Igor P. Ivanov, Dmitry V. Karlovets
Vortex electrons, i.e. freely propagating electrons whose wavefunction has helical wavefronts, could emerge as a novel tool for the physics of electromagnetic (EM) radiation. They carry non-zero intrinsic orbital angular momentum (OAM) $\ell$ and, for $\ell \gg 1$, a large OAM-induced magnetic moment, $\mu \approx \ell \mu_B$ ($\mu_B$ is the Bohr magneton), which affects the radiation of EM waves. Here, we consider in detail its influence on two forms of polarization radiation, namely on Cherenkov and transition radiation. Due to large $\ell$, we can neglect quantum or spin-induced effects, which are of order $\hbar \omega/E_e \ll 1$, but retain the magnetic moment contribution $\ell \hbar \omega/E_e \lesssim 1$, which makes the quasiclassical approach to polarization radiation applicable. We discuss magnetic moment contribution to polarization radiation, which has never been experimentally observed, and study how its visibility depends on kinematical parameters and permittivity of the medium. In particular, it is shown that this contribution can, in principle, be detected in azimuthally non-symmetrical problems, for example when vortex electrons obliquely cross a metallic screen (transition radiation) or move nearby it (diffraction radiation). We predict left-right asymmetry of the transition radiation, which appears due to effective interference between the charge and the magnetic moment radiation fields. Numerical values of this asymmetry for vortex electrons with $E_e = 300$ keV and $\ell = {\cal O}(100-1000)$ are ${\cal O} (0.1-1%)$, and we argue that this effect could be detected with today's technology. Finite conductivity of the target and a frequency dispersion play the crucial roles in these predictions.
View original:
http://arxiv.org/abs/1305.6592
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