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Dynamics of multiphoton processes in nonlinear optics and x-ray spectroscopy
KTH, School of Biotechnology (BIO), Theoretical Chemistry. (Theoretical Chemistry)
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

New generations of ultrashort and intense laser pulses as well ashigh power synchrotron radiation sources and x-ray free electronlasers have promoted fast developments in nonlinear optics andx-ray spectroscopy.The new experimental achievements and the appearance of varieties of novelnonlinear phenomena call for further development of theories. The objective of this thesis is to develop and apply thetheories to explain existing experimental data and to suggest new experiments.

The first part of the thesis is devoted to nonlinear propagation of optical pulses. It is shown that the vibrational levels can be selectively populated by varying the duration, shape and intensity of the pump pulse. We obtained a strict analytical solution for the resonant two-photon interaction in a multilevel system beyond rotating wave approximation. Simulations show that the polarization anisotropy of the two-photon excitation affects strongly the anisotropy of photobleaching.The two-photon area theorem is reformulated with taking into account the dynamical Stark shift and the contribution from the permanent dipole moments. In general the dynamical Stark shift does not allow complete population of the excited state, but it can be compensated by detunings in atoms. A dynamical theory of the sequential two-photon absorption of  microsecond pulses  is developed to explore the role of transverse inhomogeneity of the light beam on optical limiting properties.  The propagation of ultrashort laser pulses in nondipolar and dipolar media is investigated with special attention to the generation of superfluorescence and supercontinuum and the formation of attosecond pulses.

The second part of the thesis addresses the interaction of molecules with x-ray radiation.  We explore here the role of nuclear dynamics in resonant Auger scattering. Multimode simulations of the Auger spectra of ethylene molecule explain the main spectral features of the experimental spectra and show that the spectral profiles are formed mainly due to six vibrational modes. We predict the Doppler splitting of the atomic peak in resonant Auger scattering from SF6 molecule for circularly polarized x-rays. This effect is confirmed by the recent experiment. A new scheme of x-ray pump-probe spectroscopy, namely, resonant inelastic x-ray scattering accompanied by core-hole hopping induced by strong laser fields is suggested. The laser-induced promotion of core holes opens the symmetry forbidden scattering channels and gives rise to new spectral lines in the x-ray scattering spectrum. The strength of the symmetry forbidden lines becomes strong when  the time of Rabi flopping is shorter than the lifetime of the core-excited state. We study the role of propagation of femtosecond x-ray free-electron pulses on the Auger process. Simulations show  that there exists a strong competition between Auger decay and stimulated emission. The Auger yield and Auger branching ratio are strongly suppressed in the course of pulse propagation.

Place, publisher, year, edition, pages
Stockholm: KTH , 2009. , x, 76 p.
Series
Trita-BIO-Report, ISSN 1654-2312 ; 2009:25
Keyword [en]
nonlinear optics, x-ray spectroscopy, multiphoton processes, pulse propagation
National Category
Atom and Molecular Physics and Optics Physical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-11394ISBN: 978-91-7415-480-1 (print)OAI: oai:DiVA.org:kth-11394DiVA: diva2:275330
Public defence
2009-12-09, FA32, Roslagstullsbacken 21, Albanova University Center, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
QC 20100729Available from: 2009-11-11 Created: 2009-11-04 Last updated: 2010-07-29Bibliographically approved
List of papers
1. Coherent control of population and pulse propagation beyond the rotating wave approximation
Open this publication in new window or tab >>Coherent control of population and pulse propagation beyond the rotating wave approximation
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2008 (English)In: Journal of Physics B: Atomic, Molecular and Optical Physics, ISSN 0953-4075, E-ISSN 1361-6455, Vol. 41, no 7, 074016Article in journal (Refereed) Published
Abstract [en]

