Electron beam Modulated by Laguerre-Gaussian Modes

YUNG CHIANG LAN^1*, Ming-Chieh Lin²

¹Department of Photonics, National Cheng Kung University, Tainan, Taiwan
²Department of Electrical and Biomedical Engineering, Hanyang University, Seoul, Korea

* Presenter:YUNG CHIANG LAN, email:lanyc@mail.ncku.edu.tw

Light can have spin angular momentum (SAM) and orbital angular momentum (OAM). SAM is associated with right- and left-circularly polarized lights. The light with OAM indicates that it has an azimuthal phase dependence of exp(-imφ) and independent of the polarized state (m is an integer number and its sign indicates the handedness with respect to light’s propagation direction). The cylindrical Laguerre-Gaussian (LG) laser modes have an azimuthal phase dependence and carry OAM. Recently, microwave OAM beams are generated by using dielectric and metasurface antenna. On the other hand, the electron pulse can obtain OAM to form electron vortices through OAM transfer from photons to electrons. In this work, modulation of electron beam by LG modes is investigated by particle-in-cell finite-difference time-domain simulations (PIC-FDTD). An electron beam is irradiated by focused circular-polarized LG modes with different OAMs at microwave regime. The collective and individual behaviors of macro-electrons will be investigated. Dependence of electron beam size and current on electric field amplitude and gradient of LG modes are also explored. The simulated azimuthal velocity profiles of macro-particles display 3-D spiral patterns with the number of strands equal to sum of state number of SAM and OAM of LG modes. These spiral patterns move with the electron beams like a wave. The macro-particles also exhibit both orbit revolution and small rotation motion. The former is very similar to the gyromotion of electrons with applying an external magnetic field. The rotation motion has the same frequency as the LG mode while the orbit revolution frequency is far lower. Both the size and current of electron beams at the focal plane decrease with the increase of the electric field amplitude and gradient. This work can provide a new type of electron beams manipulated by LG waves only.

Keywords: Laguerre-Gaussian (LG) Modes, Light’s Orbital Angular Momentum, Particle-in-cell Simulation