SwePub - search: WFRF:(Varjú Katalin)

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1.	Major, Balázs, et al. (author) Effect of plasma-core-induced self-guiding on phase matching of high-order harmonic generation in gases 2019 In: Journal of the Optical Society of America B: Optical Physics. - 0740-3224. ; 36:6, s. 1594-1601 Journal article (peer-reviewed)abstract In this work, we numerically study a self-guiding process in which ionization plays a dominant role and analyze its effect on high-order harmonic generation (HHG) in gases. Although this type of self-guiding-termed "plasmacore- induced self-guiding" in previous works-limits the achievable cutoff by regulating the intensity of the laser beam, it provides favorable conditions for phase matching, which is indispensable for high-flux-gas highharmonic sources. To underline the role of self-guiding in efficient HHG, we investigate the time-dependent phase-matching conditions in the guided beam and show how the spatiotemporally constant fundamental intensity contributes to the constructive buildup of the harmonic field in a broad photon energy range up to the provided cutoff.
2.	Rivas, Daniel, et al. (author) Propagation-enhanced generation of intense high-harmonic continua in the 100-eV spectral region 2018 In: Optica. - 2334-2536. ; 5:10, s. 1283-1289 Journal article (peer-reviewed)abstract The study of core electron dynamics through nonlinear spectroscopy requires intense isolated attosecond extremeultraviolet or even X-ray pulses. A robust way to produce these pulses is high-harmonic generation (HHG) in agas medium. However, the energy upscaling of the process depends on a very demanding next-generation laser technologythat provides multi-terawatt (TW) laser pulses with few-optical-cycle duration and controlled electric field.Here, we revisit the HHG process driven by 16-TW sub-two-cycle laser pulses to reach high intensity in the 100-eVspectral region and beyond. We show that the combination of above barrier-suppression intensity with a long generationmedium significantly enhances the isolation of attosecond pulses compared to lower intensities and/or shortermedia and this way reduces the pulse duration as well as field-stability requirements on the laser driver. This novelregime facilitates the real-time observation of electron dynamics at the attosecond timescale in atoms, molecules, andsolids.
3.	Biegert, Jens, et al. (author) Control of the frequency chirp rate of high harmonic pulses 2004 In: Springer Series in Chemical Physics. - 0172-6218. ; 79, s. 204-206 Journal article (peer-reviewed)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.
4.	Charalambidis, Dimitris, et al. (author) The extreme light infrastructure—attosecond light pulse source (ELI-ALPS) project 2017. - 9783319648392 In: Springer Series in Chemical Physics. - Cham : Springer International Publishing. - 0172-6218. ; :9783319648392, s. 181-218 Book chapter (peer-reviewed)abstract Globally, large international research infrastructures have over many decades promoted excellence in science and technology. Aligned with the international practice, the Europe Strategy Forum for Research Infrastructures (ESFRI) has developed and keeps updating a roadmap for research infrastructures. The Extreme Light Infrastructure (ELI) is one of the two large scale Laser Research Infrastructures (RI) proposed in the ESFRI Roadmap published in 2006. ELI aims to provide access to some of the most intense world-wide lasers for the international scientific user community, as well as secondary radiation and particle sources driven by them, offering to the users new interdisciplinary research opportunities. ELI is currently implemented as a distributed infrastructure in three pillars: ELI-Beamlines (ELI-BL) in Dolní Břežany, Czech Republic, ELI-Attosecond Light Pulse Source (ELI-ALPS) in Szeged, Hungary and ELI-Nuclear Physics (ELI-NP) in Magurele, Romania. This chapter is devoted to introduce the Hungarian pillar, ELI-ALPS, which will be operational in Szeged in 2018, with the primary mission to provide to the users the highest laboratory spatiotemporal resolution and a secondary mission to contribute to the technological development towards 200 petawatt (PW) lasers for high-field science, which is the ultimate goal of the ELI project. The chapter includes descriptions of the primary and secondary sources, while emphasis is given to selected examples of the scientific case of ELI-ALPS, presenting unique access offered by the technologies to be hosted in the infrastructure.
5.	Coudert-Alteirac, Hélène, et al. (author) Micro-focusing of broadband high-order harmonic radiation by a double toroidal mirror 2017 In: Applied Sciences (Switzerland). - : MDPI AG. - 2076-3417. ; 7:11 Journal article (peer-reviewed)abstract We present an optical system based on two toroidal mirrors in aWolter configuration to focus broadband extreme ultraviolet (XUV) radiation. Optimization of the focusing optics alignment is carried out with the aid of an XUV wavefront sensor. Back-propagation of the optimized wavefront to the focus yields a focal spot of 3.6 × 4.0 μm2 full width at half maximum, which is consistent with ray-tracing simulations that predict a minimum size of 3.0 × 3.2 μm2. This work is important for optimizing the intensity of focused high-order harmonics in order to reach the nonlinear interaction regime.
