| 1. |
- L'Huillier, Anne, et al.
(författare)
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Applications of high-order harmonics
- 2003
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Ingår i: European Physical Journal D. Atomic, Molecular, Optical and Plasma Physics. - : Springer Science and Business Media LLC. - 1434-6060 .- 1434-6079. ; 26:1, s. 91-98
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Tidskriftsartikel (refereegranskat)abstract
- We review applications of high-order harmonic generation in different fields of physics, from spectroscopic studies of atoms and molecules, to interferometry and plasma diagnostics and nonlinear optics.
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| 2. |
- Lopez, Rodrigo, et al.
(författare)
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Time-frequency characterization of high-order harmonic pulses
- 2003
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Ingår i: European Physical Journal D. Atomic, Molecular, Optical and Plasma Physics. - : Springer Science and Business Media LLC. - 1434-6060 .- 1434-6079. ; 26:1, s. 105-109
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Tidskriftsartikel (refereegranskat)abstract
- We present energy-resolved cross-correlation measurements of extreme ultraviolet (XUV) pulses generated as high harmonics of femtosecond pulses from a 1 kHz titanium-sapphire laser. The harmonic pulses are probed by a fraction of the fundamental laser pulse at 800 nm, in a noncollinear geometry, allowing us to vary independently the parameters of the harmonic pump and near-infrared probe pulses. We measure the so-called "sidebands" in the photoelectron spectrum of argon corresponding to the absorption of a harmonic photon plus or minus one probe photon. Spectrally resolving the cross-correlation signal allows us to characterize the time-dependent frequency of the XUV pulse.
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| 3. |
- Norin, Johan, et al.
(författare)
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Time-frequency characterization of femtosecond extreme ultraviolet pulses.
- 2002
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Ingår i: Physical Review Letters. - 1079-7114. ; 88:19, s. 193901-193901
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Tidskriftsartikel (refereegranskat)abstract
- We present energy-resolved cross-correlation measurements of an extreme ultraviolet (XUV) pulse, generated as the fifth harmonic (15.5 eV) of an intense 80 fs laser pulse centered at 400 nm. Spectrally resolving the cross-correlation signal allows us to characterize the time-dependent frequency of the XUV pulse. We find that the fifth harmonic has a small negative chirp in excess of that predicted by perturbation theory. In addition, by manipulating the chirp of the driving laser we can induce and measure a positive or a negative chirp on the XUV pulse.
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| 4. |
- Zamith, S, et al.
(författare)
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The predissociation of highly excited states in acetylene by time-resolved photoelectron spectroscopy
- 2003
-
Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 119:7, s. 3763-3773
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Tidskriftsartikel (refereegranskat)abstract
- We study the dynamics of highly excited states in acetylene initiated by an ultrashort vacuum ultraviolet laser pulse. Electronic states lying in the 4s-3d Rydberg region are excited with one femtosecond pulse, and the dynamic development of the states is monitored by a second short pulse which ionizes the system. We show that even for femtosecond pulses where the bandwidth of the exciting pulse covers several electronic states, it is possible to extract short decay lifetimes through time-resolved photoelectron spectroscopy by using a frequency-modulated (chirped) excitation pulse. We report decay lifetimes for the F 4(0)(2) and E 4-5(0)(2) states in acetylene, and for the E 4(0)(2) and E 5(0)(2) states in d-acetylene. The time evolution measured in the electron spectra is compared to decay spectra measured using ion yield and the differences in these results are discussed. (C) 2003 American Institute of Physics.
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| 5. |
- Alveteg, Mattias, et al.
(författare)
-
Vad räknas som belägg för studenters måluppfyllelse?
- 2018
-
Ingår i: 10:e Pedagogiska Inspirationskonferensen 2018. - 2003-3761 .- 2003-377X. ; 10
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Konferensbidrag (refereegranskat)abstract
- Universitetskanslerämbetet (UKÄ) ställer numera krav på svenska lärosäten att vi ska utvärdera oss själva. Vi på LTH bör då gemensamt försöka hitta sätt att uppfylla UKÄs krav som även ger de verktyg vi själva behöver för att förbättra våra utbildningars kvalitét.Hur tar vi reda på våra utbildningars styrkor och svagheter och vad vi kan göra för att förbättra dem ytterligare? Ett sätt är att undersöka hur väl studenterna uppfyller examensmålen. Vad som ska räknas som belägg för studenters måluppfyllelse är dock svårt, av flera olika skäl. Vår rekommendation blir att triangulera olika typer av belägg samt att tydligt involvera institutionerna i arbetet.
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| 6. |
- Arnold, Cord L., et al.
(författare)
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A high-repetition rate attosecond pulse source for coincidence spectroscopy
- 2021
-
Ingår i: 2021 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2021. - 9781665418768
-
Konferensbidrag (refereegranskat)abstract
- The power of attosecond pump-probe spectroscopy combined with advanced detection schemes, such as photoelectron/ion coincidence spectrometers and time-resolved photoelectron emission microscopy (PEEM), can be unleashed by properly accounting for the repetition rate of the source. In this work, we present a high-repetition rate (200 kHz) attosecond pulse source that opens up for exploring phenomena, previously inaccessible to the community using attosecond interferometric and spectroscopy pump-probe techniques [1].
