Spin flips of electron beams in optical near fields

Abstract: Manipulating the spin polarization of electron beams using light is highly desirable but exceedingly challenging, as the approaches proposed in previous studies using free-space light usually require enormous laser intensities. Here, we propose the use of a transverse electric optical near field, extended on nanostructures, to efficiently induce spin flips of an adjacent electron beam by exploiting the strong inelastic electron scattering in phase-matched optical near fields. Our calculations show that the use of a dramatically reduced laser intensity ($\sim 10^{12}\,$W/cm$^2$) with a short interaction length ($16\,\mu$m) achieves an electron spin-flip probability of approximately $12\%$. Intriguingly, the two spin components of an unpolarized incident electron beam -- parallel and antiparallel to the electric field -- are spin-flipped and inelastically scattered to different energy states, providing an analog of the Stern--Gerlach experiment in the energy dimension. Our findings are important for optical control of free-electron spins, preparation of spin-polarized electron beams, and applications as varied as in material science and high-energy physics.