The dynamics of populations of the vibrational states in a NO molecule ( one-photon absorption) and of the electronic states in a 4,4'-bis( dimethylamino) stilbene molecule ( two-photon absorption) is studied versus the frequency, intensity and shape of the laser pulse. We show that specially designed infrared laser pulses can build selective populations of certain vibrational states. A detuning of light frequency from the vibrational resonance qualitatively changes the dynamics of populations. It is found out that the populations of the nonresonant levels follow the pulse shape adiabatically if the detuning significantly exceeds the inverse characteristic time of the change of the pulse. Depopulation of the nonresonant vibrational states leads to a high population of the resonant state at the end of the pulse. Complete breakdown of the standard rotating wave approximation for a two-photon absorption process is observed even for a rather small intensity of the laser pulse. An analytical solution for the interaction of a pulse with a three-level system beyond the rotating wave approximation is obtained, and it is in close agreement with the strict numerical solution of the amplitude equations. Special attention is paid to the population dynamics of randomly oriented molecules. The orientational disorder as well as the vibrations limit the coherent population transfer. Calculations show the strong role of the anisotropy of photoexcitation in the coherent control of populations which can affect the anisotropy of photobleaching.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2008
Keyword
self-induced transparency, 2-photon absorption, laser-pulses, basis-sets, density, exchange, behavior, bond
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-11454 (URN)10.1088/0953-4075/41/7/074016 (DOI)000254778700017 ()2-s2.0-42549157700 (Scopus ID)
Note

QC 20100728

Available from: 2009-11-11 Created: 2009-11-11 Last updated: 2017-12-12Bibliographically approved
2. Dynamics of multilevel molecules and pulse propagation beyond rotating wave approximation near two-photon resonance
Open this publication in new window or tab >>Dynamics of multilevel molecules and pulse propagation beyond rotating wave approximation near two-photon resonance
2007 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 76, no 4, 043422Article in journal (Refereed) Published
Abstract [en]

Dynamics of multilevel molecules and pulse propagation is studied near the two-photon resonance. We have found a strict solution of this problem beyond the rotating wave approximation. Our analytical solution is in close agreement with the strict numerical solution for the 4,4(')-bis(dimethylamino) stilbene molecule. The compensation of the dynamical Stark shift is studied for fixed-in-space molecules. It is shown that the orientational disorder does not allow complete compensation of the dynamical Stark shift.

Keyword
Self-Induced Transparency, Cross-Sections, Laser-Pulses, Density, Absorption, Superfluorescence, Spectroscopy, Excitation, Exchange
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-11455 (URN)10.1103/PhysRevA.76.043422 (DOI)000250619700139 ()2-s2.0-35548998315 (Scopus ID)
Note

QC 20100728

Available from: 2009-11-11 Created: 2009-11-11 Last updated: 2017-12-12Bibliographically approved
3. Optical limiting of short laser pulses
Open this publication in new window or tab >>Optical limiting of short laser pulses
2007 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 76, no 053804Article in journal (Refereed) Published
Abstract [en]

The dynamics of pulse propagation accompanied by harmonic generation, stimulated Raman scattering, amplified spontaneous emission, and superfluorescence is studied near the two-photon resonance. We explore the optical limiting of intense and short laser pulses. The numerical solutions of the coupled Bloch and Maxwell's equations for the 4,4(')-bis(dimethylamino) stilbene molecule are compared with the two-photon area theorem. It is shown that the area theorem explains qualitatively the major dynamical properties of pulse propagation even if the propagation is accompanied by the generation of new fields. In agreement with the area theorem, we see that the conventional dependence of the transmittance on the propagation depth is not valid for intense pulses.

Keyword
2-photon absorption; density; exchange
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-11456 (URN)10.1103/PhysRevA.76.053804 (DOI)000251326400143 ()2-s2.0-36048980222 (Scopus ID)
Note
QC 20100728Available from: 2009-11-11 Created: 2009-11-11 Last updated: 2017-12-12Bibliographically approved
4. Optical limiting for microsecond pulses
Open this publication in new window or tab >>Optical limiting for microsecond pulses
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2009 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 130, no 5, 054114Article in journal (Refereed) Published
Abstract [en]

