Spin Dynamics in the Kapitza-Dirac Effect

Abstract

Electron spin dynamics in Kapitza-Dirac scattering from a standing laser wave of high frequency and high intensity is studied. We develop a fully relativistic quantum theory of the electron motion based on the timedependent Dirac equation. Distinct spin dynamics, with Rabi oscillations and complete spin-flip transitions, is demonstrated for Kapitza-Dirac scattering involving three photons in a parameter regime accessible to future high-power X-ray laser sources. The Rabi frequency and, thus, the diffraction pattern is shown to depend crucially on the spin degree of freedom.

Controlling electron spin dynamics in bichromatic Kapitza-Dirac scattering by the laser field polarization

“…Inserting this ansatz into the Dirac equation (13) yields a coupled system of ordinary differential equations for the time-dependent expansion coefficients c s n (t) and d s n (t), with n ∈ Z and s ∈ {↑, ↓} [32]. We note that the same kind of approach was used in [17] within Pauli theory for bichromatic laser fields and in [14] within Dirac theory for a standing laser wave. The "positronic" contributions in Eq.…”

“…(14). In the regime of laser parameters under consideration, the outgoing electron state after the interaction with the laser fields can be expanded into these four states.…”

“…Previous studies of spin-dependent KD scattering have always found "symmetric" spin effects: the probabilities to scatter an incident spin-up electron into a spin-down state and vice versa coincide [14][15][16][17]. Consequently, when an unpolarized electron beam is incident, the scattered part of the beam will remain unpolarized because the spin of each single electron has flipped.…”

Spin-dependent two-color Kapitza-Dirac effects

“…Recently, it was predicted that x-ray laser light could be used to affect the electron spin of a beam of relativistic free electrons [9,10]. More generally, electron spin control can provide an additional control to ultrafast electron diffraction [11,12] and ultrafast electron microscopy [13,14], similar to the nonpulsed version of spin-polarized low-energy electron microscopy [15] (SPLEEM).…”

“…We call this the spin-Kapitza-Dirac (SKD) effect. References [9,10] inspired the idea to use the orthogonality between the polarization and the electron-beam propagation axis. In this paper we use optical frequencies instead of x rays, nonrelativistic intensities instead of relativistic ones, and close to orthogonal angles between the electron beam and the laser beams instead of angled ones.…”