Infrared Polarization Converter Based On Optical Antennas

Polarization is one of the fundamental properties of electromagnetic waves,which plays an important role in the interaction between electromagnetic waves and materials.The control over polarization is an important research area in polarization optics and has a wide range of applications,such as spectroscopy,wireless communication,and molecular sensing.To achieve polarization control,conventional method is mainly based on the Brewster effect or the Birefringence effect.The disadvantages of this traditional method are large device volume and narrow bandwidth.In recent years,artificial materials,such as metamaterials and optical antennas,have emerged as an alternative approach,which have unique electromagnetic properties and are favored for device miniaturization and integration.So far,the antenna-based polarization converters have been widely studied and demonstrated in the microwave and terahertz.In this thesis,we focus on infrared polarization converter based on optical antenna.We have designed and fabricated a novel polarization converter using a double-rod metasurface.The main contents and contributions in this thesis are listed below:(1)Based on classic sandwich structure and HFSS electromagnetic simulation software,a new infrared polarization converter is designed.The polarization converter is made of a double-rod antenna array.Broadband cross polarization conversion performance is achieved by superimposing three localized resonances excited on metasurface.The simulation results show that within the frequency range from 24.9THz to 44.2THz,the designed polarization converter exhibits a PCR exceeding 90% and an FWHM bandwidth of 21.7THz at normal incidence.(2)The mechanism of polarization conversion in the double-rod metasurface is analyzed,which is similar to the traditional anisotropic medium.A near-? phase difference between two orthogonal components of the electromagnetic wave is accumulated in reflection,which leads to the polarization rotation.The antenna-based polarization conversion has the advantages of ultrathin thickness in device size and an agility in design.By using the surface current distribution and field distribution of double-rod metasurface,we further analyze the influence of the structural parameters on the polarization conversion performance.In addition,the angular dependence of the polarization conversion performance is also studied.Simulation results show that the PCR degrades in terms of both bandwidth and conversion efficiency when the incident angle is larger than about 20°.(3)The designed double-rod metasurface has been successfully fabricated and characterized in experiments.The specific fabrication process includes using electron beam evaporation to evaporate copper and ZnSe films and patterning the metallic double-rod antenna array by using electron beam lithography.In addition,an angle-controllable FTIR measurement platform was set up to measure the polarization conversion property of the sample.The measured results show that the sample converted linearly polarized electromagnetic waves to its cross polarization in the frequency range from 26.2THz to 43 THz with PCR larger than 70% and an FWHM bandwidth of 16.9THz at an incident angle of 15°,which are in qualitative agreement with the simulation results.Finally,the experimental deviation has been analyzed.The results show that the light scattering on the structured top surface and uncertain variations of ZnSe refractive index or thickness likely cause the deviation between experiment and simulation.