arXiv subjects

Christoph H. Keitel

Publications and source records attributed to Christoph H. Keitel.

At least 217 records · Page 12Linked to original sources

arXiv2007

Photo-Emission of a Single-Electron Wave-Packet in a Strong Laser Field

The radiation emitted by a single-electron wave packet in an intense laser field is considered. A relation between the exact quantum formulation and its classical counterpart is established via the electron's Wigner function. In particular we show that the wave packet, even when it spreads to the scale of the wavelength of the driving laser field, cannot be treated as an extended classical charge distribution but rather behaves as a point-like emitter carrying information on its initial quantum state. We outline an experimental setup dedicated to put this conclusion to the test.

quant-ph↗

arXiv2007

Isomer triggering via nuclear excitation by electron capture

Triggering of long-lived nuclear isomeric states via coupling to the atomic shells in the process of nuclear excitation by electron capture (NEEC) is studied. NEEC occurring in highly-charged ions can excite the isomeric state to a triggering level that subsequently decays to the ground state. We present total cross sections for NEEC isomer triggering considering experimentally confirmed low-lying triggering levels and reaction rates based on realistic experimental parameters in ion storage rings. A comparison with other isomer triggering mechanisms shows that, among these, NEEC is the most efficient.

nucl-th↗

arXiv2007

Coherent hard x-rays from attosecond pulse train-assisted harmonic generation

High-order harmonic generation from atomic systems is considered in the crossed fields of a relativistically strong infrared laser and a weak attosecond-pulse train of soft x-rays. Due to one-photon ionization by the x-ray pulse, the ionized electron obtains a starting momentum that compensates the relativistic drift which is induced by the laser magnetic field, and allows the electron to efficiently emit harmonic radiation upon recombination with the atomic core in the relativistic regime. In this way, short pulses of coherent hard x-rays of up to 40 keV energy and 10 as duration can be brought about.

physics.atom-ph↗

arXiv2007

Zeptosecond $γ$-ray pulses

High-order harmonic generation (HHG) in the relativistic regime is employed to obtain zeptosecond pulses of $γ$-rays. The harmonics are generated from atomic systems in counterpropagating strong attosecond laser pulse trains of linear polarization. In this setup recollisions of the ionized electrons can be achieved in the highly relativistic regime via a reversal of the commonly deteriorating drift and without instability of the electron dynamics such as in a standing laser wave. As a result, coherent attosecond $γ$-rays in the 10 MeV energy range as well as coherent zeptosecond $γ$-ray pulses of MeV photon energy for time-resolved nuclear spectroscopy become feasible.

physics.atom-ph↗

arXiv2007

Particle physics with a laser-driven positronium atom

A detailed quantum-electrodynamic calculation of muon pair creation in laser-driven electron-positron collisions is presented. The colliding particles stem from a positronium atom exposed to a superintense laser wave of linear polarization, which allows for high luminosity. The threshold laser intensity of this high-energy reaction amounts to a few 10^22 W/cm^2 in the near-infrared frequency range. The muons produced form an ultrarelativistic, strongly collimated beam, which is explicable in terms of a classical simple-man's model. Our results indicate that the process can be observed at high positronium densities with the help of present-day laser technology.

hep-ph↗

arXiv2006

Relativistic ionization-rescattering with tailored laser pulses

The interaction of relativistically strong tailored laser pulses with an atomic system is considered. Due to a special tailoring of the laser pulse, the suppression of the relativistic drift of the ionized electron and a dramatic enhancement of the rescattering probability is shown to be achievable. The high harmonic generation rate in the relativistic regime is calculated and shown to be increased by several orders of magnitude compared to the case of conventional laser pulses. The energies of the revisiting electron at the atomic core can approach the MeV domain, thus rendering hard x-ray harmonics and nuclear reactions with single atoms feasible.

physics.atom-ph↗

arXiv2006

The nuclear AC-Stark shift in super-intense laser fields

The direct interaction of super-intense laser fields in the optical frequency domain with nuclei is studied. As main observable, we consider the nuclear AC-Stark shift of low-lying nuclear states due to the off-resonant excitation by the laser field. We include the case of accelerated nuclei to be able to control the frequency and the intensity of the laser field in the nuclear rest frame over a wide range of parameters. We find that AC-Stark shifts of the same order as in typical quantum optical systems relative to the respective transition frequencies are feasible with state-of-the-art or near-future laser field intensities and moderate acceleration of the target nuclei. Along with this shift, we find laser-induced modifications to the proton root-mean-square radii and to the proton density distribution. We thus expect direct laser-nucleus interaction to become of relevance together with other super-intense light-matter interaction processes such as pair creation.

