Transverse manipulation of an electron beam by circularly polarized Laguerre-Gaussian modes

Abstract

This study investigates the modulation of the azimuthal velocity of electrons in an electron beam using circularly polarized Laguerre-Gaussian (LG) modes. The finite-difference time-domain particle-in-cell (FDTD-PIC) method is employed for this purpose. After obtaining the orbital and spin angular momenta of the LG mode, the distribution of the electrons’ azimuthal velocity (i.e., the distribution of angular momentum) exhibits 3D spiral patterns. The number of strands in these spirals corresponds to the sum of the quantum numbers associated with the orbital and spin angular momenta of the LG mode. Furthermore, these spiral patterns rotate in the same direction as the LG mode and move along with it. In contrast, the electrons in the beam undergo a gyromotion along their forward direction (without the application of an external magnetic field in this study). The rotation direction of the electrons is primarily determined by the sign of their initial azimuthal velocity after acquiring angular momenta from the LG mode. Additionally, all electrons share the same gyrofrequency, which is much lower than the LG mode’s frequency. This gyrofrequency can be manipulated by the frequency, electric field strength, and beam waist size of the LG mode. Moreover, increasing the electric field strength allows a larger-current electron beam to be confined within the LG mode. The gyromotion and confinement effects of electrons are primarily due to the transverse ponderomotive force generated by the LG mode. It is demonstrated that the manipulation of an electron beam can be realized by using circularly polarized LG modes.