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| 7. |
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| 8. |
- Bengtsson, Samuel N., et al.
(författare)
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Free induction decay in the extreme ultraviolet
- 2015
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Ingår i: Proceedings of Frontiers in Optics 2015, FIO 2015. - 9781943580033
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Konferensbidrag (refereegranskat)abstract
- We present an experimental study of controlled Free Induction Decay (FID) in the extreme ultraviolet regime excited by High-order Harmonics. The control is done by applying a delayed infrared pulse.
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| 9. |
- Bengtsson, Samuel, et al.
(författare)
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Ultrafast control and opto-optical modulation of extreme ultraviolet light
- 2019
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Ingår i: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 52:6
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Tidskriftsartikel (refereegranskat)abstract
- Ultrashort, coherent light pulses in the extreme ultraviolet (XUV) wavelength region are excellent tools for a wide range of experiments. These XUV pulses are in particular used to characterize electron dynamics. Additionally, the high photon energy enables quantum control of short-lived transitions in atoms. These transitions are of interest for ultrafast quantum control as the bandwidth of the control pulses needs to be less than the transition frequency. Coherent XUV pulses would be even more useful if the tools to control and manipulate them existed. Presently, however, no modulators exist for these frequencies, which significantly reduces the number of possible ultrafast optics experiments and applications in the XUV regime. This Ph.D. tutorial addresses this problem and focuses on the control of XUV light in time and space. To enable control of XUV light we have developed a method for opto-optical modulation (OOM), which modulates the phase of XUV light emitted from a gas of atoms. The atoms are resonantly excited with a coherent XUV pulse generated through high order harmonic generation and emit light after the excitation pulse passed. By shifting the resonance frequencies through the AC Stark shift with a non-resonant infrared control pulse, the phase of the emitted light, and therefore its wavefront, is controlled. With OOM the direction of XUV light emitted from noble gases has been controlled, and the temporal dynamics of the emission studied. By varying the delay between the excitation and the control pulse, the time of redirection was controlled, which enabled high signal to noise study of the temporal dynamics of the emission. Furthermore, a proof-of-principle experiment with two control pulses was performed, resulting in a shaped XUV pulse with controlled duration. The technique of OOM presented in this article is still at a very early stage and in order to obtain complete control of the amplitude and phase of the XUV pulses further development is necessary. Such development would present new opportunities for ultrafast XUV quantum control.
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| 10. |
- Biegert, Jens, et al.
(författare)
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Control of the frequency chirp rate of high harmonic pulses
- 2004
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Ingår i: Springer Series in Chemical Physics. - 0172-6218. ; 79, s. 204-206
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Tidskriftsartikel (refereegranskat)abstract
- We measured the frequency chirp rate of harmonics 13 to 23 in argon by cross-correlation with a short femtosecond probe pulse. We directly measured the negative chirp due to the atomic dipole phase and demonstrated that an additional chirp on the pump pulse was transferred to the qth harmonic as q times the fundamental chirp.
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| 11. |
- Brizuela, Fernando, et al.
(författare)
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Efficient high order harmonic generation boosted by below threshold harmonics
- 2013
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 3
-
Tidskriftsartikel (refereegranskat)abstract
- High-order harmonic generation (HHG) in gases has been established as an important technique for the generation of coherent extreme ultraviolet (XUV) pulses at ultrashort time scales. Its main drawback, however, is the low conversion efficiency, setting limits for many applications, such as ultrafast coherent imaging, nonlinear processes in the XUV range, or seeded free electron lasers. Here we introduce a novel scheme based on using below-threshold harmonics, generated in a seeding cell, to boost the HHG process in a generation cell, placed further downstream in the focused laser beam. By modifying the fundamental driving field, these low-order harmonics alter the ionization step of the nonlinear HHG process. Our dual-cell scheme enhances the conversion efficiency of HHG, opening the path for the realization of robust intense attosecond XUV sources.
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| 12. |
- Carlström, Stefanos, et al.
(författare)
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Quantum coherence in photo-ionisation with tailored XUV pulses
- 2018
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Ingår i: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 51:1
-
Tidskriftsartikel (refereegranskat)abstract
- Ionisation with ultrashort pulses in the extreme ultraviolet (XUV) regime can be used to prepare an ion in a superposition of spin-orbit substates. In this work, we study the coherence properties of such a superposition, created by ionising xenon atoms using two phase-locked XUV pulses at different frequencies. In general, if the duration of the driving pulse exceeds the quantum beat period, dephasing will occur. If however, the frequency difference of the two pulses matches the spin-orbit splitting, the coherence can be efficiently increased and dephasing does not occur.
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| 13. |
- Carlström, Stefanos, et al.
(författare)
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Spatially and spectrally resolved quantum path interference with chirped driving pulses
- 2016
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 18:12
-
Tidskriftsartikel (refereegranskat)abstract
- We measure spectrally and spatially resolved high-order harmonics generated in argon using chirped multi-cycle laser pulses. Using a stable, high-repetition rate laser we observe detailed interference structures in the far-field. The structures are of two kinds; off-axis interference from the long trajectory only and on-axis interference including the short and long trajectories. The former is readily visible in the far-field spectrum, modulating both the spectral and spatial profile. To access the latter, we vary the chirp of the fundamental, imparting different phases on the different trajectories, thereby changing their relative phase. Using this method together with an analytical model, we are able to explain the on-axis behaviour and access the dipole phase parameters for the short (${\alpha }_{{\rm{s}}}$) and long (${\alpha }_{{\rm{l}}}$) trajectories. The extracted results compare very well with phase parameters calculated by solving the time-dependent Schrödinger equation. Going beyond the analytical model, we are also able to successfully reproduce the off-axis interference structure.