We present a dynamical theory of nonlinear absorption and propagation of laser pulses with duration in the microsecond time domain. The general theory is applied to fullerene C-60 because of its good optical limiting properties, namely, a rather low ground state absorption and a strong triplet-triplet absorption. It is shown that sequential absorption involving strong triplet-triplet transitions is the major mechanism of nonlinear absorption. The intrinsic hierarchy of time scales makes an adiabatic solution of the coupled rate equations valid, which therefore can be reduced to a single dynamical equation for the ground state population. The slow evolution of this population is defined by an effective rate of population transfer to the triplet state and by the pulse duration. The propagation effect plays an important role in the optical power limiting performance. The intensity of the field as well as the population of the triplet state decreases during the pulse propagation, and a weakened nonlinear sequential two-photon absorption is followed by a linear one-photon absorption which gradually becomes the dominating process. The competition between these qualitatively different processes depends on the field intensity, the length of the absorber, and the concentration. The pulse propagation is studied by solving numerically the two-dimensional paraxial field equation together with the effective rate equation for the ground state population.

Keyword
atomic clusters, fullerenes, ground states, laser beam effects, molecule-photon collisions, optical limiters, triplet state, two-photon processes
National Category
Industrial Biotechnology
Identifiers
urn:nbn:se:kth:diva-11434 (URN)10.1063/1.3072560 (DOI)000263167100017 ()2-s2.0-59949102448 (Scopus ID)
Note

QC 20100713

Available from: 2009-11-10 Created: 2009-11-10 Last updated: 2017-12-12Bibliographically approved
5. Dynamics of cooperative emissions in a cascade three-level molecular system driven by an ultrashort laser pulse
Open this publication in new window or tab >>Dynamics of cooperative emissions in a cascade three-level molecular system driven by an ultrashort laser pulse
2008 (English)In: Chinese Physics (Beijing), ISSN 1009-1963, E-ISSN 1741-4199, Vol. 17, no 11, 4211-4217 p.Article in journal (Refereed) Published
Abstract [en]

This paper investigates the dynamics of cooperative emissions in a cascade three-level system driven by an ultras short laser pulse by solving numerically the full-wave Maxwell-Bloch equations. The 4, 40-bis(dimethylamino) stilbene molecule is used as the model molecule because of its strong two-photon absorption property. The two-colour cooperative emissions are studied as functions of molecular number density and dephasing rate of the dipole coherence. The propagation effects on the evolution of the cooperative radiations are also taken into account. The cooperative radiations are enhanced for large number density of the molecule, while the fast dephasing of the dipole coherence reduces the intensity of the cooperative radiations and delays the emission times or even inhibits the formation of the emissions. The delay time of the radiation decreases with the increase of the molecular number density and the propagation distance.

Keyword
superfluorescence; ultra-short laser pulse; 4, 4 '-bis(dimethylamino) stilbene organic molecule; Maxwell-Bloch equations
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-11457 (URN)10.1088/1674-1056/17/11/043 (DOI)000261206700043 ()2-s2.0-62649152347 (Scopus ID)
Note
QC 20100729Available from: 2009-11-11 Created: 2009-11-11 Last updated: 2017-12-12Bibliographically approved
6. Modulation of supercontinuum generation and formation of an attosecond pulse from a generalized two-level medium
Open this publication in new window or tab >>Modulation of supercontinuum generation and formation of an attosecond pulse from a generalized two-level medium
2007 (English)In: Journal of Physics B: Atomic, Molecular and Optical Physics, ISSN 0953-4075, E-ISSN 1361-6455, Vol. 40, 1523-1534 p.Article in journal (Refereed) Published
Abstract [en]

The propagation of ultrashort pulses in a generalized two- level system, consisting of permanent dipole moments, is simulated by solving the full Maxwell - Bloch equations. Special attention has been paid to the supercontinuum generation of spectra and the formation of attosecond ( as) pulses. It is found that the supercontinuum generation is strongly modulated by both area and width of the pulse, resulting from the interference between the splitting pulses in the time domain and the implication of the time - energy uncertainty relation. The effect of the permanent dipole moment on the supercontinuum generation is discussed in detail. Calculations show that a well- shaped 132 as pulse can be generated from a 2 fs incoming pulse under the condition where the permanent dipole moment difference between two levels is equal to the transition dipole moment between them. Influences of carrier- envelope phase and time- dependent ionization on the spectral and temporal evolution of the ultrashort pulses are also discussed at length.