nucl-th↗

arXiv2006

Muon pair creation from positronium in a circularly polarized laser field

We study elementary particle reactions that result from the interaction of an atomic system with a very intense laser wave of circular polarization. As a specific example, we calculate the rate for the laser-driven reaction $e^+e^- \to μ^+μ^-$, where the electron and positron originate from a positronium atom or, alternatively, from a nonrelativistic $e^+e^-$ plasma. We distinguish accordingly between the coherent and incoherent channels of the process. Apart from numerical calculations, we derive by analytical means compact formulas for the corresponding reaction rates. The rate for the coherent channel in a laser field of circular polarization is shown to be damped because of the destructive interference of the partial waves that constitute the positronium ground-state wave packet. Conditions for the observation of the process via the dominant incoherent channel in a circularly polarized field are pointed out.

physics.atom-ph↗

arXiv2006

Nuclear quantum optics with x-ray laser pulses

The direct interaction of nuclei with super-intense laser fields is studied. We show that present and upcoming high-frequency laser facilities, especially together with a moderate acceleration of the target nuclei, do allow for resonant laser-nucleus interaction. These direct interactions may be utilized for the optical measurement of nuclear properties such as the transition frequency and the dipole moment, thus opening the field of nuclear quantum optics. As ultimate goal, one may hope that direct laser-nucleus interactions could become a versatile tool to enhance preparation, control and detection in nuclear physics.

nucl-th↗

arXiv2006

Interference in the resonance fluorescence of two incoherently coupled transitions

The fluorescence light emitted by a 4-level system in $J=1/2$ to $J=1/2$ configuration driven by a monochromatic laser field and in an external magnetic field is studied. We show that the spectrum of resonance fluorescence emitted on the $π$ transitions shows a signature of spontaneously generated interference effects. The degree of interference in the fluorescence spectrum can be controlled by means of the external magnetic field, provided that the Landé g-factors of the excited and the ground state doublet are different. For a suitably chosen magnetic field strength, the relative weight of the Rayleigh line can be completely suppressed, even for low intensities of the coherent driving field. The incoherent fluorescence spectrum emitted on the $π$ transitions exhibits a very narrow peak whose width and weight depends on the magnetic field strength. We demonstrate that the spectrum of resonance fluorescence emitted on the $σ$ transitions show an indirect signature of interference. A measurement of the relative peak heights in the spectrum from the $σ$ transitions allows to determine the branching ratio of the spontaneous decay of each excited state into the $σ$ channel.

quant-ph↗

arXiv2006

Microscopic laser-driven high-energy colliders

The concept of a laser-guided $e^+e^-$ collider in the high-energy regime is presented and its feasibility discussed. Ultra-intense laser pulses and strong static magnetic fields are employed to unite in one stage the electron and positron acceleration and their head-on-head collision. We show that the resulting coherent collisions in the GeV regime yield an enormous enhancement of the luminosity with regard to conventional incoherent colliders.

physics.atom-ph↗

arXiv2005

Ionization suppression of excited atomic states beyond the stabilization regime

A two-dimensional model atom is employed to study the ionization behavior of initially excited atomic states in highly-frequent intense laser pulses beyond the dipole approximation. An additional regime of ionization suppression is found at laser intensities where the stabilization effect is expected to break down. The appearance of this effect is due to a strong coupling of the initial wave function to the ground state of the cycle-averaged space-translated ionic potential, followed by a subsequent population transfer to the ground state during the laser pulse turn-off. Non-dipole effects are found to increase the overall ionization probabilities, but not to suppress or alter this effect.