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| 14. |
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| 15. |
- Dahlström, J. Marcus, 1982-, et al.
(författare)
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Theory of attosecond delays in laser-assisted photoionization
- 2013
-
Ingår i: Chemical Physics. - : Elsevier BV. - 0301-0104 .- 1873-4421. ; 414, s. 53-64
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Tidskriftsartikel (refereegranskat)abstract
- We study the temporal aspects of laser-assisted extreme ultraviolet (XUV) photoionization using attosecond pulses of harmonic radiation. The aim of this paper is to establish the general form of the phase of the relevant transition amplitudes and to make the connection with the time-delays that have been recently measured in experiments. We find that the overall phase contains two distinct types of contributions: one is expressed in terms of the phase-shifts of the photoelectron continuum wavefunction while the other is linked to continuum–continuum transitions induced by the infrared (IR) laser probe. Our formalism applies to both kinds of measurements reported so far, namely the ones using attosecond pulse trains of XUV harmonics and the others based on the use of isolated attosecond pulses (streaking). The connection between the phases and the time-delays is established with the help of finite difference approximations to the energy derivatives of the phases. The observed time-delay is a sum of two components: a one-photon Wigner-like delay and an universal delay that originates from the probing process itself.
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| 16. |
- Dahlström, Marcus, et al.
(författare)
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Atomic and macroscopic measurements of attosecond pulse trains
- 2009
-
Ingår i: Physical Review A (Atomic, Molecular and Optical Physics). - 1050-2947. ; 80:3
-
Tidskriftsartikel (refereegranskat)abstract
- We characterize attosecond pulses in a train using both the well established "reconstruction of attosecond beating by interference of two-photon transitions" (RABITT) technique and the recently demonstrated in situ method, which is based on a weak perturbation of the harmonic generation process by the second harmonic of the laser field. The latter technique determines the characteristics of the single atom emission, while RABITT allows one to measure attosecond pulses "on target." By comparing the results of the two methods, the influence of propagation and filtering on the attosecond pulses can be extracted.
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| 17. |
- Dahlström, Marcus, et al.
(författare)
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Quantum mechanical approach to probing the birth of attosecond pulses using a two-colour field
- 2011
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Ingår i: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 44:9
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the generation of even and odd harmonics using an intense laser and a weak second harmonic field. Our theoretical approach is based on solving the saddle-point equations within the strong field approximation. The phase of the even harmonic oscillation as a function of the delay between the fundamental and second harmonic field is calculated and its variation with energy is found to be in good agreement with recent experimental results. We also find that the relationship between this phase variation and the group delay of the attosecond pulses depends on the intensity and wavelength of the fundamental field as well as the ionization potential of the atom.
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| 18. |
- Delfin, C, et al.
(författare)
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Influence of laser pulse duration on relativistic channels
- 2002
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 9:3, s. 937-940
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Tidskriftsartikel (refereegranskat)abstract
- A high-power (10 TW) laser is employed to generate relativistic channels in an underdense plasma. The lengths of the channels are measured by imaging the Thomson-scattered light, and the gas densities are determined through the forward Raman scattered light. The laser-pulse parameters are varied and their impact on the channel formation is studied. It is found that increasing the laser pulse duration in many cases produces longer channels, even as this implies reducing the laser peak power. A theoretical discussion is presented, proposing an explanation of the experimental results. (C) 2002 American Institute of Physics.
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| 19. |
- Fushitani, Mizuho, et al.
(författare)
-
Time-resolved photoelectron imaging of complex resonances in molecular nitrogen
- 2021
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Ingår i: The Journal of chemical physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 154:14
-
Tidskriftsartikel (refereegranskat)abstract
- We have used the FERMI free-electron laser to perform time-resolved photoelectron imaging experiments on a complex group of resonances near 15.38 eV in the absorption spectrum of molecular nitrogen, N2, under jet-cooled conditions. The new data complement and extend the earlier work of Fushitani et al. [Opt. Express 27, 19702–19711 (2019)], who recorded time-resolved photoelectron spectra for this same group of resonances. Time-dependent oscillations are observed in both the photoelectron yields and the photoelectron angular distributions, providing insight into the interactions among the resonant intermediate states. In addition, for most states, we observe an exponential decay of the photoelectron yield that depends on the ionic final state. This observation can be rationalized by the different lifetimes for the intermediate states contributing to a particular ionization channel. Although there are nine resonances within the group, we show that by detecting individual photoelectron final states and their angular dependence, we can identify and differentiate quantum pathways within this complex system.
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| 20. |
- Fushitani, Mizuho, et al.
