| 1. |
- Varju, Katalin, et al.
(author)
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Experimental studies of attosecond pulse trains
- 2005
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In: Laser Physics. - 1054-660X. ; 15:6, s. 888-898
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Journal article (peer-reviewed)abstract
- We present experimental measurements of attosecond pulse trains. The characterization of the pulses uses a spectral interferometry technique that is implemented with a Mach-Zehnder interferometer. This allows us to manipulate independently the pump and probe pulses for a wide range of applications. By letting the attosecond pulses pass through metallic films, we can in particular compensate for the intrinsic chirp of the attosecond pulses corresponding to the plateau spectral region, thus getting pulses as short as 170 attoseconds-only 1.2 optical cycles at the central frequency. The measurement technique is also applicable for determination of the group delay of thin XUV-transparent films and relative delay in the photoionization process. Our experimental method is applied to attosecond pulse trains created by 35- and 9-fs laser pulses, and the shortest train observed consists of three or four pulses.
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| 2. |
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| 3. |
- Sola, I. J., et al.
(author)
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Temporal and spectral studies of high-order harmonics generated by polarization-modulated infrared fields
- 2006
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In: Physical Review A (Atomic, Molecular and Optical Physics). - 1050-2947. ; 74:1
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Journal article (peer-reviewed)abstract
- The temporal confinement of high harmonic generation (HHG) via modulation of the polarization of the fundamental pulse is studied in both temporal and spectral domains. In the temporal domain, a collinear cross-correlation setup using a 40 fs IR pump for the HHG and a 9 fs IR pulse to probe the generated emission is used to measure the XUV pulse duration. The observed temporal confinement is found to be consistent with theoretical predictions. An increased confinement is observed when a 9 fs pulse is used to generate the harmonics. An important spectral broadening, including a continuum background, is also measured. Theoretical calculations show that with 10 fs driving pulses, either one or two main attosecond pulses are created depending on the value of the carrier envelope phase.
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| 4. |
- Strelkov, V, et al.
(author)
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Attosecond pulses generation with an ellipticity-modulated laser pulse
- 2005
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In: Laser Physics. - 1054-660X. ; 15:6, s. 871-879
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Journal article (peer-reviewed)abstract
- Attosecond pulse emission using a laser field with time-dependent ellipticity is studied experimentally and theoretically. Our theoretical approach is validated by comparison with experimental VUV spectra. We show in calculations that the VUV emission can be confined into a narrow temporal window in which the fundamental polarization is quasi-linear. We find an analytical equation describing the duration of this window and compare it with numerical results. The requirements for the fundamental field that are necessary to confine this emission to a single attosecond pulse generation are formulated.
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| 5. |
- Strelkov, V, et al.
(author)
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Generation of attosecond pulses with ellipticity-modulated fundamental
- 2004
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In: Applied Physics B. - : Springer Science and Business Media LLC. - 0946-2171. ; 78:7-8, s. 879-884
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Journal article (peer-reviewed)abstract
- We have studied experimentally and theoretically high-order harmonic generation using a laser field with a time-dependent ellipticity. We show that the harmonic emission can be confined into a narrow temporal window, in which the fundamental polarization is quasi-linear. This allows a single attosecond pulse (200 as) with a fundamental field obtained from 10 fs pulse to be generated.
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| 6. |
- Strelkov, V, et al.
