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The Optical Characteristics of Planar Chiral Microstructures

Author: LinZuoFen
Tutor: LiJunQing
School: Harbin Institute of Technology
Course: Optics
Keywords: planar chiral gammadion optical activity Finite Element Method
CLC: O482.3
Type: Master's thesis
Year: 2007
Downloads: 83
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Abstract

Recently, it is found that, similar to a three dimensional chiral object, the planar chiral counterpart possess the polarization characters such as optical activity, polarization modulation of the light intensity. The planar chiral material, which is a two-dimensional matrix of microstructures distributed in order, can be obtained by an ordinary lithography or e-beam etching on the metallic or dielectric thin films. The concept of planar chirality is of importance in new-type polarization devices such as polarization transformers (or convector), polarizer, and polarization sensor, and also can be used in designing the super-thin planar bi-anisotropic materials.In this thesis, the optical properties of a planar chiral matrix composed of gammadions are studied by using a classical model– the coupled helical oscillator model. A set of Lorentz-type coupled equations is established to describe the oscillators which are abstracted to be four electric dipoles, moving along the bended smooth arc orbits with the charge centre in the hub of a gammadion. The influence of the characteristics of external field (such as frequency, polarization state, orientation azimuth, and so on) and the structural parameters of chiral elements (such as the element number, scale of gammadion, and so on) on far field radiation of chiral matrix is simulated numerically. Therefore the physical nature of such a matrix in polarization alternation and intensity modulation is revealed microscally. By comparing the right-handed and left-handed gammadion, it is found that the negative refractive orders of a right-handed gammadion sample correspond to the positive refractive orders of a left-handed one, vice versa.In this thesis, the Finite Element Method (FEM) is utilized to simulate the planar chiral structure in electromagnetic field in order to compensate the shortage of experimental conditions. Firstly, the radiation plots are obtained by FEM to compare with the results from the classical model. Then the surface current, surface field pattern of structure illumined by the electromagnetic field is simulated. In the end, the far field radiation of a chiral gammadion and achiral cross structures is compared. All above verifies the rationality and correctness of our classical model in another way.In this thesis, the theoretical system is set up to link the relation between microscopic planar chiral structures and corresponding macroscopic effects. It is proved that the optical activity of such chiral structures arises from their helical structure but the scales of the chiral structures or some physical parameters of the background materials.

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