(författare)
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Wave packet dynamics and control in excited states of molecular nitrogen
- 2024
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Ingår i: The Journal of chemical physics. - 0021-9606. ; 160:10
-
Tidskriftsartikel (refereegranskat)abstract
- Wave packet interferometry with vacuum ultraviolet light has been used to probe a complex region of the electronic spectrum of molecular nitrogen, N2. Wave packets of Rydberg and valence states were excited by using double pulses of vacuum ultraviolet (VUV), free-electron-laser (FEL) light. These wave packets were composed of contributions from multiple electronic states with a moderate principal quantum number (n ∼ 4-9) and a range of vibrational and rotational quantum numbers. The phase relationship of the two FEL pulses varied in time, but as demonstrated previously, a shot-by-shot analysis allows the spectra to be sorted according to the phase between the two pulses. The wave packets were probed by angle-resolved photoionization using an infrared pulse with a variable delay after the pair of excitation pulses. The photoelectron branching fractions and angular distributions display oscillations that depend on both the time delays and the relative phases of the VUV pulses. The combination of frequency, time delay, and phase selection provides significant control over the ionization process and ultimately improves the ability to analyze and assign complex molecular spectra.
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| 21. |
- Gisselbrecht, Mathieu, et al.
(författare)
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Exploring single-photon ionization on the attosecond time scale
- 2012
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Ingår i: XXVII International Conference on Photonic, Electronic and Atomic Collisions (Icpeac 2011), Pts 1-15. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 388, s. 032025-032025
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Konferensbidrag (refereegranskat)abstract
- One of the fundamental processes in nature is the photoelectric effect in which an electron is ripped away from its atom via the interaction with a photon. This process was long believed to be instantaneous but with the development of attosecond pulses (1 as 10(-18) s) we can finally get an insight into its dynamic. Here we measure a delay in ionization time between two differently bound electrons. The outgoing electrons are created via ionization with a train of attosecond pulses and we probe their relative delay with a synchronized infrared laser. We demonstrate how this probe field influences the measured delays and show that this contribution can be estimated with a universal formula, which allows us to extract field free atomic data..
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| 22. |
- Guenot, Diego, et al.
(författare)
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Measurements of relative photoemission time delays in noble gas atoms
- 2014
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Ingår i: Journal of Physics B. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 47:24, s. 245602-
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Tidskriftsartikel (refereegranskat)abstract
- We determine relative photoemission time delays between valence electrons in different noble gas atoms (Ar, Ne and He) in an energy range between 31 and 37 eV. The atoms are ionized by an attosecond pulse train synchronized with an infrared laser field and the delays are measured using an interferometric technique. We compare our results with calculations using the random phase approximation with exchange and multi-configurational Hartree-Fock. We also investigate the influence of the different ionization angular channels.
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| 23. |
- Guenot, Diego, et al.
(författare)
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Photoemission time delay measurements and calculations close to the 3s ionization cross section minimum in ar
- 2012
-
Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 85:5, s. 053424-
-
Tidskriftsartikel (refereegranskat)abstract
- We present experimental measurements and theoretical calculations of photoionization time delays from the 3s and 3p shells in Ar in the photon energy range of 32-42 eV. The experimental measurements are performed by interferometry using attosecond pulse trains and the infrared laser used for their generation. The theoretical approach includes intershell correlation effects between the 3s and 3p shells within the framework of the random-phase approximation with exchange. The connection between single-photon ionization and the two-color two-photon ionization process used in the measurement is established using the recently developed asymptotic approximation for the complex transition amplitudes of laser-assisted photoionization. We compare and discuss the theoretical and experimental results, especially in the region where strong intershell correlations in the 3s -> kp channel lead to an induced Cooper minimum in the 3s ionization cross section.
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| 24. |
- Guo, Chen, et al.
(författare)
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Phase control of attosecond pulses in a train
- 2018
-
Ingår i: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 51:3
-
Tidskriftsartikel (refereegranskat)abstract
- Ultrafast processes in matter can be captured and even controlled by using sequences of few-cycle optical pulses, which need to be well characterized, both in amplitude and phase. The same degree of control has not yet been achieved for few-cycle extreme ultraviolet pulses generated by high-order harmonic generation (HHG) in gases, with duration in the attosecond range. Here, we show that by varying the spectral phase and carrier-envelope phase (CEP) of a high-repetition rate laser, using dispersion in glass, we achieve a high degree of control of the relative phase and CEP between consecutive attosecond pulses. The experimental results are supported by a detailed theoretical analysis based upon the semi-classical three-step model for HHG.
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| 25. |
- He, Xinkui, et al.
(författare)
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Interference effects in two-color high-order harmonic generation
- 2010
-
Ingår i: Physical Review A (Atomic, Molecular and Optical Physics). - 1050-2947. ; 82:3
-
Tidskriftsartikel (refereegranskat)abstract
- We study high-order harmonic generation in argon driven by an intense 800 nm laser field and a small fraction of its second harmonic. The intensity and divergence of the emitted even and odd harmonics are strongly modulated as a function of the relative delay between the two fields. We provide a detailed analysis of the underlying interference effects. The interference changes drastically when approaching the cutoff region due to a switch of the dominant trajectory responsible for harmonic generation.
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| 26. |
- Heyl, Christoph, et al.
(författare)
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Macroscopic effects in noncollinear high-order harmonic generation.
