基于类表面等离子激元的太赫兹Cherenkov辐射源的研究

Terahertz (THz) sources have widespread applications in bio-medical imaging and security monitoring, etc. Combining together the plasmonics and electronics, a new scheme that employs the interactions between free-space electron beams with spoof surface plasmon (SSP) mode, efficiently extracts the electron kinetic energy and excites THz Cherenkov radiation via enhanced surface electric field of the SSP mode, is proposed in this project. The effect of the surface electric field enhancement of the SSP mode overcomes the disadvantages of weak beam-wave coupling strength and low interaction efficiency in a traditional THz vacuum electronic device. The free-space electron beam promises high power radiation. The electromagnetic characteristics of the THz-SSP mode will be studied. Manipulation of the SSP mode will be realized by modifying the micro-structure on the metal surface, as a way to control the surface electric field enhancement and mode phase velocity. During studying the mechanism of interaction between electron beam and SSP mode, the interactions under the single electron, pulse electron bunch and DC electron beam conditions, will be investigated, respectively. Long range synchronization between free electrons and THz SSP will be realized by controlling SSP mode velocity, as a way to realizing efficient extraction of electron kinetic energy and THz radiation enhancement. The eletron beam voltage will be reduced by introducing a high value effective index of the fundamental mode of the SSP wave or the excitation of higher order harmonics of the SSP wave. Taking features of both THz SSP and vacuum electron beam into consideration, new schemes of multi-channel integration and distributed interactions will be explored, which may open a new research trend for the high-power THz sources.

生物医学成像、安全监测等众多领域对太赫兹（THz）源提出了迫切需求。本项目结合等离子激元光子学和电子学，提出采用自由空间电子束与类表面等离子激元(SSP)作用，基于SSP增强的纵向电场高效提取电子束动能，激发太赫兹Cherenkov辐射的新思路。SSP的表面场增强克服了传统太赫兹真空电子器件束-波耦合阻抗小的缺点；同时由于采用自由空间电子束，具有大功率潜力。拟研究金属微结构表面THz-SSP的电磁特性，通过调整微结构来控制THz-SSP模式，实现表面电场增强和相速度调控；电子束与SSP互作用激发太赫兹辐射的机理；通过控制SSP的相速度来维持自由电子与太赫兹SSP的长程同步，实现电子束能量的高效提取；通过材料结构的设计，提高SSP基波的等效折射率，或者考虑电子束激发高次空间谐波，达到降低电子束电压的目的；结合太赫兹SSP与真空电子束的特性，探索基于多通道集成和分布互作用系统的太赫兹源新方案。

本项目结合光学表面等离子激元（Surface Plasmon Polariton，SPP）的表面电场增强特性和电子学自由空间电子束高功率优势，提出一种基于Cherenkov原理的新型高效率太赫兹辐射源。基于SSP模式增强的纵向电场和慢波特性，本项目提出一种基于Cherenkov辐射原理的真空电子束与SSP模式互作用激发太赫兹辐射的新机理。通过引入外加电子束，选择合适的结构参数，有可能使得入射电子束和太赫兹波段的表面SSP模发生强烈的耦合互作用，并导致它们之间高效的能量交换。1.对单个的 2维理想金属光栅结构，通过电磁场的模式展开方法获得太赫兹SSP的色散关系。结果表明：试用19.15 kV ，0.5 A的电子束，可以获得20多瓦特的1THz波输出。2. 通过模式展开方法以及电磁场的数值模拟方法，进一步获得了对称的2个金属光栅结构表面的太赫兹SSP的色散关系以及对称和反对称模式的特性。3.对于二维金属光栅结构的太赫兹波段类表面等离子激元（spoof surface-plasmons：SSP），可以用来应用于带电粒子束的加速。4.超薄超表面和相变或者磁性光学材料结合，可以有效的调节电磁波的偏振，可以实现基于等离子激元的电磁波偏振旋转系统。电磁波偏振的方向性和偏振的可调节性可以很好的延伸到太赫兹波段。由于课题本身的难度，以及部分成果滞后，紧凑型的高功率太赫兹源器件还需有进一步的研究和探讨。