Polarization Manipulation Characteristics Of Electrically Tunable Metasurfaces

Electromagnetic metamaterials,as an artificially constructed subwavelength periodic array structure,have become a research hotspot because of their unique electromagnetic characteristics which cannot be found in natural materials.Polarization manipulation characteristics of electromagnetic metasurfaces have important applicability in display,optical communication,medicine,biology,sensing and spectroscopy etc..In this dissertation,several polarization manipulation characteristics of metasurfaces are studied,which include circular dichroism?CD?,circular conversion dichroism?CCD?,asymmetric transmission?AT?,polarization conversion and phase modulation characteristics.The brief content of this dissertation is as follows:Similar to metallic microstructure,displacement current based dielectric array supports electric and magnetic responses with low losses.According to Huygens principle,a cascade Huygens surface based on two-layer silicon rod array is constructed.The symmetry of cascade Huygens surface in the light propagating direction is broken by rotating the bottom-layer silicon rod in two-layer silicon rod array.Thus,the high-efficiency and broadband transmitted CD,CCD and reflected CCD are obtained at the waveband of 1.3-1.7?m.The coherent model of electric and magnetic dipole radiation fields is proposed,which explains the role of electric and magnetic dipole resonances in polarization selectivity.Considering the feasibility in application,the parameters of two-layer silicon rod supported by SiO₂ substrate was further optimized.The high-efficient polarization identification and polarization filtering are achieved via the chirality of proposed cascading Huygens surface.Except for the way of rotating the bottom layer,obliquely incident electromagnetic waves can also break structure systems'symmetry.The high-efficiency and broadband AT is achieved by different electric and magnetic dipole resonances in symmetric silicon rod array,which are excited by obliquely incident circularly polarized light?CPL?.The AT coefficient of right circularly polarized?RCP?wave reaches 0.92 at 12.68 THz.The role of electric and magnetic dipole resonances in AT is analyzed.To gain tunability,a symmetric silicon rod/graphene composite microstructure is proposed,which achieves frequency-tunable AT of by electrically controlling graphene's Fermi energy.And the relationship between the change of Fermi energy and the shift of resonate frequency is also analyzed.Additionally,to enhance graphene's plasmon resonance,a GxBNy structure model is proposed.Due to the lattice matching,the combination of graphene and hBN improves graphene's mobility,which enhances local plasmon resonance.The superposition of graphene layers increases graphene's equivalent conductivity.The scattered fields of G3BN2 nanoribbon array are confined within 60 nm near graphene,so a slight refractive index change will greatly affect resonance wavelength,which leads to ultrasensitive refractive index sensing.The sensitivity of G3BN2 nanoribbon array reaches 4.21?m/RIU at the waveband of 6.79-10.43?m.To achieve functionality-tunable polarization convertibility,Mie multipole resonances are excited by elaborately designing the geometric parameters of silicon rod array.The scattering intensities of electric dipole,magnetic dipole,electric quadrupole and magnetic quadrupole resonance are decomposed by the total scattering intensities,and the contributions of each multipole's electric and magnetic resonance to the"x"-shape polarization transmission coefficient spectrum are analyzed.A frequency tunable half wave plate is achieved using the"x"shape polarization transmission coefficient curve of the proposed rod/graphene composite structure.The functionality-switchable polarization conversions based on tunable graphene is realized by skillfully utilizing the"x"shape polarization transmission coefficient curve and the wavelength-tunability.Additionally,a continuous 90°to 270°phase difference between the orthogonal electric field components of transmitted electromagnetic waves is obtained by electrically controlling graphene's Fermi energy,which means the multi-function polarization conversion from linearly polarized wave to many polarization states.In addition,to gain more freedom of tunability in graphene-based metamaterials,a cross-shape gold/graphene composite structure is constructed,which realizes two-dimensionally tunable polarization selective absorptions.Also,the binary and ternary polarization coding based on two-dimensional tunability are discussed.Tunable metamaterials with graphene only work at terahertz and mid-IR wavebands.A thin titanium nitride?TiN?film is introduced to construct tunable metasurfaces working in short wavebands.Similar to graphene,the carrier density in thin TiN films can be tuned by gating voltages in field effect transistor?FET?structures.A TiN nanoribbon array is designed at 1.55?m,which realized wavelength-tunable polarization selective reflection.In addition,a gold grating combined with thin TiN film is designed using coupled mode theory?CMT?,which achieves dynamic phase modulation.The phase characteristics of under-coupling state,the critical coupling state and the over-coupling state are analyzed by Smith Chart.When the reflections from resonant scattering channel and non-resonant channel are almost equal,the reflection phase experiences an abrupt phase change of180 degrees across 1.540?m.By controlling the carrier density in TiN films,the resonance states of the Au grating/TiN composite structure can be switched freely among the under-coupling state,the critical coupling state and the over-coupling state.Therefore,the dynamic phase modulation in a wide range of 337°is achieved at1.540?m.Compared with the reported phase modulation metasurfaces based on electrically controlled graphene and ITO,the tunable phase range of the proposed Au grating/TiN composite structure is the largest.