Design And Simulation Of Terahertz Broadband Dual-Function Device Based On Vanadium Dioxide

In the past decades,terahertz(THz)waves have attracted the interest of many scholars due to their broad application prospects in the fields of wireless communication,sensors and imaging.With the development of THz technology,scholars have proposed various functional devices based on metamaterials in THz region,such as filters,absorbers and polarisation converters,etc.Therein,the integration of metamaterial absorption and polarisation conversion into a single device plays a very important role in the THz range.On account of dual-function devices can not only switch functions according to the real-time needs of the system,adapting more intelligently to the application scenario and improving environmental sustainability,but also greatly reduce the device development cycle and cost.In addition,dual-functional devices can provide guidance for the study of tunable,intelligent and multifunctional devices with a wide range of applications in many fields,such as energy harvesting,optical sensing and stealth technology,etc.Vanadium dioxide(VO₂)is an ideal material for thermal control systems with the characteristics of a phase change from insulator to metal.Under external stimulation,its conductivity can be increased by 4-5 orders of magnitude and the phase transition of VO₂ can occur at an ultra-fast time scale(¹00fs).Based on the unique transition properties of VO₂,we design and simulate a dual-function device realizing the switchable function from absorption to polarisation transition by external heat.Simulations of the dual-functional device have been carried out via the finite element method in this thesis.The main content includes:Firstly,by designing a six-layer structure of hybrid"L"-shaped gold and"cross"-shaped VO₂,the dual function of broadband polarization conversion and perfect absorption is achieved,and the corresponding working principles is also analyzed.The simulation results show that when the temperature is about 298K,the VO₂ is insulator.The incident wave is converted into a cross-polarized reflection wave in a wide frequency range of 0.62 to 1.69 THz with the polarization conversion rate higher than80%.Once the temperature reaches approximately 358 K,the VO₂ switches to metallic state.The designed device behaves as a tunable broadband perfect absorption performance over a wide range from 0.77 to 2.06 THz,with the corresponding absorption rate higher than 80%and relative bandwidth of 91.2%.Secondly,the model is simplified on the basis of the above six-layer structure,and a terahertz broadband dual-function device based on three-layer structure is proposed,which is composed of"L"-shaped gold and inverted"L"-shaped VO₂.Similarly,when the temperature is about 298K,VO₂ is insulator.At this time,the"L"-shaped gold plays a dominant role,thus realizing the polarization conversion function.In the frequency range of 0.83 to 1.92 THz,the polarisation conversion rate reaches more than 80%,with a relative bandwidth of 79.7%.When the temperature reaches about 358 K,VO₂is metal and the dual-function device behaves as a broadband perfect absorption performance with the absorption rate of more than 80%and relative bandwidth of 99.3%in the frequency range of 0.71 to 2.11 THz.This terahertz dual-function switching device has the advantages of simple structure,wide bandwidth and high efficiency.The research results of the thesis provide a new way to develop functionally switching photonic devices using phase change materials,which overcomes the shortcomings of single function,narrow band width and non-tunable frequency.It has relatively important scientific significance and potential application value.