- 2014
-
Ingår i: Physical Review Letters. - 1079-7114. ; 112:14
-
Tidskriftsartikel (refereegranskat)abstract
- We study two-color high-order harmonic generation using an intense driving field and its weak second harmonic, crossed under a small angle in the focus. Employing sum- and difference-frequency generation processes, such a noncollinear scheme can be used to measure and control macroscopic phase matching effects by utilizing a geometrical phase mismatch component, which depends on the noncollinear angle. We further show how spatial phase effects in the generation volume are mapped out into the far field allowing a direct analogy with temporal carrier envelope effects in attosecond pulse generation.
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| 27. |
- Heyl, Christoph, et al.
(författare)
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Noncollinear optical gating
- 2014
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 16
-
Tidskriftsartikel (refereegranskat)abstract
- We present a novel scheme for high-order harmonic generation, enabling the production of spatially separated isolated attosecond pulses. This can be achieved by driving the generation process with two identical, but temporally delayed laser pulses, which are noncollinearly overlapping in the generation medium. Our approach provides intense attosecond pulses directly separated from the fundamental field, which is left undistorted. The method is therefore ideally suited for pump-probe studies in the extreme ultraviolet regime and promises new advances for intra-cavity attosecond pulse generation. We present a theoretical description of noncollinear optical gating, with an analytical derivation and simulations using the strong field approximation.
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| 28. |
- Ibrakovic, Neven, et al.
(författare)
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Achromatic dual-waveplate for inline two color high-order harmonic generation
- 2018
-
Ingår i: European Physical Journal D. Atomic, Molecular, Optical and Plasma Physics. - : Springer Science and Business Media LLC. - 1434-6060. ; 72:9
-
Tidskriftsartikel (refereegranskat)abstract
- Abstract: We propose a design of an achromatic dual-waveplate intended to align the polarization of orthogonal two-color pulses in an inline configuration. The waveplate acts as a half-waveplate for pulses with a carrier wavelength of 1300 nm over a bandwidth of 300 nm. For the second harmonic of the pulses the waveplate acts as a full-waveplate, centered at 650 nm with a bandwidth of 100 nm. By experimentally measuring the transmission of this optic when placed between two parallel linear polarizers we show that the polarization directions are aligned as expected. This experiment is then verified by calculating the transmission through the optic using Jones calculus. This waveplate could be useful in two-color experiments with few-cycle pulses, or pulses with a tunable wavelength, that need to be aligned with their second harmonics. Graphical abstract: [Figure not available: see fulltext.].
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| 29. |
- Jimenez-Galan, A., et al.
(författare)
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Phase Measurement of a Fano Resonance Using Tunable Attosecond Pulses
- 2015
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 635, s. 092137-092137
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Konferensbidrag (refereegranskat)abstract
- We study photoionization of argon atoms close to the 3s(2)3p(6) -> 3s(1)3p(6)4p Fano resonance using an attosecond pulse train and a weak infrared probe field. An interferometric technique combined with tunable attosecond pulses allows us to determine the phase of the photoionization amplitude as a function of photon energy. We interpret the experimental results using an analytical two-photon model based on the Fano formalism and obtain quantitative agreement.
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| 30. |
- Johnsson, Per, et al.
(författare)
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Attosecond control of ionization by wave-packet interference
- 2007
-
Ingår i: Physical Review Letters. - 1079-7114. ; 99:23
-
Tidskriftsartikel (refereegranskat)abstract
- A train of attosecond pulses, synchronized to an infrared (IR) laser field, is used to create a series of electron wave packets (EWPs) that are below the ionization threshold in .helium. The ionization probability is found to strongly oscillate with the delay between the IR and attosecond fields twice per IR laser cycle. Calculations that reproduce the experimental results demonstrate that this ionization control results from interference between transiently bound EWPs created by different pulses in the train. In this way, we are able to observe, for the first time, attosecond wave-packet interference in a strongly driven atomic system.
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| 31. |
- Klünder, Kathrin, et al.
(författare)
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Probing Single-Photon Ionization on the Attosecond Time Scale
- 2011
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Ingår i: Physical Review Letters. - 1079-7114. ; 106:14
-
Tidskriftsartikel (refereegranskat)abstract
- We study photoionization of argon atoms excited by attosecond pulses using an interferometric measurement technique. We measure the difference in time delays between electrons emitted from the 3s(2) and from the 3p(6) shell, at different excitation energies ranging from 32 to 42 eV. The determination of photoemission time delays requires taking into account the measurement process, involving the interaction with a probing infrared field. This contribution can be estimated using a universal formula and is found to account for a substantial fraction of the measured delay.
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| 32. |
- Klünder, Kathrin, et al.
(författare)
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Reconstruction of attosecond electron wave packets using quantum state holography
- 2013
-
Ingår i: Physical Review A (Atomic, Molecular and Optical Physics). - 1050-2947. ; 88:3
-
Tidskriftsartikel (refereegranskat)abstract
- We present a method for performing quantum state holography, with which we completely characterize the amplitude and phase of an attosecond electron wave packet. Our approach is an extension of a recent publication [J. Mauritsson et al., Phys. Rev. Lett. 105, 053001 (2010)] in which we demonstrated experimentally that the energies and amplitudes of an attosecond electron wave packet can be characterized using attosecond electron interferometry. Here we show theoretically that attosecond electron interferometry can be extended to retrieve the phases of all the states that make up the wave packet. We demonstrate the feasibility of our method by analyzing a wave packet created by a shake-up process. We show that our method can successfully retrieve arbitrary phases and/or lifetimes added to the component eigenstates.
