Abstract
Strong rotating magnetic fields may cause a precession of the electronʼs spin around the rotation axis of the magnetic field. The superposition of two counterpropagating laser beams with circular polarization and opposite helicity features such a rotating magnetic field component but also carries spin. The laserʼs spin density, which can be expressed in terms of the laserʼs electromagnetic fields and potentials, couples to the electronʼs spin via a relativistic correction to the Pauli equation. We show that the quantum mechanical interaction of the electronʼs spin with the laserʼs rotating magnetic field and with the laserʼs spin density counteract each other in such a way that a net spin rotation remains with a precession frequency that is much smaller than the frequency one would expect from the rotating magnetic field alone. In particular, the frequency scales differently with the laserʼs electric field strength depending on whether relativistic corrections are taken into account or not. Thus, the relativistic coupling of the electronʼs spin to the laserʼs spin density changes the dynamics not only quantitatively but also qualitatively as compared to the nonrelativistic theory. The electronʼs spin dynamics are a genuine quantum mechanical relativistic effect.
Highlights
Driven by recent developments of novel light sources that envisage to provide field intensities in excess of 1020 W/cm2 and field frequencies in the x-ray domain [1,2,3,4,5,6] relativistic light matter interaction has become an active field of experimental and theoretical research in recent years [7,8,9]
We show that the quantum mechanical interaction of the electron’s spin with the laser’s rotating magnetic field and with the laser’s spin density counteract each other in such a way that a net spin rotation remains with a precession frequency that is much smaller than the frequency one would expect from the rotating magnetic field alone
Distinct spin dynamics has been found in the relativistic Rabi effect [12] as well as in the relativistic Kapitza-Dirac effect [13, 14]; in [15] collapse-and-revival dynamics for the spin evolution of laser-driven electrons was predicted at 1018 W/cm2, and the dynamics of plasmas can be modified by spin effects [16]
Summary
Driven by recent developments of novel light sources that envisage to provide field intensities in excess of 1020 W/cm and field frequencies in the x-ray domain [1,2,3,4,5,6] relativistic light matter interaction has become an active field of experimental and theoretical research in recent years [7,8,9]. Spin effects for electrons in intense linearly polarized laser fields were studied in [10, 11]. In a pioneering work by Beth [26] In this contribution, we examine electrons and the quantum dynamics of their spin in a standing light wave formed by two counterpropagating laser beams with elliptical polarization. We will show that the electromagnetic wave’s spin density couples to the electron’s spin causing electron spin precession. This precession is identified as a genuine relativistic effect that can only be explained by treating the electron quantum mechanically and taking into account relativistic corrections to the Pauli equation. The predicted spin effect may be realized by employing hard x-ray laser pulses of ultra high power with a stable pulse shape over a few hundred or more cycles
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