Researchs On The Excitation And Modulation Of Graphene Surface Plasmon Polaritions

Owing to its two dimentional properties and the zero band gap,graphene is thought of the rising star on the horizon of materials science and condensed-matter physics,and has attacted a lot of attention in many fields like electrics and optoelectronics.Since doped graphene can support the resonances of surface plasmon polaritons(SPPs),this strictly two-dimensional material provides a platform for strongly enhanced light-graphene interactions.Besides,graphene surface plasmons(GSPs)presents some appealing properties such as high tunability,extreme confinement,and low losses in the mid-infared,opening up a new avenue towards future plasmonic applications including refractive biosensing,nonlinear and slowing light effects.However,one of the important challenges that graphene must overcome before it can legitimately declare its irreplaceable position among the fields of plasmonic materials is to achieve efficiently coupling to external light.In this paper,we will discuss how to achieve efficiently excitation and modulation of the graphene surface plasmons with refractive gratings.The main results of this thesis are listed blow.(1)A scheme capable of exciting GSPs in in-plane bended gratings that are formed by elastic vibrations of graphene nanoribbons(GNRs).The gratings enable the light polarized perpendicularly to the GNRs to two kinds of GSP modes,of which the field concentrations are within the grating crest(crest mode,C-M)and trough(trough mode,T-M),respectively.These two kinds of modes will individually cause notches in the transmission spectrum and permit fast off-on switching and tuning of their excitation dynamically(elastic vibration,Fermi energy)and geometrically(ribbon width).The performance of this device is analyzed by finite-difference time-domain simulations,which demonstrates a good agreement with the quasi-static analysis theory.The proposed concept expands our understanding of plasmons in GNRs and offers a platform for realizing of 2D graphene plasmonic devices with broadband operations and multichannel modulations.(2)A concept that is capable of exciting localized SPPs in flat gratings formed by sinusoidally shaping GNRs is proposed.These gratings enable the parallel-polarized light to couple into SPPs,creating a sharp notch with ultrahigh Q-factor on the transmission spectrum.Besides,the excited SPPs can be tuned not only by adjusting the geometrical parameters(arc length of sinusoidal grating and the ribbon width),but also by changing the Fermi level of graphene.After reasonably considering the amplitude and period of the 2D grating,both the theoretical analyses and the numerical results demonstrate the applicable properties of this structure.This work provides a framework for understanding the mechanism of plasmon excitations and designing tunable 2D plasmonic devices such as filters,switches,and sensors.(3)An effective method to excite localized SPPs on graphene-coated nanowire arrays(GCNAs).These SPPs are analyzed by introducing a universal scaling law that considerably simplifies the understanding of these modes.Meanwhile,numerical experiments are carried out to demonstrate the theoretical analysis of plasmon excitations.The excited SPPs permits the control through both geometrical and physical properties.The proposed structure can be used as a tunable optical filter,highly sensitive refractive index sensor and other plasmonic modulation devices.(4)An approach to capturing light efficiently into GSPs by patterning the sinusoidal dielectric metasurfaces above and below the graphene sheet is proposed.The presence of plasmonic resonance is demonstrated by means of an analytical model based on transformation optics through the extraction of effective graphene conductivity,which is further revealed via numerical study of the optical spectra as a function of grating parameters.Besides,the resonant position is found to be sensitive to the dielectric contacted with graphene.These findings can deepen our understanding of plasmon resonances and pave the way to the design of graphene plasmonic devices like refractive index sensors.