| Chirality refers to the property that a structure itself cannot overlap its mirror structure.The chiral signal of chiral material is characterized by circular Dichroism(CD),which is defined as the difference in structure absorption between left-handed circularly polarized light and right-handed circularly polarized light.The difference in absorption rate is the size of the CD signal.Since the chiral signal of the chiral structure existing in nature is very weak,the researchers applied a chiral metal micro-nano structure that was manually set to generate a chiral signal.Generally,the chiral metal micro/nano structure is manually arranged in a relatively regular shape,and the CD signal is large.The main reason is the generating of the LSP(localized surface plasmon)and SPPs(Surface Plasmon Polariton)when the circularly polarized light are illuminating the surface of the metal structure.This is crucial for further understanding of chirality and the causes of CD signals generated by chiral structures.Therefore,based on the research status of metal chiral micro-nanostructures,in order to further enhance the CD signal intensity of chiral structures with strong circular dichroism signals,three kinds of planar chiral metal micro-nano structures are proposed.The main research contents are as follows:(1)A T-type micro/nano structure/metal gap composite structure is designed,and the mechanism of the circular dichroism of the structure is studied based on the finite element method.Due to the conduction and limitation of the metal gap,the electric field can be localized on the metal film on both sides of the gap,so that two electric dipoles are formed on the structure and form an Anti-bonding mode,resulting in a larger CD signal.Under the vertical illumination of left-handed circularly polarized light and right-handed circularly polarized light,a clearly visible vibration mode was observed in the simulated spectrum of the T-type micro/nano structure/metal gap composite structure,and the current distribution map was used to study the mode.The physical mechanism of the CD effect.It was found that the CD effect of the T-type micro/nano structure/metal gap composite structure was enhanced by 15.1 times compared to the absorption spectrum of the T-type micro/nano structure alone.This study can be used to guide the circular dichroism of metal chiral micro-nanostructures.(2)A rectangular block structure with square holes was designed,and the mechanism of the CD signal generation and the influence of the control parameters on the CD signal were studied.The rectangular block itself is an achiral structure,so the structure achieves the CD effect by introducing a square hole to destroy the symmetry of the achiral structure.The results show that the chiral structure formed by the symmetry of the achiral structure can produce a strong CD signal.At the same time,changing the variation of structural parameters within a certain range,such as adjusting the size or position of the square hole,can effectively realize the regulation of the CD signal.Obviously,when the position of the square hole is far away from the geometric center of the rectangular block,the CD signal is obviously enhanced;when the square hole size becomes larger,the CD signal is also enhanced to some extent;and in the process of changing,the parameter of the square hole into a rectangular hole,The CD signal is gradually smaller,because the rectangular holes cause the structure as a whole to gradually become achiral.The circular dichroism is achieved by destroying the symmetry of the achiral structure.This research not only realizes the controllable metal chiral micro-nano structure,but also provides guidance for the design of achiral structure to achieve circular dichroism.(3)A planar asymmetric U-shaped structure was designed and the mechanism of the circular dichroism of the structure was explored.In this structure,a gold rod is added to the L-shaped structure to enhance the asymmetry,and then the enhancement of CD effect is realized.The length and width of the three gold bars constituting the structure itself from left to right are completely inconsistent,and the widths of the three gold bars from left to right are gradually increased,so that the difference in the sensitivity of the structure to the LCP and RCP irradiated on the surface is increased to enhance the CD.signal.In addition.,by analyzing the current distribution,the short-wavelength resonance mode is constrained by the structure to form an electric dipole,and the long-wavelength resonance mode is constrained by the structure to form a magnetic dipole.In this chapter,by investigating the circular dichroism of the planar asymmetric U-shaped structure and the law of parameter variation,the parameters of different parts of the structure are changed to regulate the CD signal,which has certain guiding significance for the future design of chiral structure enhancement and adjustment of circular dichroism. |
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