频率可切换太赫兹涡旋波束产生器

Switchable frequency terahertz vortex beam generator

已报道的太赫兹涡旋波束产生器大多数是在固定频率产生涡旋波束,限制了它的实际应用场景.本文提出一种频率可切换太赫兹涡旋波束超表面,通过改变外部温度,二氧化钒相态也随之改变,该超表面可以实现在单频模式和双频模式的自由切换.室温下,所设计的超表面在频率1.1 THz处可以产生具有不同拓扑电荷数的涡旋波束,而且它们的模式纯度均在85%以上.外部温度变为68℃时,该超表面工作频率切换到两个频率点0.7和1.23 THz,产生不同拓扑荷数的涡旋波束,模式纯度均大于60%.设计的频率可切换太赫兹涡旋产生器为无线太赫兹通信中工作频率调制提供了一个新的设计思路.

Most of the reported vortex beam generators generate vortex beams at a fixed frequency,which limits the practical applications.Therefore,it is inevitable to explore a vortex beam generator,which can actively control the operating frequency.We propose a switchable frequency terahertz vortex beam metasurface,it is freely switchable under single-frequency mode and dual-frequency mode by changing the external temperature,the phase state of vanadium dioxide(VO)is also changeable.External temperature changes will cause VOto transform from insulating state to metallic state.Generally,VOconductivity can increase by several orders of magnitude as operating temperature changes.By using the phase change property of VO,we can obtain a metasurface with switchable operating frequencies.For operating at room temperature,the proposed metasurface behaves as a single-frequency terahertz vortex generator.When(left-handed circularly polarized,LCP)terahertz wave is vertically incident on the metasurface,it generates vortex beams with different topological charge numbers at a frequency of 1.1 THz,and the mode purity is above 85%.The simulation results show that the mode purity of the vortex beam with the topological charge l=1 is 90%,and the mode purity is about 91.1%for the vortex beam with l=2,and 85.4%for the vortex beam with l=3.When the external temperature is of 68℃,the designed metasurface becomes a dual-frequency vortex beam generator.At this time,the operating frequencies of vortex beams with different topological charges(l=1,2,3)are 0.7 and 1.23 THz,whose mode purities are both above 60%.That is to say,the corresponding mode purities at topological charge with l=1 for two operating frequencies are 89.1%and 71.6%,respectively.The mode purities are 83.2%and 94.4%with topological charge l=2,respectively.The mode purities are 62.4%and 68.2%with topological charge l=3,respectively.Therefore,the proposed switchable frequency terahertz vortex generator provides a new design idea for working frequency modulation in wireless terahertz communication.