Researchs On The Theoretical Design Of Graphene-based Plasmonic Electromagnetic Coupling Devices And Their Property Modulation Mechanism

Surface plasmon polaritons(SPPs)are supported by the graphene in the mid-infrared region becsuse of its unique electronic band structures.Since the gra-phene SPPs are confirmed experimentally in 2012,a growing number of research foci have been pointing to the graphene-based plasmonic nano-photonic devices in the past years.Graphene is emeraging as new patform for the sub-wavelength plasmonic pho-tonics.In this paper,based on the finite-difference time-domain(FDTD)method and coupled-mode theoty(CMT),we study the graphene-based plasmonic electromagnetic coupling filter,Bragg reflector,plasmonically induced transparency(PIT)device,and perfect absorber.The theoretical design principle and property modulation mechansim of these coupling devices are investigated both theoretically and numerically.The main results of this thesis are listed blow.(1)The monolayer graphene with two cavities is proposed theoretically.Silula-tion results present that the propagating SPP wave will be reflected at the interface between the graphene and the air cavity.The graphene strip sandwiched between two air cavities will work as a novel Fabry-Perot(F-P)resonator.The proposed single graphene sheet hence exhibits an outstanding mid-infrared band-pass filtering effect.The spectral position of the transmission peak is tuned dynamically not only via changing the length of the graphene strip sandwiched in cavities but also by a small change in the chemical potential of graphene.The quality factor of transmission peak is also optimized by changing the width of the cavity.In addition,the wavelength of the transmission peak tends to exhibit a linear red shift as the refractive index of the substrate increases,and the simple structure of the single graphene sheet with two cavities thus has potential applications in mid-infrared plasmonic sensors and optical filtering.(2)We construct theoretically the structure of the parallel double-layer graphene waveguide with a sandwiched uniform dielectric grating.The wave vectors and elec-trical field distributions of the symmetric plasmonic mode and anti-symmetric plas-monic mode supported by the parallel double-layer graphene are analyzed.We study the effect of the periodic dielectric grating on the two kinds of plasmonic modes.Si-mulation results reveal that this simple structure working as an original plasmonic Bragg reflector can produce an ultra-broadband band-stop filtering effect in the mid-infrared region,because the propagating symmetric SPP mode can be modulated regularly by the sandwiched uniform dielectric grating.The corresponding physical mechanism is presented.In addition to varying the structural parameters of the di-electric grating,a small change in the chemical potential of graphene is also used for tuning dynamically the transmission spectrum.On the other hand,the bandwidth of the stopband can be modulated by varying the refractive index of the dielectric grating.The bandwidth of the stopband up to 4.3 μm is achieved.As an application,a defect is introduced into the uniform dielectric grating,and modulation of the transmission spectrum is desmonstrated.The whole structure without doubt supports the fabrication of nano-plasmonic integrated circuits in the mid-infrared region.(3)We study theoretically the plasmonically induced transparency(PIT)effect based on the planar structure of the single graphene ribbon bus waveguide coupled with short graphene nanoribbons.Simulation results exhibit that the short graphene strip behaves as a perfect Fabry-Perot(F-P)resonator based on the unique edge mode propagation.For the single graphene bus waveguide coupled with a short graphene nanoribbon,the obvious band-stop filtering effect is obtained.For the single graphene ribbon bus waveguide with two identical short graphene nanoribbons paralleled lo-cated on the same side of the waveguide at different coupling distances,the closer and farther short ribbons act as the bright and dark modes,respectively.The extreme de-structive interference between bright and dark modes gives rise to the outstanding mid-infrared PIT response.The central wavelength of the transparency window is tuned dynamically by a small change in the chemical potential of graphene,without re-fabricating new structures.The quality factor of the PIT window can be controlled by varying the coupling distance between two short graphene strips.Numerical results are in excellent agreement with the coupled-mode theory(CMT)and the three-level system.The PIT window with the group time delay up to 0.28 ps is achieved,which may find utility in plasmonic devices for the slow light.(4)We study a wide-angle plasmonic narrowband perfect absorber based on the periodic double-layer graphene ribbon arrays.Simulation results exhibits that the an-ti-symmetric dipole-dipole coupling resonance supported by double-layer graphene ribbons acts as the electrical resonance to suppress the reflection,due to its impedance matching to that of vacuum.The ultra-thick metallic substrate is used for restricting the transmission.The outstanding perfect absorption peak with the FWHM of 300 nm is obtained,which is in excellent aggrement with that predicted by the versatile coupled-mode theory(CMT).In addition to varying geometrical parameters of the absorber,a small change in the chemical potential of graphene can be used for con-trolling the spectral position of the absorption peak.Meanwhile,this device exhibits good absorption stability over a wide angle range of incidence around ±60° at least.Such absorber will play an important role in the mid-infrared optical filtering and so-lar cells.