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Relativistic light-matter interaction
Stockholm University, Faculty of Science, Department of Physics.
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

During the past decades, the development of laser technology has produced pulses with increasingly higher peak intensities. These can now be made such that their strength rivals, and even exceeds, the atomic potential at the typical distance of an electron from the nucleus. To understand the induced dynamics, one can not rely on perturbative methods and must instead try to get as close to the full machinery of quantum mechanics as practically possible. With increasing field strength, many exotic interactions such as magnetic, relativistic and higher order electric effects may start to play a significant role. To keep a problem tractable, only those effects that play a non-negligible role should be accounted for. In order to do this, a clear notion of their relative importance as a function of the pulse properties is needed. 

In this thesis I study the interaction between atomic hydrogen and super-intense laser pulses, with the specific aim to contribute to the knowledge of the relative importance of different effects. I solve the time-dependent Schrödinger and Dirac equations, and compare the results to reveal relativistic effects. High order electromagnetic multipole effects are accounted for by including spatial variation in the laser pulse.

The interaction is first described using minimal coupling. The spatial part of the pulse is accounted for by a series expansion of the vector potential and convergence with respect to the number of expansion terms is carefully checked. A significantly higher demand on the spatial description is found in the relativistic case, and its origin is explained. As a response to this demanding convergence behavior, an alternative interaction form for the relativistic case has been developed and presented.

As a guide mark for relativistic effects, I use the classical concept of quiver velocity, vquiv, which is the peak velocity of a free electron in the polarization direction of a monochromatic electromagnetic plane wave that interacts with the electron. Relativistic effects are expected when vquiv reaches a substantial fraction of the speed of light c, and in this thesis I consider cases up to vquiv=0.19c. For the present cases, relativistic effects are found to emerge around vquiv=0.16c .

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2017. , 70 p.
Keyword [en]
Time-dependent Dirac equation, Beyond dipole effects, Relativistic effects, High-Intensity laser-matter interaction
National Category
Atom and Molecular Physics and Optics
Research subject
Theoretical Physics
Identifiers
URN: urn:nbn:se:su:diva-147749ISBN: 978-91-7797-008-8 (print)ISBN: 978-91-7797-009-5 (electronic)OAI: oai:DiVA.org:su-147749DiVA: diva2:1148577
Public defence
2017-11-24, sal FB42, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:15 (English)
Opponent
Supervisors
Available from: 2017-10-31 Created: 2017-10-11 Last updated: 2017-11-01Bibliographically approved
List of papers
1. Ionization dynamics beyond the dipole approximation induced by the pulse envelope
Open this publication in new window or tab >>Ionization dynamics beyond the dipole approximation induced by the pulse envelope
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2016 (English)In: Physical Review D, ISSN 2469-9926, Vol. 93, no 5, 053411Article in journal (Refereed) Published
Abstract [en]

When atoms and molecules are ionized by laser pulses of finite duration and increasingly high intensities, the validity of the much-used dipole approximation, in which the spatial dependence and magnetic component of the external field are neglected, eventually breaks down. We report that, when going beyond the dipole approximation for the description of atoms exposed to ultraviolet light, the spatial dependence of the pulse shape, the envelope, provides the dominant correction, while the spatial dependence of the carrier is negligible. We present a first-order beyond-dipole correction to the Hamiltonian which accounts exclusively for nondipole effects stemming from the carrier envelope of the pulse. We demonstrate by ab initio calculations for hydrogen that this approximation, which we refer to as the envelope approximation, reproduces the full interaction beyond the dipole approximation for absolute and differential observables and proves to be valid for a broad range of high-frequency fields. This is done both for the Schrodinger and the Dirac equation. Moreover, it is demonstrated that the envelope approximation provides an interaction-term which gives rise to faster numerical convergence in terms of partial waves compared to its exact counterpart.

National Category
Physical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-131525 (URN)10.1103/PhysRevA.93.053411 (DOI)000376239900011 ()
Available from: 2016-07-04 Created: 2016-06-21 Last updated: 2017-10-11Bibliographically approved
2. Relativistic ionization dynamics for a hydrogen atom exposed to superintense XUV laser pulses
Open this publication in new window or tab >>Relativistic ionization dynamics for a hydrogen atom exposed to superintense XUV laser pulses
2017 (English)In: Physical Review A: covering atomic, molecular, and optical physics and quantum information, ISSN 2469-9926, E-ISSN 2469-9934, Vol. 95, no 4, 043403Article in journal (Refereed) Published
Abstract [en]

We present a theoretical study of the ionization dynamics of a hydrogen atom exposed to attosecond laser pulses in the extreme ultraviolet region at very high intensities. The pulses are such that the electron is expected to reach relativistic velocities, thus necessitating a fully relativistic treatment. We solve the time-dependent Dirac equation and compare its predictions with those of the corresponding nonrelativistic Schrodinger equation. We find that as the electron is expected to reach about 20% of the speed of light, relativistic corrections introduce a finite yet small decrease in the probability of ionizing the atom.

National Category
Physical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-143604 (URN)10.1103/PhysRevA.95.043403 (DOI)000399378500004 ()
Available from: 2017-05-31 Created: 2017-05-31 Last updated: 2017-10-16Bibliographically approved
3. Alternative gauge for the description of the light-matter interaction in a relativistic framework
Open this publication in new window or tab >>Alternative gauge for the description of the light-matter interaction in a relativistic framework
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2017 (English)In: Physical Review A: covering atomic, molecular, and optical physics and quantum information, ISSN 2469-9926, E-ISSN 2469-9934, Vol. 96, no 2, 023426Article in journal (Refereed) Published
Abstract [en]

We present a generalized velocity gauge form of the relativistic laser-matter interaction. In comparison with the (equivalent) regular minimal coupling description, this form of light-matter interaction results in superior convergence properties for the numerical solution of the time-dependent Dirac equation. This applies both to the numerical treatment and, more importantly, to the multipole expansion of the laser field. The advantages of the alternative gauge is demonstrated in hydrogen by studies of the dynamics following the impact of superintense laser pulses of extreme ultraviolet wavelengths and subfemtosecond duration.

Keyword
Multiphoton or tunneling ionization & excitation, Strong electromagnetic field effects, Ultrashort pulses
National Category
Physical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-147051 (URN)10.1103/PhysRevA.96.023426 (DOI)000408565900008 ()
Available from: 2017-09-20 Created: 2017-09-20 Last updated: 2017-10-11Bibliographically approved

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