The Circular Dichroism of Chiral Metamaterials

• Main Authors: Shi-KangTseng, 曾士綱
• Other Authors: Shih-Hui Chang
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/46950971583285616473

碩士 === 國立成功大學 === 光電科學與工程學系 === 100 === Metamaterial is an artificial material with tailored dielectric or metallic nanostructure at optical sub wavelength size to mimic the nature atoms or molecules. It leads to extraordinary optical property such as negative refraction index, negative permeability, strong chirality, etc. Chirality means different optical response for right-hand circular-polarized (RCP) light and left-hand circular-polarized (LCP) light. This phenomenon of having different optical responses for LCP and RCP is called circular dichroism (CD). The chiral metamaterial CMM, a structure in the absence of mirror-symmetry, can distinguish LCP and RCP. Recently some design of CMM possess observable CD effect, for example, the conjugated gammadion structure.[33] In this thesis we first numerically analyze the CD properties of conjugated gammadion (CG) CMM by Finite-Difference Time-Domain (FDTD) simulations. Our results indicate that the CD of CG CMM is much stronger than that of a single-layer gammadion and bi-layered gammadion CMM. Significant CD effect is located at reflection band edge of the reflection spectrum caused by frequency selective surface (FSS) effect. But when the metallic part of CG CMM is replaced as perfect electric conductor (PEC), the CD effect is vanished. The main distinguish between PEC and metal is the existence of surface plasma (SP) or not. One might suspect that the CD of CG CMM is caused by SP. Next, we trim down the design of CG structure to conjugated Z (CZ) shape structure to unveil the role of SP in CD. However, PEC CZ did demonstrate CD even without SP. This indicates that SP does not play a critical role in the CD effect for CZ system. Finally we employ Lagrange-Jones model to explain our previous findings. The permittivity of the PEC CZ obtained via Lagrange-Jones model, is a complex asymmetry anisotropic tensor. The transmission polarized state is then referred by Jones matrix from complex permittivity tensor. The effective permittivity of CG is a quasi-symmetry form. When the damping (loss) component is ignored, the permittivity tensor becomes a real symmetry matrix. In a lossless CG system, the transmission coefficient of incident LCP and RCP light is indistinguishable. And then through the virtual experiment by removing the damping term in the metallic Drude model, SPP provides little effect on CD. The Lagrange-Jones model and our FDTD simulations verify that damping loss is crucial to generate phase retardation in CG system and thus leads to strong CD effect.