(author)
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Single attosecond pulse production with an ellipticity-modulated driving IR pulse
- 2005
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In: Journal of Physics B: Atomic, Molecular and Optical Physics. - 0953-4075. ; 38:10, s. 161-167
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Journal article (peer-reviewed)abstract
- We theoretically study attosecond pulse production via high-harmonic generation using a driving laser pulse with a time-dependent ellipticity. The theoretical approach produces results that agree with our experimental data obtained using 35 fs driving laser pulses and is further used to study the generation of single attosecond pulses with shorter laser pulses. We find an equation for the duration of the temporal window created by the time-varying driving laser polarization in which high-harmonic emission can occur. We formulate the necessary requirements concerning the driving laser field in order to confine the high-harmonic emission in the form of a single attosecond pulse. Indeed, we show that using incident 12 fs laser pulses single attosecond pulses can be produced for certain carrier-envelope phase (CEP) values of the driving pulse. For 6 fs incident laser pulses, single attosecond pulses are produced for all values of the CEP (the intensity of the attosecond pulse still depends on the actual value of the CEP). If implemented with state-of-the-art 5 A laser pulses, this technique can even lead to the production of sub-100 as pulses.
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| 7. |
- Zaïr, A., et al.
(author)
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Confinement of attosecond train pulses by using a modulated polarization IR pulse
- 2005
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In: 2005 Conference on Lasers and Electro-Optics Europe. - 0780389743 - 9780780389748
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Conference paper (peer-reviewed)abstract
- We study the temporal and spectral behaviour of high order harmonics generated by pulses with temporally modulated polarization. We observe a harmonic temporal confinement and a harmonic spectral broadening, compatible with 1 -or-2 attosecond pulse emission.
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| 8. |
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| 9. |
- Balcou, P, et al.
(author)
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High-order-harmonic generation: towards laser-induced phase-matching control and relativistic effects
- 2002
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In: Applied Physics B. - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 74:6, s. 509-515
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Journal article (peer-reviewed)abstract
- We present a review of some recent results on high-order-harmonic generation, aiming at optimizing the photon flux to allow for future applications in extreme-ultra-violet non-linear optics. We first present new schemes to control phase matching of high harmonics in gases, by using the effect of the spatially varying atomic phase displayed by the high harmonics. An enhancement by a factor of 50 is observed in neon in conditions for which the gradient of the atomic dispersion balances the electronic dispersion. A new scheme to manipulate the laser field was demonstrated, and shown to improve phase matching. We then turn to high-harmonic generation by solid targets, and show that high harmonics generated by an intense 30-fs laser pulse remain collimated even at the threshold of the relativistic regime.
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| 10. |
- Erman, P, et al.
(author)
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Autoionization Widths of the No Rydberg-valence State Complex In the 11-12 Ev Region
- 1995
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In: Chemical Physics Letters. - : Elsevier BV. - 0009-2614. ; 239:1-3, s. 6-10
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Journal article (peer-reviewed)abstract
- The photoion excitation spectra of (NO)-N-14 and (NO)-N-15 have been studied in the 105-112 Mn excitation region using high-order harmonic laser spectroscopy. Utilizing the small bandwidth offered by this technique, the natural widths of prominent lines in this region have been measured. These lines originate from interactions between high-lying Rydberg levels converging to the NO+ electronic ground state and a 'new' NO valence state situated close to the latter state. The present measurements show broad resonances corresponding to a decay time of 20-50 fs. This supports the interpretation that the decay takes place via rapid electronic autoionization.
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| 11. |
- Lopez, Rodrigo, et al.
(author)
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Time-resolved ellipticity gating of high-order harmonic emission
- 2004
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In: Physical Review A. ; 69, s. 1-053811
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Journal article (peer-reviewed)abstract
- We present time-resolved cross-correlation measurements of extreme ultraviolet (xuv) pulses generated as high-order harmonics of intense 35 fs pulses, using a short (12 fs) probe pulse. We modulate the ellipticity of the laser driving the generation process such that the polarization is linear for short times around the temporal peak of the pulse. Since harmonic generation is strongly suppressed for very small amounts of driving laser ellipticity, the emission of xuv radiation can therefore be confined to times much shorter than the laser pulse duration. In addition, our setup allows us to continuously confine the xuv emission as well as to determine its frequency sweep during the pulse.
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| 12. |
- Roos, L, et al.