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| 33. |
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| 34. |
- Kotur, Marija, et al.
(författare)
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Spectral phase measurement of a Fano resonance using tunable attosecond pulses
- 2016
-
Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 7
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Tidskriftsartikel (refereegranskat)abstract
- Electron dynamics induced by resonant absorption of light is of fundamental importance in nature and has been the subject of countless studies in many scientific areas. Above the ionization threshold of atomic or molecular systems, the presence of discrete states leads to autoionization, which is an interference between two quantum paths: direct ionization and excitation of the discrete state coupled to the continuum. Traditionally studied with synchrotron radiation, the probability for autoionization exhibits a universal Fano intensity profile as a function of excitation energy. However, without additional phase information, the full temporal dynamics cannot be recovered. Here we use tunable attosecond pulses combined with weak infrared radiation in an interferometric setup to measure not only the intensity but also the phase variation of the photoionization amplitude across an autoionization resonance in argon. The phase variation can be used as a fingerprint of the interactions between the discrete state and the ionization continua, indicating a new route towards monitoring electron correlations in time.
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| 35. |
- Kroon, David, et al.
(författare)
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Attosecond pulse walk-off in high-order harmonic generation
- 2014
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Ingår i: Optics Letters. - 0146-9592. ; 39:7, s. 2218-2221
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Tidskriftsartikel (refereegranskat)abstract
- We study the influence of the generation conditions on the group delay of attosecond pulses in high-order harmonic generation in gases. The group delay relative to the fundamental field is found to decrease with increasing gas pressure in the generation cell, reflecting a temporal walk-off due to the dispersive properties of the nonlinear medium. This effect is well reproduced using an on-axis phase-matching model of high-order harmonic generation in an absorbing gas. (C) 2014 Optical Society of America
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| 36. |
- Kumar Maroju, Praveen, et al.
(författare)
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Attosecond pulse shaping using a seeded free-electron laser
- 2020
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 578, s. 386-391
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Tidskriftsartikel (refereegranskat)abstract
- Attosecond pulses are central to the investigation of valence- and core-electron dynamics on their natural timescales. The reproducible generation and characterization of attosecond waveforms has been demonstrated so far only through the process of high-order harmonic generation. Several methods for shaping attosecond waveforms have been proposed, including the use of metallic filters, multilayer mirrors and manipulation of the driving field. However, none of these approaches allows the flexible manipulation of the temporal characteristics of the attosecond waveforms, and they suffer from the low conversion efficiency of the high-order harmonic generation process. Free-electron lasers, by contrast, deliver femtosecond, extreme-ultraviolet and X-ray pulses with energies ranging from tens of microjoules to a few millijoules. Recent experiments have shown that they can generate subfemtosecond spikes, but with temporal characteristics that change shot-to-shot. Here we report reproducible generation of high-energy (microjoule level) attosecond waveforms using a seeded free-electron laser. We demonstrate amplitude and phase manipulation of the harmonic components of an attosecond pulse train in combination with an approach for its temporal reconstruction. The results presented here open the way to performing attosecond time-resolved experiments with free-electron lasers.
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| 37. |
- Larsen, Esben Witting, et al.
(författare)
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Sub-cycle ionization dynamics revealed by trajectory resolved, elliptically-driven high-order harmonic generation
- 2016
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- The sub-cycle dynamics of electrons driven by strong laser fields is central to the emerging field of attosecond science. We demonstrate how the dynamics can be probed through high-order harmonic generation, where different trajectories leading to the same harmonic order are initiated at different times, thereby probing different field strengths. We find large differences between the trajectories with respect to both their sensitivity to driving field ellipticity and resonant enhancement. To accurately describe the ellipticity dependence of the long trajectory harmonics we must include a sub-cycle change of the initial velocity distribution of the electron and its excursion time. The resonant enhancement is observed only for the long trajectory contribution of a particular harmonic when a window resonance in argon, which is off-resonant in the field-free case, is shifted into resonance due to a large dynamic Stark shift.
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| 38. |
- Liu, Yi, et al.
(författare)
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Unexpected Sensitivity of Nitrogen Ions Superradiant Emission on Pump Laser Wavelength and Duration
- 2017
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Ingår i: Physical Review Letters. - 0031-9007. ; 119:20
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Tidskriftsartikel (refereegranskat)abstract
- Nitrogen molecules in ambient air exposed to an intense near-infrared femtosecond laser pulse give rise to cavity-free superradiant emission at 391.4 and 427.8 nm. An unexpected pulse duration-dependent cyclic variation of the superradiance intensity is observed when the central wavelength of the femtosecond pump laser pulse is finely tuned between 780 and 820 nm, and no signal occurs at the resonant wavelength of 782.8 nm (2ω782.8 nm=ω391.4 nm). On the basis of a semiclassical recollision model, we show that an interference of dipolar moments of excited ions created by electron recollisions explains this behavior.
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| 39. |
- Lopez, Rodrigo, et al.
