Relativistic high-power laser–matter interactions

Electrons are diffracted by a standing light wave of light, a phenomenon known as the Kapitza–Dirac effect. A generalization of this effect opens perspectives for the manipulation of ultrashort electron wavepackets by intense laser fields. In the early days of quantum mechanics Kapitza and Dirac predicted that matter waves would scatter off the optical intensity grating formed by two counter-propagating light waves1. This interaction, driven by the ponderomotive potential of the optical standing wave, was both studied theoretically and demonstrated experimentally for atoms2 and electrons3,4,5. In the original version of the experiment1,5, only the transverse momentum of particles was varied, but their energy and longitudinal momentum remained unchanged after the interaction. Here, we report on the generalization of the Kapitza–Dirac effect. We demonstrate that the energy of sub-relativistic electrons is strongly modulated on the few-femtosecond timescale via the interaction with a travelling wave created in vacuum by two colliding laser pulses at different frequencies. This effect extends the possibilities of temporal control of freely propagating particles with coherent light and can serve the attosecond ballistic bunching of electrons6, or for the acceleration of neutral atoms or molecules by light.

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Inelastic ponderomotive scattering of electrons at a high-intensity optical travelling wave in vacuum

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.

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Spin Dynamics in the Kapitza-Dirac Effect

Production of electron-positron pairs in the collision of a high-energy photon with a high-intensity few-cycle laser pulse is studied. By utilizing the frameworks of laser-dressed spinor and scalar quantum electrodynamics, a comparison between the production of pairs of Dirac and Klein-Gordon particles is drawn. Positron energy spectra and angular distributions are presented for various laser parameters. We identify conditions under which predictions from Klein-Gordon theory either closely resemble or largely differ from those of the proper Dirac theory. In particular, we address the question to which extent the relevance of spin effects is influenced by the short duration of the laser pulse.

Strong-field Breit-Wheeler pair production in short laser pulses: Relevance of spin effects

A relativistic description of the Kapitza-Dirac e ect in the so-called Bragg regime with two and three interacting photons is presented by investigating both numerical and perturbative solutions of the Dirac equation in momentum space. We demonstrate that spin-flips can be observed in the two-photon and the three-photon Kapitza-Dirac e ect for certain parameters. During the interaction with the laser field the electron’s spin is rotated, and we give explicit expressions for the rotation axis and the rotation angle. The o -resonant Kapitza-Dirac e ect, that is, when the Bragg condition is not exactly fulfilled, is described by a generalized Rabi theory. We also analyze the in-field quantum dynamics as obtained from the numerical solution of the Dirac equation.

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V.M. Haroutunian

Papers

An analogue of the Kapitza--Dirac effect

“Reflection” and catching of electrons in the field of opposite electromagnetic waves

Quantum modulation of an electron beam when it is “reflected” from a plane electromagnetic wave in a medium