Keyword
femtosecond optical pulses, laser-pulses, ionization
National Category
Industrial Biotechnology
Identifiers
urn:nbn:se:kth:diva-11458 (URN)10.1088/0953-4075/40/8/006 (DOI)000246010300007 ()2-s2.0-34247138000 (Scopus ID)
Note
QC 20100630Available from: 2009-11-11 Created: 2009-11-11 Last updated: 2017-12-12Bibliographically approved
7. Multimode resonant Auger scattering from the ethylene molecule
Open this publication in new window or tab >>Multimode resonant Auger scattering from the ethylene molecule
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(English)Manuscript (preprint) (Other academic)
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-14256 (URN)
Note
QC 20100728Available from: 2010-07-28 Created: 2010-07-28 Last updated: 2010-07-29Bibliographically approved
8. Symmetry-forbidden x-ray Raman scattering induced by a strong infrared-laser field
Open this publication in new window or tab >>Symmetry-forbidden x-ray Raman scattering induced by a strong infrared-laser field
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2008 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 77, no 4, 043405- p.Article in journal (Refereed) Published
Abstract [en]

Resonant inelastic x-ray scattering accompanied by core-hole hopping induced by a strong infrared-laser field is studied for the nitrogen molecule. This process involves a strong laser-field-induced promotion of ungerade core holes created by a weak x-ray pulse to a gerade core level, which opens symmetry-forbidden scattering channels and gives rise to new features in the x-ray scattering spectrum. The core-hole hopping within the short lifetime of the core-excited state required for observation of the described process can be achieved at moderate intensities of the infrared field (similar to 10(12) W/cm(2)) because of the large transition dipole moment between the relevant core levels. The dynamics of resonant inelastic x-ray scattering assisted by change of core-hole parity is studied in detail versus the intensity, detuning, phase, and duration of the incident infrared-laser and x-ray pulses.

Keyword
Dipole moment, Nitrogen, Phase transitions, Raman scattering, X ray scattering
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-11459 (URN)10.1103/PhysRevA.77.043405 (DOI)000255457100107 ()2-s2.0-41749124114 (Scopus ID)
Note
QC 20101001Available from: 2009-11-11 Created: 2009-11-11 Last updated: 2017-12-12Bibliographically approved
9. Ultrafast dissociation of core-excited SF6 probed by Auger-Doppler e_ect in the _eld of circularly polarized X-rays
Open this publication in new window or tab >>Ultrafast dissociation of core-excited SF6 probed by Auger-Doppler e_ect in the _eld of circularly polarized X-rays
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(English)Manuscript (preprint) (Other academic)
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-14257 (URN)
Note
QC 20100728Available from: 2010-07-28 Created: 2010-07-28 Last updated: 2010-07-29Bibliographically approved
10. Auger effect in the presence of strong x-ray pulses
Open this publication in new window or tab >>Auger effect in the presence of strong x-ray pulses
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2010 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, ISSN 1050-2947, Vol. 81, no 4, 043412- p.Article in journal (Refereed) Published
Abstract [en]

We study the role of propagation of strong x-ray free-electron laser pulses on the Auger effect. When the system is exposed to a strong x-ray pulse the stimulated emission starts to compete with the Auger decay. As an illustration we present numerical results for Ar gas with the frequency of the incident x-ray pulse tuned in the 2p(3/2)-4s resonance. It is shown that the pulse propagation is accompanied by two channels of amplified spontaneous emission, 4s-2p(3/2) and 3s-2p(3/2), which reshape the pulse when the system is inverted. The population inversion is quenched for longer propagation distances where lasing without inversion enhances the Stokes component. The results of simulations show that the propagation of the strong x-ray pulses affect intensively the Auger branching ratio.

Keyword
Amplified spontaneous emissions, Auger decays, Auger effects, Branching ratio, Lasing without inversion, Numerical results, Population inversions, Propagation distances, Pulse propagation, Stokes component, Two channel, X ray pulse, X-ray free electron lasers
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-14258 (URN)10.1103/PhysRevA.81.043412 (DOI)000277227300128 ()2-s2.0-77951125476 (Scopus ID)
Funder
Swedish Research CouncilCarl Tryggers foundation
Note

QC 20100728

Available from: 2010-07-28 Created: 2010-07-28 Last updated: 2017-12-12Bibliographically approved

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