(författare)
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Characterization of high-order harmonic radiation on femtosecond and attosecond time scales
- 2004
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Ingår i: Applied Physics B. - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 78:7-8, s. 835-840
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Tidskriftsartikel (refereegranskat)abstract
- We characterize the temporal structure of high-order harmonic radiation on both the femtosecond and attosecond time scales. The harmonic emission is characterized by mixed-color two-photon ionization with an infrared femtosecond laser using a Mach-Zehnder interferometer where both pump and probe arms travel completely separate paths. In a first experiment, we measure the duration and chirp of individual harmonics. In a second experiment, we resolve, for the first time with this type of setup, the attosecond beating of several harmonics generated under conditions similar to the first experiment. We suggest that the results of both measurements can be combined to determine the full attosecond time structure of the harmonic emission.
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| 40. |
- Lopez, Rodrigo, et al.
(författare)
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Time-resolved ellipticity gating of high-order harmonic emission
- 2004
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Ingår i: Physical Review A. ; 69, s. 1-053811
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Tidskriftsartikel (refereegranskat)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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| 41. |
- Lorek, Eleonora, et al.
(författare)
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High-order harmonic generation using a high-repetition-rate turnkey laser.
- 2014
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Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 1089-7623 .- 0034-6748. ; 85:12
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Tidskriftsartikel (refereegranskat)abstract
- We generate high-order harmonics at high pulse repetition rates using a turnkey laser. High-order harmonics at 400 kHz are observed when argon is used as target gas. In neon, we achieve generation of photons with energies exceeding 90 eV (∼13 nm) at 20 kHz. We measure a photon flux of up to 4.4 × 10(10) photons per second per harmonic in argon at 100 kHz. Many experiments employing high-order harmonics would benefit from higher repetition rates, and the user-friendly operation opens up for applications of coherent extreme ultra-violet pulses in new research areas.
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| 42. |
- Lorek, Eleonora, et al.
(författare)
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Size and shape dependent few-cycle near-field dynamics of bowtie nanoantennas
- 2015
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Ingår i: Optics Express. - 1094-4087. ; 23:24, s. 31460-31471
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Tidskriftsartikel (refereegranskat)abstract
- Metal nanostructures can transfer electromagnetic energy from femtosecond laser pulses to the near-field down to spatial scales well below the optical diffraction limit. By combining few-femtosecond laser pulses with photoemission electron microscopy, we study the dynamics of the induced few-cycle near-field in individual bowtie nanoantennas. We investigate how the dynamics depend on antenna size and exact bowtie shape resulting from fabrication. Different dynamics are, as expected, measured for antennas of different sizes. However, we also detect comparable dynamics differences between individual antennas of similar size. With Finite-difference time-domain simulations we show that these dynamics differences between similarly sized antennas can be due to small lateral shape variations generally induced during the fabrication.
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| 43. |
- Louisy, Maite, et al.
(författare)
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Gating attosecond pulses in a noncollinear geometry
- 2015
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Ingår i: Optica. - 2334-2536. ; 2:6, s. 563-566
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Tidskriftsartikel (refereegranskat)abstract
- The efficient generation of isolated attosecond pulses (IAPs), giving access to ultrafast electron dynamics in various systems, is a key challenge in attosecond science. IAPs can be produced by confining the extreme ultraviolet emission generated by an intense laser pulse to a single field half-cycle or, as shown recently, by employing angular streaking methods. Here, we experimentally demonstrate the angular streaking of attosecond pulse trains in a noncollinear geometry, leading to the emission of angularly separated IAPs. The noncollinear geometry simplifies the separation of the fundamental laser field and the generated pulses, making this scheme promising for intracavity attosecond pulse generation, thus opening new possibilities for high-repetition-rate attosecond sources. (C) 2015 Optical Society of America
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| 44. |
- Mansten, Erik, et al.
(författare)
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Spectral shaping of attosecond pulses using two-colour laser fields
- 2008
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 10
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Tidskriftsartikel (refereegranskat)abstract
- We use a strong two-colour laser field composed of the fundamental (800 nm) and the second harmonic (400 nm) of an infrared (IR) laser field to generate attosecond pulses with controlled spectral and temporal properties. With a second-harmonic intensity equal to 15% of the IR intensity the second-harmonic field is strong enough to significantly alter and control the electron trajectories in the generation process. This enables us to tune the central photon energy of the attosecond pulses by changing the phase difference between the IR and the second-harmonic fields. In the time domain the radiation is emitted as a sequence of pulses separated by a full IR cycle. We also perform calculations showing that the effect of even stronger second-harmonic fields leads to an extended tunable range under conditions that are experimentally feasible.
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| 45. |
- Mansten, Erik, et al.
(författare)
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Spectral signature of short attosecond pulse trains.
- 2009
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Ingår i: Physical Review Letters. - 1079-7114. ; 102:8
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Tidskriftsartikel (refereegranskat)abstract
- We report experimental measurements of high-order harmonic spectra generated in Ar using a carrier-envelope-offset (CEO) stabilized 12 fs, 800 nm laser field and a fraction (less than 10%) of its second harmonic. Additional spectral peaks are observed between the harmonic peaks, which are due to interferences between multiple pulses in the train. The position of these peaks varies with the CEO and their number is directly related to the number of pulses in the train. An analytical model, as well as numerical simulations, support our interpretation.
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| 46. |
- Maroju, Praveen K., et al.