6.	Dacasa, Hugo, et al. (author) Single-shot extreme-ultraviolet wavefront measurements of high-order harmonics 2019 In: Optics Express. - 1094-4087. ; 27:3, s. 2656-2670 Journal article (peer-reviewed)abstract We perform wavefront measurements of high-order harmonics using an extreme-ultraviolet (XUV) Hartmann sensor and study how their spatial properties vary with different generation parameters, such as pressure in the nonlinear medium, fundamental pulse energy and duration as well as beam size. In some conditions, excellent wavefront quality (up to 휆/11) was obtained. The high throughput of the intense XUV beamline at the Lund Laser Centre allows us to perform single-shot measurements of both the full harmonic beam generated in argon and individual harmonics selected by multilayer mirrors. We theoretically analyze the relationship between the spatial properties of the fundamental and those of the generated high-order harmonics, thus gaining insight into the fundamental mechanisms involved in high-order harmonic generation (HHG).
7.	Johnsson, Per, et al. (author) Attosecond electron wave packet dynamics in strong laser fields 2005 In: Physical Review Letters. - 1079-7114. ; 95:1, s. 1-013001 Journal article (peer-reviewed)abstract We use a train of sub-200 attosecond extreme ultraviolet (XUV) pulses with energies just above the ionization threshold in argon to create a train of temporally localized electron wave packets. We study the energy transfer from a strong infrared (IR) laser field to the ionized electrons as a function of the delay between the XUV and IR fields. When the wave packets are born at the zero crossings of the IR field, a significant amount of energy (similar to 20 eV) is transferred from the field to the electrons. This results in dramatically enhanced above-threshold ionization in conditions where the IR field alone does not induce any significant ionization. Because both the energy and duration of the wave packets can be varied independently of the IR laser, they are valuable tools for studying and controlling strong-field processes.
8.	Johnsson, Per, et al. (author) Trains of attosecond electron wave packets 2006 In: Journal of Modern Optics. - : Informa UK Limited. - 0950-0340 .- 1362-3044. ; 53:1-2, s. 233-245 Journal article (peer-reviewed)abstract We study temporally localized electron wave packets, generated using a train of extreme ultraviolet (XUV) attosecond pulses to ionize the target atoms. Both the electron wave packets and the attosecond pulse train ( APT) are characterized using the same technique, based on interference of two-photon transitions in the continuum. We study, in particular, the energy transfer from a moderately strong infrared (IR) field to the electron wave packets as a function of time delay between the XUV and the IR fields. The use of an APT to generate the electron wave packets enables the generation at times not accessible through tunneling ionization. We find that a significant amount of energy is transferred from the IR field to the electron wave packets, when they are generated at a zero-crossing of the IR laser field. This energy transfer results in a dramatically enhanced above-threshold ionization even at IR intensities that alone are not strong enough to induce any significant ionization.
9.	Kühn, Sergei, et al. (author) The ELI-ALPS facility : The next generation of attosecond sources 2017 In: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 50:13 Research review (peer-reviewed)abstract This review presents the technological infrastructure that will be available at the Extreme Light Infrastructure Attosecond Light Pulse Source (ELI-ALPS) international facility. ELI-ALPS will offer to the international scientific community ultrashort pulses in the femtosecond and attosecond domain for time-resolved investigations with unprecedented levels of high quality characteristics. The laser sources and the attosecond beamlines available at the facility will make attosecond technology accessible for scientists lacking access to these novel tools. Time-resolved investigation of systems of increasing complexity is envisaged using the end stations that will be provided at the facility.
10.	Lepine, F., et al. (author) Short XUV pulses to characterize field-free molecular alignment 2007 In: Journal of Modern Optics. - : Informa UK Limited. - 0950-0340 .- 1362-3044. ; 54:7, s. 953-966 Journal article (peer-reviewed)abstract We present experiments on field-free molecular alignment of N-2 and CO2 probed with short XUV pulses that are obtained via high-harmonic generation. The XUV pulses induce a dissociative ionization or a Coulomb explosion of the molecule, where the fragment ion recoil (measured using the velocity map imaging technique) provides information on the alignment of the parent molecule at the time of ionization. We discuss how photoelectron detection may in future allow the determination of molecular-frame photoelectron angular distributions and molecular structure.
