Investigation On The Mechanism Of The Plasmonic Chirality Based On The Coupled Electric And Magnetic Modes

Chirality exists ubiquitously in a variety of living and nonliving systems in nature. For the most chiral molecules, the physiological activity and poisonous are different with different configuration. So the detection of chiral signals,the recongnition and separation of the configurations are very important at those field such as biological pharmacy, organic chemistry, high polymer material and medicinal chemistry etc. The CD signal of natural compounds such as organic and biological chiral molecules are typically very weak and occur in the ultraviolet region, which have limited greatly those applications. With the development of the nanotechnology and plasmonic, it has been suggested to use plasmonic nanostructures to boost the sensitivity of the method by generating superchiral electromagnetic near-fields or exploiting their acute response to their immediate environment. Since the status of suface plasmons in metallic nanostructures is sensitive to the shape, size, material and configuration of structures, it offers a flexible way to tune optical activity. So the optical activity of plasmonic nanostructures has attracted increasing attention as an emerging area in recent years.Hence, according to the present hotspots and difficulties on plasmonic, in this thesis, we present an analytical model, and quantitatively analyze the giant CD in extrinsic plasmonic chiral nanostructure through theoretical means. The interplay of the electric and magnetic modes of the meta-structure is analyzed and considerd to be responsible for the giant CD. Inspirited by the chiral molecule theory, we present a similar analytical model and quantitatively analyze the plasmonic CD of 3D chiral nanostructures. Then we studied the Fano resonance assisting plasmonic circular dichroism from nanorice heterodimers for extrinsic chirality. And the last, we analyzed the the sensitivity of the CD to the surroundings of the Au-Ag nanorices dimer and the chiral near-field properties and sensing. The conclusions are drawn as following:(1) The generation mechanism of extrinsic plasmonic chirality is quantitatively presented using an analytical model of coupled electric and magnetic dipoles. The strong interplay of electric and induced magnetic dipoles will cause a mixed electric and magnetic polarizability. We can quantitatively analyzing the CD mechanism with the imagery part ofmixed electric and magnetic polarizability. The model is verified using the numerical FEM result of splitting rectangle rings. The hybridization of the electric mode and the magnetic mode results in a mixing of the two modes, coincident with the above analytical model. The parameter-dependent CD response of the splitting rectangle ring is also investigated. When the right arm becomes shorter, the total magnetic mode becomes stronger, increasing the CD of the structure. The corresponding CD response is markedly enhanced as the thickness increases. When the thickness changes to 60 nm, the structure exhibits total left-handedness.(2) The strong interplay of the electric and magnetic modes result in the CD of plasmonic 3D intrinsic chiral structures and the modes represented e and m dipoles can be use quantitatively analyze the CD mechanism with the imagery part of mixed e and m polarizability. The model is presented with coupled dipole approximation and verified with numerical FEM result in Born-Kuhn model(which is a classic model for chirality) and other 3D plasmonic chiral nanostructures. It is not suitable for higher order modes. The quantitative model is expected to be applied to all the 3D chiral structures.(3) The heterodimer composed of Au and Ag nanorices have Fano resonance and strong CD effect. The Fano resonance has an enhancement effect for the CD. A simple quantitative analysis shows that the structure with larger Fano asymmetry factor has stronger CD. The stronger Fano resonance and CD effect in Au-Ag heterodimer was attributed to the much larger free charge density in metal compared to dielectrics. When the gap decreases, the structure with larger Fano asymmetry factor(absolute value) in spectrum has larger CD value. With the size increasing the intensity of CD signals increase dramatically, higher-order modes especially.With decreasing the aspect ratio that the CD response become stronger.(4) When the refractive index varied from 1.1 to 1.4 are considered around the Au-Ag heterodimer, all CD peaks exhibit significant red-shift. Consider the three main CD peak, the FOM is from 6.1 to 6.8, 11.9 to 12.4, and 20.7 to 21.6, respectively. It can see that the CD sensing has around 5 times of the FOMlarger than Fano sensing for the dipole hybrid mode 3, and the other modes keep similar sensitivity compared with the Fano extinction profile.(5) We investigated the superchiral distributions of the splitting rectangle. The field enhancement in the two gaps has different selectivity for the handedness of the exciting light and for the resonant modes. The upper gap enhances the RCP chiral field at 1610 nm, 840 nm, and the LCP chiral field at 1080 nm, 840 nm. The lower gap enhances the RCP chiral field at 1080 nm, and the LCP chiral field does the same at 1610 nm. If one focuses on one resonant peak, e.g., the 1610 nm peak, the RCP light will excite a strong chiral field in the upper gap, and the LCP light will excite a stronger chiral field in the lower gap. The case is reversed for the 1080 nm peak. The selective switching enhancement of the chiral field for CPL is very useful in chiral molecule sensing and catalysis.