J. Z. Kamiński

Fundamental processes of quantum electrodynamics in laser fields of relativistic power

In this review we summarize our progress in the investigation of fundamental processes of quantum electrodynamics in laser fields of relativistic power in view of the more recent experimental progress in the generation of laser field intensities, yielding ponderomotive energy shifts Up of the order of magnitude mc2 and beyond. In particular, the generation of electron–positron pairs during the collision of laser pulses with ions or protons appears to become feasible.

Compton process in intense short laser pulses

The spectra of Compton radiation emitted during electron scattering off an intense laser beam are calculated using the framework of strong-field quantum electrodynamics. We model these intense laser beams as finite length plane-wave-fronted pulses, similar to Neville and Rohrlich [Phys. Rev. D {\bf 3}, 1692 (1971)], or as trains of such pulses. Expressions for energy and angular distributions of Compton photons are derived such that a comparison of both situations becomes meaningful. Comparing frequency distributions for both an isolated laser pulse and a laser pulse train, we find a very good agreement between the results for long pulse durations which breaks down however for ultrashort laser pulses. The dependence of angular distributions of emitted radiation on a pulse duration is also investigated. Pronounced asymmetries of angular distributions are found for very short laser pulses, which gradually disappear with increasing the number of laser field oscillations. Those asymmetries are attributed to asymmetries of the vector potential describing an incident laser beam.

Breit-Wheeler Process in Intense Short Laser Pulses

Energy-angular distributions of electron-positron pair creation in collisions of a laser beam and a nonlaser photon are calculated using the

Global phase and frequency comb structures in nonlinear Compton and Thomson scattering

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Coulomb scattering in intense, high-frequency laser fields☆

-matrix formalism. The laser field is modeled as a finite pulse, similar to the formulation introduced in our recent paper in the context of Compton scattering [Phys. Rev. A {\bf 85}, 062102 (2012)]. The nonperturbative regime of pair creation is considered here. The energy spectra of created particles are compared with the corresponding spectra obtained using the modulated plane wave approximation for the driving laser field. A very good agreement in these two cases is observed, provided that the laser pulse is sufficiently long. For short pulse durations, this agreement breaks down. The sensitivity of pair production to the polarization of a driving pulse is also investigated. We show that in the nonperturbative regime, the pair creation yields depend on the polarization of the pulse, reaching their maximal values for the linear polarization. Therefore, we focus on this case. Specifically, we analyze the dependence of pair creation on the relative configuration of linear polarizations of the laser pulse and the nonlaser photon. Lastly, we investigate the carrier-envelope phase effect on angular distributions of created particles, suggesting the possibility of phase control in relation to the pair creation processes.

Electron-positron pair creation by powerful laser-ion interaction

The Compton and Thomson radiation spectra, generated in collisions of an electron beam with a powerful laser beam, are studied in the framework of quantum and classical electrodynamics, respectively. We show that there are frequency regimes where both radiation spectra are nearly identical, which for Compton scattering relates to the process which preserves the electron spin. Although the radiation spectra are nearly identical, the corresponding probability amplitudes exhibit different global phases. This has pronounced consequences, which we demonstrate by investigating temporal power distributions in both cases. We show that, contrary to Thomson scattering, it is not always possible to synthesize short laser pulses from Compton radiation. This happens when the global phase of the Compton amplitude varies in a nonlinear way with the frequency of emitted photons. We also demonstrate that while the Compton process driven by a non-chirped laser pulse can generate chirped bursts of radiation, this is not the case for the Thomson process. In principle, both processes can lead to a generation of coherent frequency combs when single or multiple driving laser pulses collide with electrons. Once we synthesize these combs into short bursts of radiation, we can control them, for instance, by changing the time delay between the driving pulses.

Off-shell effects in laser-assisted electron scattering at low frequency

Abstract A recently developed theory for free-free transitions in intense, high-frequency laser fields is applied to study the laser-modified elastic scattering in a Coulomb potential (case of linear polarization). Under the conditions of the theory the scattering can be viewed as happening in a time-independent dressed Coulomb potential. This is modelled here by its spherical average so that a phase-shift analysis can be applied. We study the change of the Rutherford cross section as a function of the electron energy, the laser parameters, and the nuclear charge Z . Large modifications are shown to appear for values of the parameters corresponding to presently existing high-frequency lasers. Characteristic Coulomb-interference oscillations are displayed at small angles.

Coherent combs of antimatter from nonlinear electron-positron-pair creation

Presently available high-power laser pulses of ponderomotive energy Up ≫ 2mc2 should permit the fundamental processes of quantum electrodynamics in such fields, in particular, the formation of electron-positron pairs in impacts of laser pulses with highly charged ions, to be observed. We evaluate the highly nonlinear production rates of this process and investigate the most favorable conditions of pair production, in particular, either along the direction of linear polarization or in the propagation direction of the laser pulse. For femtosecond radiation pulses, it is possible to represent the laser beam by a monochromatic and linearly polarized electromagnetic plane wave. This approximation considerably simplifies the calculations required.

X-ray generation by Compton scattering of elliptically polarized laser light at relativistic radiation powers

Differential cross sections for the scattering of electrons by a short-range potential in the presence of an intense, linearly polarized, low-frequency field are obtained in three different ways: within the Kroll-Watson approximation, within the impulse approximation and through an exact Floquet calculation. Examples are given where the cross section for small-angle scattering with emission or absorption of photons is larger by several orders of magnitude than predicted by the Kroll-Watson approximation. The enhancement can be ascribed to off-shell contributions and is correctly reproduced by the impulse approximation. However, no such enhancement is found in model calculations for helium. The accuracy of the impulse approximation in the vicinity of a resonance is also studied.

Coherent phase control in laser-assisted radiative recombination and x-ray generation

Electron-positron pair creation in collisions of a modulated laser pulse with a high-energy photon (nonlinear Breit-Wheeler process) is studied by means of strong-field quantum electrodynamics. It is shown that the driving pulse modulations lead to appearance of comb structures in the energy spectra of produced positrons (electrons). It is demonstrated that these combs result from a coherent enhancement of probability amplitudes of pair creation from different modulations of the laser pulse. Thus, resembling the Young-double slit experiment for anti-matter (matter) waves.