Lan Yung-Chiang, Shen Chia-Hui, Chen Chih-Min
Department of Photonics and Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan, 701, Taiwan.
Sci Rep. 2020 Oct 7;10(1):16768. doi: 10.1038/s41598-020-73725-6.
In this work, surface plasmons (SPs) on a germanium (Ge) thin film in terahertz (THz) region that are excited by electron cyclotron motion (ECM) and the subsequent SP emission (SPE) by adding Ge gratings on the film are explored by finite-difference time-domain (FDTD) and particle-in-cell FDTD (PIC-FDTD) simulations. The optical properties of ECM-excited SPs are the same as those of SPs that are excited by electron straight motion (ESM). For operating at the flat band of SPs' dispersion curve on the Ge film, changing the electron energy will only change the wavevector of SPs and hence the number of periods of SPs on the circular orbital. When the periodic gratings are deposited on the Ge film along the circular orbital of electrons, the emitted SPE contains the orbital angular momentum (OAM). The number of arms and chirality of the spiral patterns in phase map (i.e. the quantum number of OAM) of SPE are determined by the difference between the number of SPs' periods and the number of gratings. Manipulations of the quantum number of OAM by changing the number of gratings for a fixed electron energy and by changing the electron energy for a fixed number of gratings are also demonstrated. This work provides an active OAM source and it is not required to launch circularly polarized beams or pumping beams into the structure.
在这项工作中，通过有限时域差分（FDTD）和粒子模拟有限时域差分（PIC - FDTD）方法，研究了太赫兹（THz）波段锗（Ge）薄膜上由电子回旋运动（ECM）激发的表面等离子体激元（SPs），以及通过在薄膜上添加锗光栅产生的后续表面等离子体激元发射（SPE）。由ECM激发的表面等离子体激元的光学性质与由电子直线运动（ESM）激发的表面等离子体激元的光学性质相同。为了在锗薄膜上表面等离子体激元色散曲线的平带处工作，改变电子能量只会改变表面等离子体激元的波矢，从而改变圆形轨道上表面等离子体激元的周期数。当沿着电子的圆形轨道在锗薄膜上沉积周期性光栅时，发射的SPE包含轨道角动量（OAM）。SPE相位图中螺旋图案的臂数和手性（即OAM的量子数）由表面等离子体激元的周期数与光栅数之间的差值决定。还展示了通过在固定电子能量下改变光栅数以及在固定光栅数下改变电子能量来操控OAM量子数的方法。这项工作提供了一个有源OAM源，并且无需向结构中注入圆偏振光束或泵浦光束。
