Research On Terahertz Devices For Wavefront Manipulation Based On Polarization Multiplexed Metasurface

The rapid progress of terahertz science and technology has brought new technical solutions to the next-generation wireless communication,astronomical observation,medical imaging,biochemical sensing and other fields.The rise and vigorous development in research of metasurface has provided important technical support for functional devices and systems in the terahertz band.However,due to the limitations of material types or design mechanisms,the current terahertz metasurface devices generally show many shortcomings such as single parameters control,limited design channels,and low efficiency,which make it difficult to match the rapidly developing of terahertz radiation,detection and adapt to the growing requirements of the applications.This dissertation mainly studies terahertz metasurface devices with multiple polarization channels,which realize independent wavefront control,multi-parameter manipulation and dynamic modulation of orthogonal linearly or circularly polarized waves.The specific research contents are as follows:1.Combine the principles of linear polarization and spatial multiplexing,make full use of the low crosstalk characteristics and linear birefringence effect of all-dielectric anisotropic meta-atoms,we realize the multi-channel conversion of plane waves to complex helical wavefronts using a single device,and superposition of orbital angular momentum is obtained.By introducing a focused phase profile,the effective manipulation of the transverse and longitudinal electric field components is simultaneously achieved.Furthermore,through the spatial multiplexing arrangement of the heterogeneous units,the co-and cross-polarization components of linear birefringent unit are combined,the wavefront control and multi-channel polarization conversion are simultaneously realized by one device.2.Based on the joint design of geometric and dynamic phase,the independent phase control of orthogonal circularly polarized terahertz waves is realized,and the chirality-like response of transmitted waves is obtained based on wavefront manipulation.Pseudo-random phase arrangement is introduced into one of the circularly polarized components,and the peak value of circular dichroism in transmission is observed to be about 0.4,and the polarization conversion efficiency reaches 80%.In addition,based on this method,arbitrary chiral wavefront control is also demonstrated,and the generation of terahertz vortex beams is realized for one of the circularly polarized components.3.A reflective chiral metasurface with metal-insulator-metal structure is designed to achieve high-efficiency spin-selective terahertz absorption.Arbitrary chiral wavefront control is achieved by adding geometric phases to the unabsorbed components through rotation of the units.By introducing structured monolayer graphene as an active material,switching between the chiral and isotropic structure of the hybrid unit is realized by adjusting its electrical properties,and then the dynamic wavefront modulation is obtained.Furthermore,the chiral arbitrary wavefront control is realized by using the metal-semiconductor composite chiral structure and geometric phase.Asymmetric all-optical modulation of orthogonal circularly polarized terahertz wavefronts is achieved by adjusting the semiconductor conductivity via optical pumping,where the chiral structure is converted into an anisotropic one.