Generating "vortex Smith-Purcell radiation" by having periodically bunched free electrons pass on a holographic grating
Generating "vortex Smith-Purcell radiation" by having periodically bunched free electrons pass on a holographic grating
Smith-Purcell radiation (SPR) is the electromagnetic wave generated by free electrons passing above a diffraction grating, which has played an important role in free electron light sources and particle accelerators. Since SPR is derived from the scattering of the evanescent wave around the free electrons by grating, the characteristics of SPR, including radiation wavelength, intensity, and polarization, could be controlled by manipulating the free electrons and the structure interacting with electrons. In recent years, orbital angular momentum (OAM) has been known as a new degree of freedom of light, which shows promising application in increasing the bandwidth of optical communication, manipulating micro-particles, fabrication of nano-structures, and so on.
In order to generate SPR carrying OAM, Nano OptoElectronics Laboratory led by Prof. Yidong Huang, Department of Electronic Engineering, Tsinghua University, proposes an integratable method to flexibly generate "vortex Smith-Purcell radiation" by having periodically bunched free electrons pass on a holographic grating. The simulation results illustrate that coherent OAM wave with different topological charges and in different frequency regions, including ultraviolet, visible light, infrared, terahertz, and microwave region, could be derived by designing different holographic gratings. It is also found that the radiation angle of OAM wave could be tuned easily and the topological charge would be multiplied when high-order SPR is generated. Related research results are published in Photonics Research, Volume 8, No. 8, 2020 (M. Wang, et al., Vortex Smith-Purcell Radiation Generation with Holographic Grating).
The corresponding author of this paper, associate Prof. Fang Liu, believes that this work introduces a new way to manipulate the OAM of free-electron-induced radiation and to develop an integratable and tunable vortex wave source in different frequency regions.
Further work will focus on observing the vortex SPR experimentally and realizing the on-chip integrated vortex SPR emitter.
可集成方法产生携带轨道角动量的史密斯-帕塞尔辐射
基于平面全息光栅的 “涡旋Smith-Purcell辐射”产生
史密斯-帕塞尔辐射（Smith-Purcell radiation，SPR）是自由电子飞过衍射光栅表面时产生的电磁辐射，其在自由电子光源和电子加速器等领域扮演着重要的角色。SPR的产生机制可以理解为飞行自由电子周围的倏逝场被光栅散射到自由空间的过程。因此，通过改变自由电子的形态或与电子相互作用的微纳结构，可以调控SPR的波长、功率和偏振等特性。
近年来，研究人员发现了电磁波的一个新自由度，即轨道角动量（OAM），它使得光束具有涡旋形的截面光场和螺旋形的等相位面。OAM波束有趣的特性吸引了众多研究者的关注，其在光通信、微粒子操控和纳米结构加工等领域具有重要的应用前景。
清华大学电子工程系黄翊东教授领导的微纳光电子实验室提出了利用周期群聚的自由电子与全息光栅相互作用产生携带OAM的“涡旋SPR”的可集成方法。相关研究结果发表在Photonics Research 2020年第8期上（Mengxuan Wang, Fang Liu, Yuechai Lin, et al. Vortex Smith–Purcell radiation generation with holographic grating[J]. Photonics Research, 2020, 8(8): 08001309）。
利用倏逝波作为参考光，与OAM波束干涉可以得到平面全息光栅结构，其可以保存OAM波束的波前信息。当周期群聚的自由电子飞越全息光栅表面时，自由电子周围的倏逝场与全息光栅相互作用，可还原出OAM光束的波前，形成涡旋SPR。
仿真结果表明，该方法适用于产生微波、太赫兹、红外、可见光和紫外波段的不同拓扑数的涡旋SPR；此外，只需极低能量的自由电子便可以产生该辐射，为实现片上集成涡旋SPR提供了可能。研究工作还同时发现了高阶SPR的OAM拓扑数等于一阶SPR的OAM拓扑数乘以辐射阶次这一重要规律。
论文通讯作者刘仿副教授认为，该工作不仅可以实现SPR的OAM维度的操控，而且提供了一种适用于多个波段、波长可调谐、可集成的自由电子OAM光源的新方法。
未来的工作将致力于实验观测携带OAM的SPR辐射，并研制出自由电子OAM光源芯片。