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Optical limiting for microsecond pulses
KTH, School of Biotechnology (BIO), Theoretical Chemistry (closed 20110512). (Theoretical Chemistry)
KTH, School of Biotechnology (BIO), Theoretical Chemistry (closed 20110512). (Theoretical Chemistry)
KTH, School of Biotechnology (BIO), Theoretical Chemistry (closed 20110512).ORCID iD: 0000-0002-1763-9383
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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.

Place, publisher, year, edition, pages
2009. Vol. 130, no 5, 054114
Keyword [en]
atomic clusters, fullerenes, ground states, laser beam effects, molecule-photon collisions, optical limiters, triplet state, two-photon processes
National Category
Industrial Biotechnology
Identifiers
URN: urn:nbn:se:kth:diva-11434DOI: 10.1063/1.3072560ISI: 000263167100017Scopus ID: 2-s2.0-59949102448OAI: oai:DiVA.org:kth-11434DiVA: diva2:276056
Note

QC 20100713

Available from: 2009-11-10 Created: 2009-11-10 Last updated: 2016-07-21Bibliographically approved
In thesis
1. Molecular electronic,  vibrational and rotational motion in optical and x-ray fields
Open this publication in new window or tab >>Molecular electronic,  vibrational and rotational motion in optical and x-ray fields
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The subject of this theoretical  study is the role ofelectronic structure as well as of rotational and vibrational motionson interactions between molecules and electromagnetic radiation,ranging from optical to x-ray. The thesis concerns both linear and nonlinear regimes of the light-matter interaction. The first part of the thesis is devoted to propagation of opticalpulses with different time-structure through various nonlinear absorbers.First we explain the double-exponential decay of fluorescence caused by photobleaching of pyrylium  salt irradiated by a train of short (100 fs) optical pulses. The main reason for this effect is the transversal inhomogeneity of the light beam which makes the dynamics of the photobleaching differ in the core of the pulse and on its periphery. We also explore the optical power limitingof C60 fullerene irradiated by either microsecond optical pulses or a picosecond pulse trains. Enhancement of nonlinear absorption is caused by strong triplet-triplet absorption that becomes important due toelongation of the interaction time.Here we show the importance of the repetitionrate for the optical power limiting performance.The second part of the thesis addresses the interaction of optical and x-rayfields with rotational degrees of freedom of molecules. In this part the main attention is paid to the rotational heating caused by the recoil, experienced by molecules due to the ejection of photoelectrons. We have quantitatively explained two qualitatively different experiments with the N2 molecule.We predict the interference modulation of the recoil-induced shift,which is a shift of the photoelectron line caused by the rotational recoil effect, as a function of the photon energy.The developed theory also explains the rotational heating ofmolecules observed in the optical fluorescence induced by x-ray radiation.Based on this explanation, we suggest a new scheme of the optical fluorescence induced by x-rays that allows to detect the recoil effect via the recoil-inducedsplitting of the optical resonance.The last part of the thesis focuses on multi-mode nuclear dynamics of the resonant Auger scattering from the C2H2 molecule, that was the subject of a recent experimental study.Here we develop a theory that explains the observed vibrationalscattering anisotropy. We have found that three qualitatively different mechanisms are responsible for this phenomenon. The first mechanism is the interference of the direct and resonance scattering channels. The second mechanismis the interference of the resonant scattering channels through core excitedstate with the orthogonal orientation of the vibrational modes of core excitedstate. The Young's double slit like interference of the quantum pathways through the double-well potential of the bending motion of core excited state is the third mechanism of the vibrational scattering anisotropy.

Place, publisher, year, edition, pages
Stockholm: KTH, 2009. viii, 67 p.
Series
Trita-BIO-Report, ISSN 1654-2312 ; 2009:24
Keyword
x-ray, nonlinear optics, photobleaching, recoil, anisotropy, Auger
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-11386 (URN)978-91-7415-483-2 (ISBN)
Public defence
2009-12-01, FB 53, AlbaNova, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20100713Available from: 2009-11-10 Created: 2009-11-03 Last updated: 2011-11-23Bibliographically approved
2. Dynamics of multiphoton processes in nonlinear optics and x-ray spectroscopy
Open this publication in new window or tab >>Dynamics of multiphoton processes in nonlinear optics and x-ray spectroscopy
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
nonlinear optics, x-ray spectroscopy, multiphoton processes, pulse propagation
National Category
Atom and Molecular Physics and Optics Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-11394 (URN)978-91-7415-480-1 (ISBN)
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

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Ågren, Hans

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