Research On Terahertz Metamaterials In Optical Chirality Tuning

Metamaterials is one of the most concerned research directions in the field of optics since the 21 st century.It has been repeatedly selected as one of the ten scientific advances in “Science” magazine these years.The periodic unit structure and half-wavelength size of metamaterials give it the ability to effectively manipulate electromagnetic waves.The research band of metamaterials covers a wide range from microwave to ultraviolet,and there are still many unknown areas to be explored in the terahertz band.Metamaterials in the terahertz region can make up for the shortcomings of natural materials' weak response to terahertz waves,and are expected to become an important support for the application and development of terahertz technology.The specific electromagnetic effect caused by chiral structure is one of the research directions of terahertz metamaterials,and its application fields are still being explored,including negative refraction materials,photoelectric detection,chemical analysis and polarization conversion.The research in this paper is based on metamaterials,and studies the generation mechanism and Modulation methods of optical chirality in the terahertz region.The full text is divided into five chapters,the main content of each chapter is as follows:The first chapter is the introduction,which introduces the research background and content.First,the basic concepts,characteristics and research significance of metamaterials and terahertz technology are briefly summarized.Secondly,we introduce the core content of our research “optical chirality”,and explain its basic principles and significance.Finally,the development and research status of the chiral metamaterials are reviewed from the two characteristics of the chiral materials.The second chapter is a summary of the research methods.Firstly,from the important theoretical support of metamaterials "surface plasmon" theory and the universal field equation in chiral media,the physical knowledge needed to study optical chirality is summarized.Secondly,our main simulation software CST studio is introduced,which is widely used to solve the equation based on the finite integral time domain method.Finally,the main preparation technology(lithography)of metamaterials is briefly introduced.In the third chapter,we present a detailed work on multilayer chiral metamaterials,which focuses on the circular dichroism of optical chirality.In this chapter,we first use the metamaterial with double Ta Chi structure to achieve strong circular dichroism,and find that the occurrence of circular dichroism is the result of two-layer structure.Secondly,the chirality is tuned by changing the incident angle and interlaminar twist angle,and the variation of circular dichroism is summarized.In addition,we propose a physical model to explain these phenomena according to the surface current distribution of metamaterials,and describe the generation and change of circular dichroism with the theory of electric dipole and magnetic dipole coupling.This part of work has guiding significance in the design of multi-layer super surface and the study of circular dichroism.It has been published in“Applied Optics”,a SCI Journal of engineering technology.In the fourth chapter,we design a kind of temperature controlled chiral switch based on phase change material vanadium dioxide.This design is inspired by the phenomenon that the conductivity of phase change materials can be controlled.When designing the metasurface,metal and vanadium dioxide are used to form the super material unit.This structure can realize the switch control of optical chirality(asymmetric transmission)at high temperature and room temperature.We also prepared vanadium dioxide thin films and plotted the conductivity curve with temperature,which verified the feasibility of the temperature control switch.Through the optimization of the structure,the metasurface has the characteristics of broadband asymmetric transmission in a large range of incidence angle,and the effect of external chirality on asymmetric transmission performance under oblique incidence is studied.This work is of great significance to the design of controllable metamaterials,the study of asymmetric transmission characteristics and the application of metamaterials in devices.This part of the work has been published in the physics SCI journal“Optics Express”.The fifth chapter is a summary and Prospect of the whole work.It is mainly the refining and thinking of the content,innovation and research significance of the two works.At last,it puts forward some prospects in this field.The prospect part includes the future development of chiral work and the application of metamaterials in holography.This part of work is expected to combine with the finished modulation work of metamaterials to realize a dynamically controllable holography.