Research On Circular Dichroism Of Metal Micro-nano Structure

Chirality refers to the property that a structure cannot completely overlap its mirror image,and it is the aspect of geometric description of the symmetry of matter.Most of the biomolecules in nature have chirality,and chiral substances are closely related to the human life.Therefore,to detect the chiral signal of substances has become a hot spot for researchers.The chiral signal of chiral material is characterized by Circular Dichroism(CD),which excited different optical properties,such as absorbance,and transmission,under left circularly polarized light(LCP)and right circularly polarized light(RCP).With the excitation,the difference in absorbance is defined as the CD of the chiral structure,and the difference value is the intensity of the CD signal.The chiral signal of the chiral structure existing in nature is very weak and located in the near-ultraviolet band,so it is not convenient to use the original experimental equipment to detect and research.The artificial chiral metal structure has such a strong CD signal is that the localized surface plasmon(LSP)was produced in the surface of the metal structure,and with the interaction of the strong electric dipole moment LSP resonance in the surface of the metal structure and the incident electromagnetic field,the chiral metal structure produce a strong CD signal,however,for the artificial mental structures.Therefore,the artificial chiral metal nanostructure is supposed to design.And by studying its CD,the chirality and the mechanism of chiral structure producing the CD can be further understood.In order to further enhance the CD with strong circular dichroism and explore its potential mechanism,four chiral metal nano structures are proposed in this work.The finite element numerical calculation method is used to design the structures.The main research work is as follows:Firstly,this part proposes a combined array structure of nanowire and crossed nanorods,which can explain the mechanism of the CD of its combined structure,and the mechanism of the connected nanowires to enhance the circular dichroism signal of the crossed nanorods.The numerical results show that the separation of resonant wavelengths of transmission under LCP and RCP increases because the electric interaction between the upper nanorod and nanowire under LCP incidence is different from that under RCP incidence.The increased separation of resonant wavelengths,in turn,enhances the CD signals.In addition,two new CD modes appear,and these modes can be tuned by changing the structural parameters of the proposed structure.The present results will guide the design of plasmonic chiral nanostructures for enhancing the CD signal.Secondly,this paper designs a connected semi-annular planar chiral array structure,and studies the circular dichroism effect of the connected semi-annular structure under normal incident circularly polarized light.The mechanism of the CD by the connected semi-annular planar chiral structure and the influence of the geometric parameters of the connected semi-annular structure on the CD effect are discussed.The numerical results show that due to the electronic oscillation of the surface of the structure,the electric dipole with the largest electric dipole moment is formed at the long wavelength,and the quadrupole and the octapole with small electric dipole moments are formed at the short wavelength.At the minimum wavelength mode,an electric dipole perpendicular to the direction of the connecting line is formed,and t The asymmetric electric dipole resonance causes a difference in the absorption of the structure,thereby generating a circular dichroic signal.Thirdly,the square S-shaped metal array structure is proposed in this paper.The absorption and circular dichroism of the square S-shaped array structure in the visible light band are studied.The mechanism of the circular dichroism of the structure is discussed.The square S-shaped structure itself has a planar chirality,so it has different absorption for different circularly polarized lights.However,when changing the different parameters of the square S-shaped structure,not only the intensity and position of the original circular dichroic signal mode can be adjusted,but also the sign of the circular dichroic signal is changed.Computational studies show that the structure generates different resonance modes under the excitation of different polarized lights,which makes the structure,have different absorption intensities in the same mode,resulting in a circular dichroism signal.In addition,by changing the parameters of the square S-shaped,it is found that when a certain parameter of the structure is increased,a new mode of resonant coupling of the electric dipole and the magnetic dipole appears at a certain mode,so that the symbol of the CD signal of the structure changes.Fourthly,this paper proposes an oblique I-shaped array structure,and studies the absorption rate and circular dichroism of the oblique I-shaped array structure under the excitation of circularly polarized light,and studies the parameter pairs of different parts of the array structure,the influence and mechanism of its circular dichroism.The planar oblique I-shaped array structure itself has chirality and has different absorption efficiencies for different circularly polarized lights.However,when the structural parameters of the nanorods of the structure are changed,the intensity and position of the circular dichroism mode also change accordingly.That is,when changing the tilted nanorod,long wave mode intensity and position change;when changing the horizontal nanorod,short wave mode intensity and position change;when changing the vertical nanorod connecting the tilted nanorod and the horizontal nanorod,long wave and short wave modes didn't change much.In summary,four chiral metal nanostructure arrays are designed in this paper.CD signal mechanisms are studied,which deepens the understanding of the physical mechanism of chiral structures producing the CD signals,and has certain guiding significance for the design of planar chiral metal structures with large CD signals in the future.In addition,it provides the idea of using connected nanowires to enhance the chiral structure of the CD.It also provides a theoretical basis for the design of optical devices,such as optical filters and polarizers for future applications.