(author)
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Controlling phase matching of high-order harmonic generation by manipulating the fundamental field
- 1999
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In: Physical Review A (Atomic, Molecular and Optical Physics). - 1050-2947. ; 60:6, s. 5010-5018
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Journal article (peer-reviewed)abstract
- We study experimentally how to control and improve phase matching of high-order harmonic generation. We use a birefringent lens and a birefringent compensator to obtain a fundamental laser pulse (150 fs, 800 nm, similar to 4 mJ) With two foci Separated by 6.2 mm along the propagation axis and with a controllable phase delay between the polarizations along the optic axes of the birefringent optical components. This enables us to enhance the high-order harmonic conversion efficiency for the high-order harmonics in neon to 3 x 10(-8), a factor of 4 higher compared to a single-focus setup in similar conditions. The enhancement is achieved by improving the phase matching and at the same time maintaining a high intensity in a large generating volume. [S1050-2947(99)04512-6].
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| 13. |
- Tcherbakoff, O., et al.
(author)
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Controlling the duration of XUV high order harmonic pulses
- 2003
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In: 2003 Conference on Lasers and Electro-Optics Europe, CLEO/EUROPE 2003. - 0780377346 - 9780780377349
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Conference paper (peer-reviewed)abstract
- We modulate temporally the polarization of a ∼ 30 fs, 800 nm IR pulse and use it to generate high order harmonics. The harmonic emission can clearly be confined and the XUV pulse duration can be continuously tuned from - 5 - 7 fs to 50 fs.
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| 14. |
- Veyrinas, K., et al.
(author)
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Chromatic aberrations correction of attosecond high-order harmonic beams by flat-top spatial shaping of the fundamental beam
- 2023
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In: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 25:2
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Journal article (peer-reviewed)abstract
- Attosecond pulses created by high-order harmonic generation in gases often exhibit strong chromatic aberrations, arising from the broad bandwidth and wavelength-dependent nonlinear light-matter interaction. When the driving laser intensity varies spatially, as for Gaussian driving beams, the apparent source position of the harmonics differs significantly from one order to the next, thus affecting the achievable intensity and duration of the attosecond pulses when they are focused on a target. We show that these chromatic aberrations can be reduced by spatially shaping the fundamental beam to generate high-order harmonics with a driver having a flat-top profile inside the gas medium. By measuring both the intensity profile and wavefront for each harmonic in a plane, we access the extreme ultra-violet (XUV) beam properties and investigate these properties near focus. We observe that controlling chromatic aberrations by flat-top spatial shaping strongly reduces the variation of the XUV spectrum on the beam axis during propagation and, in return, the longitudinal sensitivity of both the temporal profiles and the temporal shifts of the focused attosecond pulses.
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| 15. |
- Veyrinas, K., et al.
(author)
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High order harmonic generation with spatially shaped flat top driver to control XUV chromatic aberrations
- 2023
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In: 2023 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2023. - 9798350345995
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Conference paper (peer-reviewed)abstract
- The XUV beams generated via high order harmonic generation (HHG) in gases have spatial properties evolving with the harmonic order. It leads to chromatic aberrations when the harmonics are focussed so that, locally, the spectral content can change significantly during propagation [1-3] especially near focus.
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| 16. |
- Zair, A, et al.
(author)
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Time-resolved measurements of high order harmonics confined by polarization gating
- 2004
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In: Applied Physics B. - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 78:7-8, s. 869-872
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Journal article (peer-reviewed)abstract
- We investigate the temporal confinement of high order harmonic pulses generated by a femtosecond (fs) infrared (IR) pulse with a time varying polarization. We use a set of two birefringent quartz plates to modulate the IR polarization. It produces a short temporal gate of linear polarization where harmonics are efficiently generated during a small fraction of the IR pulse. By rotating one of the plates, the gate width can be continuously varied between 70 fs down to 7 fs. The XUV pulse duration is measured by cross-correlation with a probe IR pulse of 12 fs. When the gate width is decreased, a clear temporal confinement of the XUV emission is observed through the cross correlation signal. This experiment is the first direct experimental evidence in the temporal domain that the polarization gating technique can be used to significantly shorten the harmonic pulse duration.