(författare)
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A Novel Attosecond Timing Tool for Free-Electron Laser Experiment
- 2020
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Ingår i: High Intensity Lasers and High Field Phenomena 2020. - 9781943580736
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Konferensbidrag (refereegranskat)abstract
- We demonstrate a novel timing tool for Free-Electron Lasers to determine the delay between an attosecond pulse train and infrared pulse with sub-optical-cycle resolu-. tion.
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| 47. |
- Maroju, Praveen Kumar, et al.
(författare)
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Attosecond coherent control of electronic wave packets in two-colour photoionization using a novel timing tool for seeded free-electron laser
- 2023
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Ingår i: Nature Photonics. - : Springer Science and Business Media LLC. - 1749-4885 .- 1749-4893. ; 17, s. 200-207
-
Tidskriftsartikel (refereegranskat)abstract
- In ultrafast spectroscopy, the temporal resolution of time-resolved experiments depends on the duration of the pump and probe pulses, and on the control and characterization of their relative synchronization. Free-electron lasers operating in the extreme ultraviolet and X-ray spectral regions deliver pulses with femtosecond and attosecond duration in a broad array of pump-probe configurations to study a wide range of physical processes. However, this flexibility, together with the large dimensions and high complexity of the experimental set-ups, limits control of the temporal delay to the femtosecond domain, thus precluding a time resolution below the optical cycle. Here we demonstrate a novel single-shot technique able to determine the relative synchronization between an attosecond pulse train-generated by a seeded free-electron laser-and the optical oscillations of a near-infrared field, with a resolution of one atomic unit (24 as). Using this attosecond timing tool, we report the first example of attosecond coherent control of photoionization in a two-colour field by manipulating the phase of high-order near-infrared transitions.
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| 48. |
- Maroju, Praveen Kumar, et al.
(författare)
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Complex attosecond waveform synthesis at fel fermi
- 2021
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Ingår i: Applied Sciences (Switzerland). - : MDPI AG. - 2076-3417. ; 11:21
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Tidskriftsartikel (refereegranskat)abstract
- Free-electron lasers (FELs) can produce radiation in the short wavelength range extending from the extreme ultraviolet (XUV) to the X-rays with a few to a few tens of femtoseconds pulse duration. These facilities have enabled significant breakthroughs in the field of atomic, molecular, and optical physics, implementing different schemes based on two-color photoionization mechanisms. In this article, we present the generation of attosecond pulse trains (APTs) at the seeded FEL FERMI using the beating of multiple phase-locked harmonics. We demonstrate the complex attosecond waveform shaping of the generated APTs, exploiting the ability to manipulate independently the amplitudes and the phases of the harmonics. The described generalized attosecond waveform synthesis technique with an arbitrary number of phase-locked harmonics will allow the generation of sub-100 as pulses with programmable electric fields.
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| 49. |
- Matyschok, Jan, et al.
(författare)
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Compact, high-repetition-rate OPCPA system for high harmonic generation
- 2014
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Ingår i: Frontiers in Ultrafast Optics : Biomedical, Scientific, and Industrial Applications XIV - Biomedical, Scientific, and Industrial Applications XIV. - : SPIE. - 9780819498854 ; 8972
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Konferensbidrag (refereegranskat)abstract
- A compact, high-repetition rate optical parametric chirped pulse amplifier system emitting CEP-stable, few-cycle pulses with 10 μJ of pulse energy is reported for the purpose of high-order harmonic generation. The system is seeded from a commercially available, CEP-stabilized Ti:sapphire oscillator, delivering an octave-spanning spectrum from 600-1200 nm. The oscillator output serves on the one hand as broadband signal for the parametric amplification process and on the other hand as narrowband seed for an Ytterbium-based fiber preamplifier with subsequent main amplifiers and frequency doubling. Broadband parametric amplification up to 17 μJ at 200 kHz repetition rate was achieved in two 5 mm BBO crystals using non-collinear phase matching in the Poynting-vector-walk-off geometry. Efficient pulse compression down to 6.3 fs is achieved with chirped mirrors leading to a peak power exceeding 800 MW. We observed after warm-up time a stability of < 0.5 % rms over 100 min. Drifts of the CE-phase in the parametric amplifier part could be compensated by a slow feedback to the set point of the oscillator phase lock. The CEP stability was measured to be better than 80 mrad over 15 min (3 ms integration time). The experimentally observed output spectra and energies could be well reproduced by simulations of the parametric amplification process based on a (2+1)-dimensional nonlinear propagation code, providing important insight for future repetition rate scaling of OPCPA systems. The system is well-suited for attosecond science experiments which benefit from the high repetition rate. First results for high-order harmonic generation in argon will be presented.
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| 50. |
- Mauritsson, Johan, et al.
(författare)
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Attosecond electron interferometry
- 2013
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Ingår i: Attosecond Physics : Attosecond Measurements and Control of Physical Systems - Attosecond Measurements and Control of Physical Systems. - Berlin, Heidelberg : Springer Berlin Heidelberg. - 1556-1534 .- 0342-4111. - 9783642376238 - 9783642376221 ; 177, s. 121-134
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Bokkapitel (refereegranskat)abstract
- Attosecond extreme ultraviolet light pulses have the potential to resolve the ultrafast electron dynamics that govern basic properties of atoms, molecules, and solids. Here we present three different interferometric pump-probe methods aiming to access not only the temporal dynamics, but also state specific phase information after excitation/ionization using attosecond pulses.
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