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arXiv:1701.05951 · Physical Review A

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

Matthias M. Dellweg, Carsten Müller

Published Apr 20, 2017• 28 citations
arXivHTMLPDF

Abstract

Electron spin dynamics in counterpropagating, bichromatic laser fields are studied by solving the Dirac equation with different laser polarization configurations. It is found that the spin-dependent Kapitza-Dirac effect in the dynamical processes may be used to design spin filters for free electrons.
DOI
10.1103/physreva.95.042124

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We discuss electron diffraction from two standing waves with two different frequencies. The effects of increasing the frequency of the second laser beam and changing the laser amplitudes on the form and period of the Rabi oscillation are studied theoretically. The corresponding scattering probabilities for certain incident electron momenta are obtained by an analytical Rabi matrix and numerical solution of the Dirac equation. We show that at high intensities, $\ensuremath{\ge}{10}^{20}$ W ${\mathrm{cm}}^{\ensuremath{-}2}$, the process with an even number of photons involved in the Kapitza-Dirac effect can be used as a spin filter for free electrons. On the other hand, the process with an odd number photons and an electron with momentum along the laser polarization preserves the initial spin of the electron.
We demonstrate the theoretical feasibility of spin-dependent diffraction and spin polarization of an electron in two counterpropagating, circularly polarized laser beams. The spin dynamics appears in a two-photon process of the Kapitza-Dirac effect in the Bragg regime. We show the spin dependence of the diffraction process by comparison of the time evolution of spin-up and spin-down electrons in a relativistic quantum simulation. We further discuss the spin properties of the scattering by studying an analytically approximated solution of the time-evolution matrix. A classification scheme in terms of unitary or nonunitary propagation matrices is used for establishing a generalized and spin-independent description of the spin properties in the diffraction process.
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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 time-dependent 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.