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| 17. |
- Altucci, C, et al.
(author)
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Influence of atomic density in high-order harmonic generation
- 1996
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In: Optical Society of America. Journal B: Optical Physics. - 0740-3224. ; 13:1, s. 148-156
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Journal article (peer-reviewed)abstract
- We have investigated how high-order harmonics generated in rare gases depend on the atomic density. The peak and the profile of the atomic density in the interaction region were measured as a function of the backing pressure and the distance from the nozzle by a differential interferometry technique, The conversion efficiency for the harmonics in the plateau was found to increase approximately quadratically over the entire range of peak pressures investigated (3-80 mbar). The intensity of the harmonics in the cutoff region, in contrast, increased only until an optimum peak pressure was reached, beyond which it decreased. This optimum peak pressure was found to be dependent on both the laser intensity and the process order. To understand this effect, we have performed extensive propagation calculations of both the fundamental and the harmonic fields, using ionization rates and dipole moments from a tunnel ionization model. We obtained good agreement with the experimental results. The observed effect is attributed to ionization-induced defocusing of the fundamental laser beam, which reduces the peak intensity obtained in the medium and shortens the extent of the plateau. (C) 1996 Optical Society of America
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| 18. |
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| 19. |
- Johansson, Allan, et al.
(author)
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Two-color pump-probe experiments in helium using high-order harmonics
- 2003
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In: European Physical Journal D. Atomic, Molecular, Optical and Plasma Physics. - : Springer Science and Business Media LLC. - 1434-6060 .- 1434-6079. ; 22:1, s. 3-11
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Journal article (peer-reviewed)abstract
- A pump-probe technique has been applied for measuring the lifetimes and absolute photoionization cross-sections of excited He states. The 1s2p P-1 and 1s3p P-1 states of He are excited by using the 13th and the 14th harmonic, respectively, of a tunable 70 ps dye laser generated in a Kr gas jet. The states are ionized after a varying time delay, by absorption of probe photons with energies between 1.6 and 4.5 eV. Lifetimes of tau(1s2p) = 0.57 ns and tau(1s3p) = 1.76 ns are determined with a precision of about 15%. A significant enhancement of the number of ions present in the lifetime curves at zero time delay for pressures above 6 x 10(-5) mbar is attributed to direct two-photon ionization of He in combination with AC Stark broadening of the excited state and absorption of the XUV light in the medium. Absolute photoionization cross-sections from the He 1s2p P-1 and He 1s3p P-1 states in the threshold region are determined by measuring the saturation of the ionization process with a precision of similar to 25%. In addition, the variation of the relative orientation between the polarization vectors of the pump and probe beams enables the determination of partial photoionization cross-sections.
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| 20. |
- Larsson, Jörgen, et al.
(author)
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Two-color Time-resolved Spectroscopy of Helium Using High-order Harmonics
- 1995
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In: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 28:3
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Journal article (peer-reviewed)abstract
- The radiative lifetime of the 1s2p(1)P state of helium is measured in a two-colour ionization experiment with a 5% accuracy. The state is excited by the 13th harmonic of a tunable 80 ps laser and ionized by a synchronous ultraviolet laser, with a variable time delay. This experiment demonstrates that the high harmonics generated in a jet of rare gas exposed to an intense laser field provide a tunable short-pulse xuv source ideally suited for pump/probe type of studies and, in particular, short lifetime measurements.
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| 21. |
- Larsson, J., et al.