physics.atom-ph↗

arXiv2005

Radiative Corrections to Multi-Level Mollow-Type Spectra

This paper is concerned with two rather basic phenomena: the incoherent fluorescence spectrum of an atom driven by an intense laser field and the coupling of the atom to the (empty) modes of the radiation field. The sum of the many-photon processes gives rise to the inelastic part of the atomic fluorescence, which, for a two-level system, has a well-known characteristic three-peak structure known as the Mollow spectrum. From a theoretical point of view, the Mollow spectrum finds a natural interpretation in terms of transitions among laser-dressed states which are the energy eigenstates of a second-quantized two-level system strongly coupled to a driving laser field. As recently shown, the quasi-energies of the laser-dressed states receive radiative corrections which are nontrivially different from the results which one would expect from an investigation of the coupling of the bare states to the vacuum modes. In this article, we briefly review the basic elements required for the analysis of the dynamic radiative corrections, and we generalize the treatment of the radiative corrections to the incoherent part of the steady-state fluorescence to a three-level system consisting of 1S, 3P and 2S states.

quant-ph↗

arXiv2004

Relativistic and Radiative Corrections to the Mollow Spectrum

The incoherent, inelastic part of the resonance fluorescence spectrum of a laser-driven atom is known as the Mollow spectrum [B. R. Mollow, Phys. Rev. 188, 1969 (1969)]. Starting from this level of description, we discuss theoretical foundations of high-precision spectroscopy using the resonance fluorescence light of strongly laser-driven atoms. Specifically, we evaluate the leading relativistic and radiative corrections to the Mollow spectrum, up to the relative orders of (Z alpha)^2 and alpha(Z alpha)^2, respectively, and Bloch-Siegert shifts as well as stimulated radiative corrections involving off-resonant virtual states. Complete results are provided for the hydrogen 1S-2P_{1/2} and 1S-2P_{3/2} transitions; these include all relevant correction terms up to the specified order of approximation and could directly be compared to experimental data. As an application, the outcome of such experiments would allow for a sensitive test of the validity of the dressed-state basis as the natural description of the combined atom-laser system.

quant-ph↗

arXiv2004

Radiation spectra of laser-driven quantum relativistic electrons

A procedure to calculate the radiation spectrum emitted by an arbitrarily prepared Dirac wave packet is developed. It is based on the Dirac charge current and classical electrodynamic theory. Apart from giving absolute intensity values, it is exact in terms of relativistic retardation effects and angular dependence. We employ a laser driven free electron to demonstrate the advantages of our method as compared to traditional ones that merely rely on the Fourier transform of the dipole operator's expectation value. Classical reference calculations confirm the results obtained for the low-frequency part of the spectrum, especially in terms of the observed red-shifts, which clearly deviate from non-relativistic calculations. In the high-frequency part of the spectrum, we note appreciable deviations to the purely classical calculations which may be linked to quantum averaging effects.

physics.atom-ph↗

arXiv2004

Double-EIT ground-state laser cooling without blue-sideband heating

We discuss a laser cooling scheme for trapped atoms or ions which is based on double electromagnetically induced transparency (EIT) and makes use of a four-level atom in tripod configuration. The additional fourth atomic state is coupled by a strong coupling laser field to the usual three-level setup of single-EIT cooling. This effectively allows to create two EIT structures in the absorption spectrum of the system to be cooled, which may be controlled by the coupling laser field parameters to cancel both the carrier- and the blue-sideband excitations. In leading order of the Lamb-Dicke expansion, this suppresses all heating processes. As a consequence, the double-EIT scheme can be used to lower the cooling limit by almost two powers of the Lamb-Dicke parameter as compared to single-EIT cooling.

quant-ph↗

arXiv2003

Two-Loop QED Bound-State Calculations and Squared Decay Rates

We discuss the epsilon-method as used in various recent QED bound-state calculations by considering mathematical model examples. Recently obtained results for higher-order self-energy binding corrections at the two-loop level are reviewed. Problems associated with the interpretation of squared decay rates as radiative bound-state energy level shifts are discussed. We briefly expand on the relation of squared decay rates to nonresonant and radiative corrections to the Lorentzian line shape, including their dependence on the experimental process under study.

hep-ph↗

arXiv2003

Quantum signatures in laser-driven relativistic multiple-scattering

The dynamics of an electronic Dirac wave packet evolving under the influence of an ultra-intense laser pulse and an ensemble of highly charged ions is investigated numerically. Special emphasis is placed on the evolution of quantum signatures from single to multiple scattering events. We quantify the occurrence of quantum relativistic interference fringes in various situations and stress their significance in multiple-particle systems, even in the relativistic range of laser-matter interaction.

physics.comp-ph↗