11.	Lopez, Rodrigo, et al. (author) Amplitude and phase control of attosecond light pulses 2005 In: Physical Review Letters. - 1079-7114. ; 94:3 Journal article (peer-reviewed)abstract We report the generation, compression, and delivery on target of ultrashort extreme-ultraviolet light pulses using external amplitude and phase control. Broadband harmonic radiation is first generated by focusing an infrared laser with a carefully chosen intensity into a gas cell containing argon atoms. The emitted light then goes through a hard aperture and a thin aluminum filter that selects a 30-eV bandwidth around a 30-eV photon energy and synchronizes all of the components, thereby enabling the formation of a train of almost Fourier-transform-limited single-cycle 170 attosecond pulses. Our experiment demonstrates a practical method for synthesizing and controlling attosecond waveforms.
12.	Lopez, Rodrigo, et al. (author) Characterization of high-order harmonic radiation on femtosecond and attosecond time scales 2004 In: Applied Physics B. - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 78:7-8, s. 835-840 Journal article (peer-reviewed)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.
13.	Mauritsson, Johan, et al. (author) Measurement and control of the frequency chirp rate of high-order harmonic pulses 2004 In: Physical Review A (Atomic, Molecular and Optical Physics). - 1050-2947. ; 70:2 Journal article (peer-reviewed)abstract We measure the chirp rate of harmonics 13 to 23 in argon by cross correlation with a 12 femtosecond probe pulse. Under low ionization conditions, we directly measure the negative chirp due to the atomic dipole phase, and show that an additional chirp on the pump pulse is transferred to the qth harmonic as q times the fundamental chirp. Our results are in accord with simulations using the experimentally measured 815 nm pump and probe pulses. The ability to measure and manipulate the harmonic chirp rate is essential for the characterization and optimization of attosecond pulse trains.
14.	Mauritsson, J, et al. (author) Probing temporal aspects of high-order harmonic pulses via multi-colour, multi-photon ionization processes 2005 In: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 38:13, s. 2265-2278 Journal article (peer-reviewed)abstract High-order harmonics generated through the interaction of atoms and strong laser fields are a versatile, laboratory-scale source of extreme ultraviolet (XUV) radiation on a femtosecond or even attosecond time-scale. In order to be a useful experimental tool, however, this radiation has to be well characterized, both temporally and spectrally. In this paper we discuss how multi-photon, multi-colour ionization processes can be used to completely characterize either individual harmonics or attosecond pulse trains. In particular, we discuss the influence of the intensity and duration of the probe laser, and how these parameters effect the accuracy of the XUV characterization.
15.	Morlens, AS, et al. (author) Design and characterization of extreme-ultraviolet broadband mirrors for attosecond science 2006 In: Optics Letters. - 0146-9592. ; 31:10, s. 1558-1560 Journal article (peer-reviewed)abstract A novel multilayer mirror was designed and fabricated based on a recently developed three-material technology aimed both at reaching reflectivities of about 20% and at controlling dispersion over a bandwidth covering photon energies between 35 and 50 eV. The spectral phase upon reflection was retrieved by measuring interferences in a two-color ionization process using high-order harmonics produced from a titanium: sapphire laser. We demonstrate the feasibility of designing and characterizing phase-controlled broadband optics in the extreme-ultraviolet domain, which should facilitate the manipulation of attosecond pulses for applications.
16.	Nayak, Arjun, et al. (author) Saddle point approaches in strong field physics and generation of attosecond pulses 2019 In: Physics Reports. - : Elsevier BV. - 0370-1573. ; 833, s. 1-52 Research review (peer-reviewed)abstract Attoscience is the emerging field that accesses the fastest electronic processes occurring at the atomic and molecular length scales with attosecond (1 as = 10−18 s) time resolution having wide ranging physical, chemical, material science and biological applications. The quintessential and one of the most fundamental processes in this domain is the generation of phase locked XUV attosecond pulses. The theoretical approach to understand the process incorporates a fully quantum or semi classical or relativistic description of coherent charge dynamics in intense ultrashort electromagnetic fields driving a quantum system (an atom, a molecule, solid band gap materials or surface plasmas). Modelling of such physical and dynamical systems in science and also in many other branches often leads to equations represented in terms of complex multi-dimensional integrals. These integrals can often be solved using the stationary phase approximation, which leads to a series of equations identifying the points in the multi-dimensional space, having most significant contributions in their evaluation. These points are usually indicated as saddle points. The description of the dynamics of quantum mechanical or relativistic systems that results from such an approach enables near to classical physics intuitive perceptions of the processes under investigation. Thus, the saddle point methods are very powerful and valuable general theoretical tools to obtain asymptotic expressions of such solutions and help also to gain physical insights on the underlying phenomena. Such techniques developed in the past have been adapted to study the emission of as pulses by different physical systems and have been widely employed in calculating and estimating the response of matter to intense electromagnetic pulses on ultrafast time scales. Here we provide an extensive disposition of the saddle point approaches unifying their ubiquitous applications within the domain of attoscience valid for simple atomic to more complex condensed matter systems undergoing ultrafast dynamics and present current trends and advancements in the field. In this review we would delineate the methodology, present a synthesis of seminal works and describe the state of the art applications. Finally we also address ultrashort time dynamics of novel materials that have gained much attention recently, namely lower dimensional material systems and micro-plasma systems.
