Circular Dichroism Induced By Near-field Interactions Between Chiral Molecules And Achiral Nanostructures

Circular dichroism(CD)refers to the absorption difference between right-handed circular polarization and left-handed circular polarization.Circular dichroism technique is one of the important methods to detect the chirality of molecules.However,due to their weak circular dichroism,it is difficult to detect the handiness of chiral molecules.In addition,the circular dichroism signal of chiral molecules can only be detected in ultraviolet region,but ultraviolet exposure will seriously damage chiral molecular samples.Previous studies have enhanced the circular dichroism of molecules by surface plasmon enhancement technology.It is found that the induced circular dichroism(ICD)induced by the interaction between chiral molecules and achiral plasma nanostructures is more sensitive to the detection of molecular chirality.Therefore,induced circular dichroism is considered as a potential method to detect the chirality of single molecule.However,induced circular dichroism needs further study in the aspects of generation mechanism,enhancement mechanism and design of Nano-structures with better enhancement effect.(1)Previous researchers have studied the role of near field in the interaction between isotropic chiral molecules and achiral nanostructures,but the influence of near field on circular dichroism induced by the interaction between anisotropic chiral molecules and achiral nanostructures needs further study.(2)Although a large number of experimental results show that inducing circular dichroism can improve the sensitivity of molecular chiral detection by two orders of magnitude,the mechanism of inducing circular dichroism needs further research.(3)In order to detect the chirality of a single molecule,it is necessary to further enhance the induced circular dichroism in order to improve the sensitivity of molecular chirality.Therefore,it is necessary to further study the mechanism of inducing the enhancement of circular dichroism and to design nanostructures with better enhancement effect.The main work of this research is as follows.1.A Nano-structure composed of chiral molecule and nanorod dimers was designed,and the circular dichroism properties of induced by anisotropic chiral molecules was studied.The results show that the induced circular dichroism reaches the maximum when the orientation of chiral molecules is parallel to the maximum component of electric field intensity.In addition,different directional components of electric field strength will produce opposite ICD signals,which will lead to the inversion of ICD spectrum when molecules are oriented in a specific angle.2.Based on the dipole approximation of chiral molecules and the near-field interaction approximation between chiral molecules and achiral nanostructures,the mechanism of inducing circular dichroism is studied.The results show that the electric field intensity radiated by chiral molecular dipoles has different increments under the excitation of right-handed circular polarization and left-handed circular polarization.Therefore,the interaction between chiral molecules and achiral nanostructures can induce circular dichroism.In addition,induced circular dichroism is directly proportional to the chirality parameters and chiral molecular density,and increases with the decrease of distance between molecules and nanostructure.The induced circular dichroism also increases with the increase of electric field intensity in nanostructure and magnetic induction intensity in molecular regions.In order to verify this theory,the circular dichroism caused by the near-field interaction between chiral molecules and graphene nanohole composite structure was studied.The theoretical analysis results are in good agreement with the simulation results.3.A quadrilateral nanohole array is designed,the induced circular dichroism properties are studied,and the enhancement mechanism of induced circular dichroism is put forward.By analyzing the surface plasmon resonance modes,it is found that the induced circular dichroism is enhanced by the overlapping of electric dipole and magnetic dipole resonance,and when the electric dipole resonance or magnetic dipole resonance disappears,the induced circular dichroism also disappears.4.The cross-shaped nanohole array is designed to enhance the sensitivity of molecular chiral detection.By adjusting the thickness of the cross-shaped nanohole structure,the enhancement effect can reach 450 times.In addition,the influences of the length of long arm and short arm,the dislocation of short arm and the period of crossshaped nanohole structure on induced circular dichroism were also studied.The results show that the enhancement factor of molecular chiral detection sensitivity can be controlled by changing the parameters of cross-shaped nanohole structure.