Research On Polarzation Effects Of Chiral Metamaterials

Varity of artificial materials have been made to manipulate electromagenetic waves, such as fiber, diffraction gratings, photonic cystals and so on. Since the early2000s, a new class of artifical materials named as metamaterials have emerged as a new frontier of science involving material science, optics, mechanics and nanoscience. Metamaterials are periodical artificial media with a pitch smaller than the wavelength of interest and can exhibit unusual properties not found in nature or in their constituent materials.Chirality indicates that object cannot be superimposed with its mirror image. In natural, there are many structures with chitality, such as helices, protein molecules and quartz crystal structures. These chiral materials can exhibit optical activity, which has been used as a diagnostic tool in spectroscopy, analytical chemistry, crystallography and molecular biology for identifying the spacial arrangement of atoms. With careful designs, metamaterials have been proved to show chirality. Chiral metamaterials not only show much stronger optical activity than natural materials, but also exhibit some unusual properities such as negative refractive index and circular conversion dichroism. By far, numerous chiral metamaterials have been proposed. Therefore, there is a demand for summarizing the requirements of the polarization effects of chiral metamaterials, specifically circular dichroism and circular conversion dichroism.This thesis studies the circular polarization effects of the chiral metamaterials based on helices and asymmetrical split rings, which were fabricated using Direct Laser Writing and standard lithography, respectively. Futhermore, they were measured at the optical region and microwave region, respectively. Then chiral metamaterials with different symmetry are investigated. Next, the complete requirements of circular dichroism and circular conversion dichroism are proposed separately. Finally, a kind of bilayered asymmetrical splir rings is proposed to exhibit high conversions from linearly polarized waves to circularly polarized waves. To be more specific, it mainly includes:Firstly, circular dichroism of chiral metamaterials is investigated. Regarding the helical metamaterials, this thesis proposes elliptical polarizers based on elliptical helical metamaterials and improves Signal-to-Noise ratios of circular polarizers based on single helical metamaterials. The results can be used as broadband elliptical polarizers and circular polarizers at the optical region. Regarding the bilayered asymmetrical split rings, two kinds of structures are proposed to function as circular polarizers at the microwave region;Secondly, circular conversion dichroism of helical metamaterials is investigated, this thesis proposes the coupled pairs of oppositely-handed gold helices, which can exhibit giant circular conversion dichroism and be used as broadband circular converters at the infrared region;Thirdly, circular conversion dichroism of bilayered asymmetrical split rings is investigated, this thesis proposes two kinds of strutures, by which, the differences between the two circular conversions are highly imporved compared to that of monolayer asymmetrical split rings. The results can be used as circular converters at the microwave region;Fourthly, this thesis investigates the transmission matrices, eigen polarizations and transmission properities of circular polarizations of chiral metamaterials with different symmetries, and summarizes the complete requirements of circular dichroism and circular conversion dichroism separately;Fifthly, a kind of bilayered asymmetrical split rings, which can exhibit circular dichroism and circular conversion dichroism simultaneously, is proposed to exhibit high conversions from linearly polarized waves to circularly polarized waves at the microwave region.The chiral metamaterials investigated in this thesis can be used as narrowband or broadband circular polarizers, elliptical polarizers, circular converters and linear-to-circular converters. They can manipulate poalarizations of electromagnetic waves for many applications such as sensors, communications, detectors and image displays in the fields such as all-optical signal processing, integrated optics, radar and antenna technologies.