Effect of a Strong Laser on Spin Precession

The semiclassical dynamics of a charged spin-1/2 particle in an intense electromagnetic plane wave is analyzed beyond the electric dipole approximation and taking into account the leading relativistic corrections to the Pauli equation. It is argued that the adiabatic spin evolution driven by a low intensity radiation changes its character drastically as an intensity of a laser is increasing. Particularly, it is shown that a charged particle exposes a spin flip resonance at a certain pick value of a laser field strength, which is determined by a particle’s gyromagnetic ratio. The problem and result. A good deal of a considerable knowledge on a charged spinning particle interaction with a low intensity laser has been gleaned from the extensive use of the electric dipole approximation [1]. This approximation works perfectly to describe the particle’s classical trajectory as well as to understand the adiabatic evolution of spin, represented by the intrinsic angular momentum [2]. With the growing intensity of a radiation different relativistic corrections to the charge motion become relevant [3], [4]. This demands to refuse the electric dipole approximation and to take into account the influence of the magnetic part of the Heaviside-Lorentz force. Entering to this non-dipole region a new physics become tangible. In this context, the present talk aims to report on the manifestation of a such non-dipole physics: a charged particle’s spin flip resonance induced by a strong laser field. It is arguably the best to describe the spin-flip resonance in the so-called average rest frame, frame where the mean particle’s velocity vanishes. In this frame, as our calculations show, the probability to flip for a spin, that is initially polarised along the direction of propagation of the circularly polarised monochromatic plane wave, is given by an analog of the well-known formula from the Rabi magnetic resonance problem [5]: P↓↑ = A↓↑(η) sin2(ωS t) , ωS := ωL|1− g| 8 √ κη + (η − η ∗)2 , (1) The frequency ωS differs from a laser circular frequency ωL and depends nonlinearly on a particle’s gyromagnetic ratio g , as well as on a laser field strength parameter [6]: η = −2 e 2 mc 〈A〉 , 〈· · ·〉 − time average, A− a laser gauge potential . (2) The flipping amplitude A↑↓(η) in (1) has the following resonance form A↓↑(η) = κη κη + (η − η ∗)2 , κ := 2 g (1− g) , η 2 ∗ := 4 g − 1 . (3)