(author)
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Two-colour time-resolved spectroscopy of helium using high-order harmonics
- 1995
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In: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 28:3, s. 53-58
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Journal article (peer-reviewed)abstract
- The radiative lifetime of the ls2p1P state of helium is measured in a two-colour ionization experiment with a 5% accuracy. The state is excited by the 13th harmonic of a tunable SO ps laser and ionized by a synchronous ultraviolet laser, with a variable time delay. This experiment demonstrates that the high harmonics generated in a jet of rare gas exposed to an intense laser field provide a tunable short-pulse xuv source ideally suited for pump/probe type of studies and, in particular, short lifetime measurements.
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| 22. |
- L'Huillier, Anne, et al.
(author)
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High-order Harmonics - A Coherent Source In the Xuv Range
- 1995
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In: Journal of Nonlinear Optical Physics & Materials. - 0218-1991. ; 4:3, s. 647-665
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Journal article (peer-reviewed)abstract
- We review the main results concerning high-order generation processes from the point of view of a potential user of this new source of XUV radiation. The perspectives for optimizing the source, both in efficiency and in spectral range, its characteristics and in particular, its coherence properties, are discussed. Finally, we describe two experiments, which demonstrate the usefulness of the harmonics as a short-pulse, coherent source in the XUV domain.
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| 23. |
- Mercer, I, et al.
(author)
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Spatial mode control of high-order harmonics
- 1996
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In: Physical Review Letters. - 1079-7114. ; 77:9, s. 1731-1734
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Journal article (peer-reviewed)abstract
- We demonstrate that the spatial mode of high-order harmonics can be continuously controlled. The control is achieved by spatially modulating the degree of elliptical polarization of the fundamental field using birefringent optics. A highly sensitive relationship between the efficiency of harmonic generation and the degree of laser elliptical polarization leads to atoms emitting harmonics mainly in regions of linear polarization. The harmonics are emitted as annular beams whose angles of divergence can be continuously varied.
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| 24. |
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| 25. |
- van't Westeinde, A, et al.
(author)
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Sex differences in brain structure: a twin study on restricted and repetitive behaviors in twin pairs with and without autism
- 2019
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In: Molecular autism. - : Springer Science and Business Media LLC. - 2040-2392. ; 11:1, s. 1-
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Journal article (peer-reviewed)abstract
- BackgroundFemales with autism spectrum disorder have been reported to exhibit fewer and less severe restricted and repetitive behaviors and interests compared to males. This difference might indicate sex-specific alterations of brain networks involved in autism symptom domains, especially within cortico-striatal and sensory integration networks. This study used a well-controlled twin design to examine sex differences in brain anatomy in relation to repetitive behaviors.MethodsIn 75 twin pairs (n= 150, 62 females, 88 males) enriched for autism spectrum disorder (n= 32), and other neurodevelopmental disorders (n= 32), we explored the association of restricted and repetitive behaviors and interests—operationalized by the Autism Diagnostic Interview-Revised (C domain) and the Social Responsiveness Scale-2 (Restricted Interests and Repetitive Behavior subscale)—with cortical volume, surface area and thickness of neocortical, sub-cortical, and cerebellar networks.ResultsCo-twin control analyses revealed within-pair associations between RRBI symptoms and increased thickness of the right intraparietal sulcus and reduced volume of the right orbital gyrus in females only, even though the mean number of RRBIs did not differ between the sexes. In a sub-sample of ASD-discordant pairs, increased thickness in association with RRBIs was found exclusively in females in the orbitofrontal regions, superior frontal gyrus, and intraparietal sulcus, while in males RRBIs tended to be associated with increased volume of the bilateral pallidum.LimitationsHowever, due to a small sample size and the small difference in RRBI symptoms within pairs, the results of this exploratory study need to be interpreted with caution.ConclusionsOur findings suggest that structural alterations of fronto-parietal networks in association with RRBIs are found mostly in females, while striatal networks are more affected in males. These results endorse the importance of investigating sex differences in the neurobiology of autism symptoms, and indicate different etiological pathways underlying restricted and repetitive behaviors and interests in females and males.
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