no cut-off transmission modes.The waveguide can realize multimode transmission under small volume and it is favourable to be applied in sensing.The transmission mode characteristics of the waveguide can be changed by adjusting the fermi level of graphene and the waveguide can break through the structural constraint to achieve tunability.2.A graphene square waveguide with the small effective mode area is proposed and investigated.The volume of this waveguide is in nano-micro scale and it is suitable to be used in optical chips with high integration.When the operating frequency of graphene square waveguide is greater than 20.8 THz,the propagation length of GPs in the graphene square waveguide is better than the nanowire waveguide with the similar size.The normalized effective mode area of the graphene square waveguide is 10-5 and it is two orders of magnitude smaller than the normalized effective mode area of nanowire waveguide.3.A single-band graphene absorber based on graphene ribbon-array and graphene sheet is proposed and investigated.Perfect absorption and easy fabricating can be both realized by this graphene absorber.By numerical simulation,an almost perfect absorption peak can be achieved with the absorptivity of 99.5%at the frequency of 10.9 THz.By adjusting structural parameters,the performance of single-band graphene absorber is further analyzed.According to the performance analysis results to optimize the structural parameters,the absorption peak is adjusted to the terahertz band.At the frequency of 6.15 THz,the absorption peak of 99.9%was obtained.In addition,the bias voltage can be used to adjust the absorption peak of the single-band graphene absorber and it is proved that the single-band graphene absorber can be used in a variety of application environments.4.A novel dual-band graphene absorber is proposed and investigated.The frequencies of the two absorption peaks are 4.95 THz and 9.2 THz and the absorption rates of the two absorption peaks reach up to 99.8%and 99.6%,respectively.The relationship between the structural parameters and the properties of the dual-band graphene absorber is analyzed and the effects of different incident angles on the absorption rates of the dual-band graphene absorber are discussed.Simultaneously,it is verified that the dual-band graphene absorber has a good tolerance for the deviation of incident angle.5.A graphene double-layer waveguide pair with field enhancement is proposed and investigated.Using the coupling principle between the two graphene double-layer waveguide,the energy of the GPs is concentrated in a very small gap between the two waveguides and the field enhancement effect is realized.The field enhancement contrast ratio(FECR)of the graphene double-layer waveguide pair can reach 108 and it is an order of magnitude larger than the previously reported FECR 107 of graphene single-layer waveguides pair.The graphene double-layer waveguide pair is also compared with the metal double-layer waveguide pair and the field enhancement effect of graphene double-layer waveguide pairs is better than that of metal double-layer waveguide pair.