Manipulating Electromagnetic Wave Polarizations By Multilayer Metamaterials

Polarization is an important characteristic of electromagnetic waves.It is always desirable to achieve flexible control of polarization.In the past decades,metamaterials have attracted enormous attentions due to their ability to tailor the effective medium properties.Metamaterials offer a platform to achieve flexible control of polarization.In recent years,metamaterials have received increasing attentions in polarization control.Anisotropic metamaterials can be applied in polarization encrypt,holography,beam shaping and microwave and optical communication systems.Compared with bulk metamaterials,planar metamaterials have the merit of easy of fabrication and have been applied in a variety of applications.In this dissertation,a bilayer asymmetrically spilt ring apertures and a four layered cascaded coding metamaterial were designed to achieve a flexible manipulation of circular polarization and linear polarization,which is a compelling evidence that multilayer metamaterial has a great potential in manipulating electromagnetic wave polarization.This dissertation will start from the theoretical analysis,numerical simulations and experimental verification to illustrate multilayer metamaterial's powerful manipulation to polarization.The main contents of this thesis are summarized as follows:A bilayer asymmetrically split ring apertures was designed to study the relationship between intrinsic and extrinsic chiral responses,the rotation angle between the top and back layers is 90°.The bilayer structure's circular dichroism exhibits triple-band resonant at normal incidence.The oblique incidence leads to giant enhancement of circular dichroism at two lowfrequency resonances while the third resonance undergoes a change from a single peak to two positive and negative resonances.We have numerically and experimentally demonstrated the angle-induced circular dichroism varies from-0.4 to 0.44.The whole circular dichroism response results from the balance between intrinsic and extrinsic chirality and the circular dichroism spectra at positive and negative angles of incidence exhibit an asymmetry due to the existence of intrinsic chirality.Importantly,the intrinsic chirality may be individually investigated since extrinsic chiral response in the bilayer 90°-twisted metamaterial can be removed from the total circular dichroism by superimposing two circular dichroism spectra at positive and negative incident angles.We also study the dependence of the chiral response on the asymmetry of the asymmetrically split aperture and the rotation angle between the top and back layers.By analyzing the surface current distributions,we explained why the bilayer 90°-twisted metamaterial has three transmission bands.Besides that,a four layered cascaded transmissive type coding metamaterial was also designed.By varying the length of stripe in x and y direction,the phase profile of x(y)polarization can be modulated.Thus we designed a 2-bit coding metamaterial which has four different digital states with 90° phase difference.Based on the full control of two orthogonal polarizations,anisotropic coding metamaterial can achieve phase control besides polarization conversion,thus a polarization beam splitter and a l = 2 Orbital Angular Momentum(OAM)beam generator have been designed.As for polarization beam splitter the x polarized and y polarized electromagnetic waves were refracted into xoz plane and yoz plane,respectively.The deviation angle is 21° which shows a good agreement with the calculated result.The strength of x and y polarized refracted EM wave are exactly the same,which reflects that the meta-atom works well in both x and y polarization.We also fabricated a sample to generate a l = 2 OAM beam.The fabricated sample can achieve both polarization conversion(linear to circular)and generated a l = 2 OAM beam.The experiment results verify the simulated near field distribution and show that the fabricated sample can generate a l = 2 OAM beam in 9-10 GHz.