Research Of Terahertz Polarization Devices Based On All-dielectric Metamaterials

Terahertz waves have attracted much attention due to their unique physical properties.In the past decade,terahertz technology was widely applied in diverse fields such as high-speed communication,biomedicine,security inspection,radar imaging and scientific research.The polarized electric field of light waves carries valuable information regarding its amplitude and phase,and plays an important role in many applications.High-performance polarization convertors are of great significance for terahertz applications.However,the development of terahertz polarization convertors is hindered due to the lack of natural materials with a high birefringence in the terahertz band.Metamaterials can overcome the shortage of natural materials in the terahertz band,but there are still some problems for the existing metamaterial polarization convertors,such as low transmission efficiency and single function.In order to solve these two problems,we have designed corresponding polarization convertors based on all-dielectric metamaterials with high transmission efficiency and multiple functions,and studied their polarization properties and working principles with simulation and experiment.The main research contents of this thesis are as follows:1.To overcome the low transmission efficiency of the existing transmission polarization convertors based on metallic metamaterials,all-dielectric anisotropic metamaterials are utilized to realize terahertz polarization conversions without Ohmic loss.A metamaterial based on a periodic sub-wavelength elliptical-hole array is designed as a terahertz quarter-wave-plate.It has a high birefringence due to the anisotropy of the elliptical holes on a silicon substrate,and can accomplish linear-tocircular polarization conversion with a high transmission efficiency.Another metamaterial based on a periodic sub-wavelength elliptical-pillar array and an antireflection layer is designed as a terahertz half-wave-plate with a higher transmission efficiency.The metamaterial includes two working layers called the wave-plate-layer and the antireflection-layer,respectively.The wave-plate-layer with an elliptical-pillar array can achieve cross-polarization conversion for linearly polarized terahertz incidence,and the antireflection-layer with a square-pillar array can reduce the reflection to obtain a higher transmission efficiency.The results show that the duallayer structure does have a higher transmission efficiency than its single-layer counterpart.2.To go beyond the general single function of the existing polarization convertors based on passive metamaterials,we present multi-functional polarization convertors with complex unit-cells.The sub-cells in a unit-cell can interfered with each other to increase the design freedoms of the metamaterials.A metamaterial based on dual-pillar complex unit-cells is designed as a terahertz dual-functional wave-plate.The metamaterial device can achieve linear-to-cross-linear and linear-to-circular polarization conversions due to the designed interference of the sub-cells.In order to enhance the functionality further,another dual-layer metamaterial based on complex unit-cells is designed as a terahertz quadruple-functional wave-plate.The combination of two single-layer complex unit-cell metamaterials leads to more design freedoms,and finally performs the expected four polarization conversions.