Study On Absorption Circular Dichroism Of Metal Chiral Nanostructures And Graphene Enhanced Circular Dichroism

When incident light is irradiated on the surface of metal nanostructures,a large number of free electrons in metal materials will cause the collective oscillation on the surface,that is the surface plasmons.Surface plasmons use metal nanostructures as carriers,which can confine light to the surface of nanostructures,break the optical diffraction limit,and manipulate light at the nanoscale.Metal nanostructures based on surface plasmons have been designed and studied extensively in many areas,such as surface enhanced spectroscopy and biosensing.By combining the surface plasmons with chirality,designing chiral surface plasmon metal nanostructures and studying their unique optical chirality have become one of the current research hotspots.This thesis designed the arrays of chiral metal nanostructures,and calculateed its electromagnetic field distribution characteristics based on finite element method(FEM)numerical simulation.The electromagnetic coupling of chiral nanostructures under the excitation of circularly polarized light with different rotation directions.By introducing two-dimensional material of graphene,the chiral response of composite chiral metal nanostructures were studied in the visible and near infrared wavelength.The main research work of this thesis is as follows:1.Vertical Q-shaped nanostructure arrays(VQNAs)were designed and their characteristics of circular dichroism(CD)was studied.Numerical calculation shows that the CD spectra of VQNAs exhibit four surface plasmon resonance modes in the visible wavelength.By adjusting the parameters such as the geometry of the structures and the angle between the two achiral nanostructures through the control variable method,it is found that the amplitude of the CD spectrum always increases first and then decreases as the geometric parameter increases,which corresponds to the non-monotonic the asymmetry factor,and proves the relative and tunnable symmetry breaking.The related research conclusions can help to understand the physical mechanism of obtaining CD spectrum through symmetry breaking deeply.2.Graphene-planar chiral metal nanostructure arrays(PHCNs@GRs)were designed by introducing graphene ribbons(GRs)into planar h-shaped-metal chiral nanostructures(PHCNs),and their CD spectral characteristics was studied.Numerical calculation result shows that the plasmon resonance coupling between PHCNs and GRs makes the absorption spectrum enhanced of PHCNs with varying degrees underleft-handed circularly polarized light(LCP)and right-handed circularly polarized light(RCP)excitation,and causes the CD signal intensity of the nanostructure in the overall spectra enhanced.The strong surface currents of GRs in the proposed hybrid chiral nanostructures have certain potential in enhancing catalytic reactions and Raman scattering spectra.3.Based on the localized surface plasmons(LSP)and surface plasmon polaritons(SPP)resonance,the hybrid plasmon chiral nanostructures of YSNAs @ GRs,composed of Y-shaped-groove semi-permeable nanostructure arrays(YSNAs)and graphene ribbons(GRs),was designed and their CD response characteristics is studied.The multi-layer planar nanostructures YSNAs was composed of planar Y-shaped-hole nanostructure arrays(YNAs)and the metal nanosubstrates.The absorption spectra and CD spectra of YNAs@GRs and YSNAs@GRs were simulated by FEM numerical calculation.The result shows that graphene can significantly enhance the CD signal of LSP resonance in the nanostructures,but not enhance the CD signal of SPP resonance.However,SPP resonance can be significantly enhanced by adding metal nanosubstrate.In addition,the resonance coupling between YSNAs and GRs is similar to the YNAs@GRs,both of which are contributed to the LSP resonance enhanced by graphene in the z direction arround the Y-shaped-grooves.At the same time,the thickness of the air column has a cavity effect on the nanostructure,which is formed a stationary wave in the air layer above it and regulated the energy distribution of the incident light.In addition,the absorption enhancement of YSNAs@GRs can be adjusted by the geometric parameters,such as changing the area of y-shaped-grooves.The related research results provide a possible way for designing the artificial chiral hybrid nanostructures with strong circular dichroism response and easy preparation.