The Design And Research Of Waveguide Coupling Devices Based On Graphene SPPs

Surface plasmon polaritons(SPPs),which are coupled electromagnetic modes of exciting photons and surface electrons in conductors or semiconductors,provide the possibility to realize sub-wavelength optical devices and integrated optical circuits for their ability of breaking the traditional diffraction limit.Recently,scientists find that graphene is capable of supporting SPPs,the SPPs supported in graphene possesses many wonderful properties such as low loss,extreme confinement and long lifetimes.Compared to noble metals,the most attractive plasmonic property of graphene is that the SPPs in graphene can be tuned by a diversity of ways such as by utilizing electric or magnetic field,chemical doping or a gate voltage.These fantastic advantages provide possibility of realizing a new generation of micro-and nano-optical devices.To date,scientists have made many achievements both theoretically and experimentally with regard to the transmitting and turning of graphene SPPs.These achievements lay a foundation for us to design and study novel graphene based SPPs waveguide coupling devices.In this paper,we have designed three types of graphene based waveguide coupling optical devices using numerical simulations: a graphene ring resonator,a graphene based terahertz directional coupler and a Bragg reflector based on graphene nano-ribbons.We have analyzed the optical properties and application potential of these optical devices by using finite element method(FEM)and finite-difference time-domain(FDTD)method.The main results are arranged as follows:(1)Considering the SPPs can transmit along curved graphene surface,we have designed an active graphene ring resonator,which is composed of a planer graphene waveguide and a monolayer graphene that is rolled into a cylindrical shape.Calculated with finite element method,we find that both a high quality factor and extinction ratio of the resonant mode can be achieved when the plasmonic waves in the ring and the input waveguide are properly coupled;the resonant frequency can be tuned over a wide frequency range by a small change of Fermi level in the graphene ring;it has a good performance in optical modulation in the infrared range.Generally speaking,this kind of resonator show high value in a couple of applications such as optical filtering,optical modulation,optical switching and optical directional coupling.(2)Taking advantage of the strong confinement of graphene on THz waves,we have designed a graphene based THz directional coupler.Calculated with FDTD method,we discover that for the frequency of 7.5THz,SPPs wave can be coupled to the right output port as long as the Fermi level in graphene is 0.6eV,however,when Fermi level is reduced to 0.3eV,the SPPs waves are channeled to the left output port.Since the Fermi level in graphene can be tuned by utilizing a biased voltage,the transmission direction of THz SPPs wave can be controlled dynamically.(3)Taking advantage of the extreme confinement of SPPs in graphene nano-ribbon,we have designed a active tunable Bragg reflector based on grapnene nano-ribbon waveguide.It's constructed by periodically stacking graphene nanoribbon waveguides with patterned Fermi levels.The ribbon width is 10 nm.Calculated with finite element method,we find that the Bragg bandgap width is 2.77?m at wavelength of 10?m;the position and width of the bandgap can be tuned by the ribbon width or the Fermi level in the ribbon.This kind of Bragg reflector exhibits extreme compactness of lateral scales and wonderful light confinement in both the longitudinal and the lateral directions,which is expected to have significant applications in constructing 3D highly integrated optical networks or systems.