Research On Graphene Plasmonic Metamaterials And Their Photonic Devices

Since the first fabrication of monolayer graphene by A.K.Geim and K.S.Novoselov in 2004,graphene has attracted enormous interest of researchers due to its distinctive and intriguing optical and electronic properties as diverse as zero-band gap,tunable Fermi level,high carrier mobility and optical transparency.In photonics and optoelectronics,a variety of high-performance graphene-integrated optoelectronic devices have been successively reported,for example,electro-optical modulators,photodetectors,and saturable absorbers.Recently,graphene has been experimentally proved to support the surface plasmons with flexible tunability at deep subwavelength scale,called graphene plasmons(GPs),which provides the opportunities and also challenges for the development of integrated photonics.Additionally,as a highly designable technique for exploiting artificial materials,metamaterials(MMs)possess exotic properties that can never be achieved by materials and devices in nature,such as negative refraction,invisible cloak,rainbow trapping,and metalens.In this dissertation,for the purpose of the functionalities and applications of grapheme-integrated photonic devices,the optical properties of GPs have been systematically investigated,and then the design methods of 1D,2D,and 3D graphene plasmonic metamaterials are proposed and demonstrated theoretically and numerically,which successively inherit the unique advantages of GPs and MMs.Finally,on the basis of the proposal,several types of graphene metamaterial photonic devices operating at mid-infrared(MIR)and terahertz(THz)range are investigated in details,including Bragg gratings,transformation optical lenses,electro-optical switches,dual-polarization optical modulators,and tunable multi-channel optical filters.The main contents of this dissertation are shown as follows:1.The optical properties of GPs are systematically investigated.According to the carrier transition processes in energy band of graphene,the basic condition for GPs with minimal loss is:?ω<0.20 eV and 2E_F>?ω.The propagation of GPs in the dielectric-graphene-dielectric waveguide model is theoretically analyzed,and the dispersion relation,propagation loss,and field confinement of TE and TM modes are presented.The filtering,polarization-selectivity,and voltage-tunability of graphene micro/nanostructures are discussed.The equivalent model of graphene in the simulation and calculation is provided and the relationship between equivalent permittivity and surface conductivity is investigated.Moreover,both the finite-difference time-domain(FDTD)and transfer-matrix method(TMM)models of grapheme are constructed.2.The tailoring and controlling approaches of surface conductivity in graphene are investigated,and the 1D Bragg grating metamaterial based on graphene plasmonic waveguide is designed.Investigations validate that this metamaterial can not only reflect waves at a certain wavelength efficiently(reflectance>90%)as the traditional fiber Bragg gratings,but the reflection wavelength can also be dynamically tuned by external voltage in the MIR region.The extremely strong slow-light phenomenon with factor of 450 is founded at the same time.Furthermore,in order to broaden the operation wavelength range,the chirped Bragg grating metamaterial is designed.The operation range of~2.1μm is achieved in the chirped Bragg grating with length of1.26μm,and a special slow-light phenomenon that called“Rainbow trapping”is also observed numerically.The flexible controllable“trapping”and“releasing”of slow light are finally realized.3.The design method of graphene plasmonic metasurfaces is proposed by effective medium theory(EMT)principle,metasurfaces with gradient index(GRIN)are investigated in the THz region,and three kinds of 2D GRIN lenses are performed.Theoretical and simulated results demonstrate that the proposed EMT metasurfaces exhibit extremely tight field confinement and subwavelength feature(effective refractive index 20~50).On the basis of the GRIN metasurfaces,2D Maxwell Fisheye lens,Luneburg lens,and self-focusing lens are simulated,showing that these metasurface lenses have extraordinary focusing abilities:the focal spot size is one-sixtieth of the incident wavelength and can also be controlled by external voltage.For example,when the wavelength of incident light is 60μm(5 THz),the focal spots of Fisheye and Luneburg lenses are 0.952 and 1.064μm respectively.In addition,the generality and versatility of the proposal are discussed,and the results predict that THz metasurfaces and 2D transformation optic devices with arbitrary GRIN profiles can be projected.4.An efficient mechanism,phase coupling,for generating electromagnetically induced transparency(EIT)-like effects in graphene micro/nanostructures has been proposed.And based on such mechanism,graphene ETI-like metamaterials and their photonic devices in the MIR region are designed and investigated.The simple model is proposed that graphene F-P microcavity is constructed by only using resonance-detuned graphene nanostructures(nanoribbon and nanopatch).Both the TMM and FDTD results demonstrate that with the assistance of phase coupling mechanism such F-P microcavity and its derived structures can produce single and multiple EIT-like effects with great ease.By optimizing the phase-match condition(φ=π,2π,3π,……),electro-optical switch with the ON/OFF ratio>90%,dual-polarization optical modulators with the modulation depths of~32 dB(X-polarization)and~28 dB(Y-polarization),and flexibly controllable multi-channel optical filters are finally achieved.