Asymmetric Transport And Circular Dichroism Of Metal Chiral Micro-Nano Structures

When a light wave is incident on a metal micro/nanostructure,free electrons in the metal and the incident light wave are coupled,and form near-field electromagnetic wave on interface of metal-dielectric that propagates along the interface.In optics,this phenomenon is called surface plasmon(SP).SP has strong coupling and highly localized electric field characteristics,and it strongly depends on the specific micro/nano structure.As a result,SP provides opportunities to control light on a sub-wavelength scale.In general,it is possible to manipulate the SP resonance characteristics by changing the geometry,shape and material of the micro/nano structure.SP characteristics already had been widely applied in negative refractive index materials,surface enhanced electric field,subwavelength waveguide,plasmon laser,plasmon color,nanophotonic photovoltaic device,metamaterial/metasurface,extraordinary optical transmission,nanophotonic circuits and so on.The asymmetric transmission(AT)and circular dichroism(CD)effect of the metal chiral micro/nano structure based on the SP characteristics have the advantages of strong signal and easy control of signals.In this thesis,a chiral metal micro/nano structures is designed and mechanism of AT and CD effect is studied by finite element method(FEM).The main research work of this thesis is as follows:1.In this thesis,tilted rectangular nanohole(TRNH)arrays in a square lattice are proposed to realize an AT effect.Numerical results show two AT modes in the transmission spectrum,and they are ascribed to the localized surface plasmon resonances around the two ends of TRNH and surface plasmon polaritons on the gold film,respectively.AT properties of the TRNH strongly depend on structural parameters,such as width,length,thickness,and tilted angle of TRNH.Results provide a novel mechanism for generating AT effect and it may provide a potential application in plasmonic device,such as asymmetric wave splitters and optical isolators.2.In this thesis,a metasurface consisting of nanoholes and tilted nanorods is proposed to achieve the CD effect.Numerical calculations show that electrical current forms between the film and the tilted nanorods under circularly polarized light illumination,and CD effects originate from the the current oscillations between the film and those on the tilted nanorods.This electrical oscillation mode provides unique coupling mechanisms for the CD effect.In addition,CD is strongly dependent on the structural parameters,and the resonant modes also can be tuned by modulating the currents on the film.These results are helpful for designing novel chiral optical structures and provide unique methods for circular polarizers.3.In this thesis,designs planar composite metal nanostructure(PCMN),which composed of infinite long nanowire and G-shaped nanostructure,and larger CD signals are realized.The absorption spectra and CD spectra,surface charge distributions at resonance wavelength of planar composite nanostructure are calculated by finite element method.For comparison,a circular dichroism signal with only G-shaped nanostructures(GNS)is also studied.The numerical results show that under circularly polarized light illumination,the planar composite nanostructure and G-shaped nanostructure exhibit electric dipole,quadrupolar,octupolar resonance modes,respectively.When the G-shaped nanostructure is connected to an infinitely long nanowire,all resonance peaks have a red shift due to infinitely long nanowire increases the local surface resonance intensity under different circularly polarized light excitation.Therefore,it significantly enhances the circular dichroism signal of the planar composite nanostructure.At the same time,the influence of geometric parameters such as the different length of each nanorod of the G-shaped nanostructure and the width of the infinitely length nanowire on the circular dichroism modes are also studied.The findings may provide some guideline and methods for improving the circular dichroism signal of planar chiral nanostructures which are widely used in circular polarizers,optical modulators and optoelectronic devices.4.In this thesis,bilayer three-crossing nanorod(BTCN)arrays was proposed and the properties and mechanisms of the enhanced CD signal have been studied in detail.The finite element method is used to characterize the optical properties of the BTCN array nanostructure.Numerical calculation results show that under the excitation of circulary polarized light,the structure shows a large transmission difference which produces a circular dichroism effect.The characteristics of charge density distribution indicate that the circular dichroism effect ascribed to a bonding mode at a long wavelength and an anti-bonding mode at a short wavelength,which results from the equivalent electric dipole coupling between top and bottom layer nanorods,respectively.In addition,the effects of the relative distance of top and bottom nanorods,length of nanorods,the relative distance of each nanorod,and the different rotation direction of nanorod on the circular dichroism were also studied.