Research On Terahertz Waveguide Devices Based On Graphene

A lot of waveguide structures have been designed in order to meet various needsrecently, such as widely-used photonic crystal waveguides. As it is well known, thephotonic crystals (PCs) are normally composed of dielectrically mismatchedstructures that can affect the propagation of light waves. The PCs have beenextensively studied for their unique optical properties, such as negative fraction,super-prism, self-collimation, etc. As a result, they have been widely applied intelecommunication and optical waveguide. At present, a great progress has been madein theoretical design and in device fabrication of photonic crystal waveguides forapplication mainly in visible and infrared bandwidth. However, the micro structuresproduced by the typical material of quartz in THz frequency range of absorption lossis higher, and not suitable for the production of THz waveguide. The design andfabrication of waveguide to be used in terahertz (THz) regime is still a challengingfield of research due to relatively long wavelength of the THz light waves. In thisstudy, we would like to explore the possibility to utilize graphene as material toproduce the THz waveguide.Graphene is a new class of nanostructure with single or few layers of carbonatoms arranged in a hexagonal lattice. In recent years graphene is replacingconventional indium tin oxide (ITO) electrodes in LCD and OLED in order to makebetter and cheaper displays, owing to its excellent properties in conductivity and lighttransmission. Very recently, graphene has been proposed to be used in infraredoptoelectronic devices due to its high electrical conductance and high lighttransmittance in infrared bandwidth. Considering that graphene has lower refractiveindex and that different dielectric structures can have great influences on theproperties of waveguide, here we intend to examine the optical properties of asemiconductor-graphene cylinder structure and to discuss its potential applications inTHz waveguide.We propose a semiconductor-graphene structure which can serve as a terahertzwaveguide. In such a structure, grapheme plays a role in achieving strong mismatch of the dielectric constant at semiconductor-graphene interface due to its two-dimensionalnature and relatively low value of dielectric constant.Two important conclusions canbe drawn from the present theoretical study.Firstly, when the radius of the outer semiconductor layer ρ１is about100μ m, thefrequencies of different photonic modes are within the THz bandwidth and they canbe tuned efficiently via altering ρ１.The transmission frequency decreases withincreasing ρ１ ，which can be used to tune the transmission frequency.Secondly, roughly linear dispersion relation can be found for photoinc modes ina semiconductor-graphene structure when propagation constantk>3×1041zm. Thus,there is a weak effect of group velocity dispersion in the semiconductor-graphenestructure over a wide range ofk z.Hence, the proposed structure can be applied for transmission of THz waves witha weak effect of light pulse broadening in the remote transmission. This study ispertinent to the application of grapheme as THz photonic devices.