17.	Remetter, Thomas, et al. (author) Attosecond electron wave packet interferometry 2006 In: Nature Physics. - : Springer Science and Business Media LLC. - 1745-2473 .- 1745-2481. ; 2:5, s. 323-326 Journal article (peer-reviewed)abstract A complete quantum-mechanical description of matter and its interaction with the environment requires detailed knowledge of a number of complex parameters. In particular, information about the phase of wavefunctions is important for predicting the behaviour of atoms, molecules or larger systems. In optics, information about the evolution of the phase of light in time(1) and space(2) is obtained by interferometry. To obtain similar information for atoms and molecules, it is vital to develop analogous techniques. Here we present an interferometric method for determining the phase variation of electronic wave packets in momentum space, and demonstrate its applicability to the fundamental process of single-photon ionization. We use a sequence of extreme-ultraviolet attosecond pulses(3,4) to ionize argon atoms and an infrared laser field, which induces a momentum shear(5) between consecutive electron wave packets. The interferograms that result from the interaction of these wave packets provide useful information about their phase. This technique opens a promising new avenue for reconstructing the wavefunctions(6,7) of atoms and molecules and for following the ultrafast dynamics of electronic wave packets.
18.	Robson, L, et al. (author) Volumetric intensity dependence on the formation of molecular and atomic ions within a high intensity laser focus 2005 In: Journal of the American Society for Mass Spectrometry. - : American Chemical Society (ACS). - 1879-1123 .- 1044-0305. ; 16:1, s. 82-89 Journal article (peer-reviewed)abstract The mechanism of atomic and molecular ionization in intense, ultra-short laser fields is a subject which continues to receive considerable attention. An inherent difficulty with techniques involving the tight focus of a laser beam is the continuous distribution of intensities contained within the focus, which can vary over several orders of magnitude. The present study adopts time of flight mass spectrometry coupled with a high intensity (8 X 10(15) Wcm(-2)), ultra-short (20 fs) pulse laser in order to investigate the ionization and dissociation of the aromatic molecule benzene-d1 (C6H5D) as a function of intensity within a focused laser beam, by scanning the laser focus in the direction of propagation, while detecting ions produced only in a "thin" slice (400 and 800 mum) of the focus. The resultant TOF mass spectra varies significantly, highlighting the dependence on the range of specific intensities accessed and their volumetric weightings on the ionization/dissociation pathways accessed.
19.	Ruchon, Thierry, et al. (author) Macroscopic effects in attosecond pulse generation 2008 In: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 10 Journal article (peer-reviewed)abstract We examine how the generation and propagation of high-order harmonics in a partly ionized gas medium affect their strength and synchronization. The temporal properties of the resulting attosecond pulses generated in long gas targets can be significantly influenced by macroscopic effects, in particular by the intensity in the medium and the degree of ionization which control the dispersion. Under some conditions, the use of gas targets longer than the absorption length can lead to the generation of compressed attosecond pulses. We show these macroscopic effects experimentally, using a 6 mm-long argon-filled gas cell as the generating medium.
20.	Sola, I. J., et al. (author) Temporal and spectral studies of high-order harmonics generated by polarization-modulated infrared fields 2006 In: Physical Review A (Atomic, Molecular and Optical Physics). - 1050-2947. ; 74:1 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.
21.	Varju, Katalin, et al. (author) Angularly resolved electron wave packet interferences 2006 In: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 39:18, s. 3983-3991 Journal article (peer-reviewed)abstract We study experimentally the ionization of argon atoms by a train of attosecond pulses in the presence of a strong infrared laser field, using a velocity map imaging technique. The recorded momentum distribution strongly depends on the delay between the attosecond pulses and the laser field. We interpret the interference patterns observed for different delays using numerical and analytical calculations within the strong field approximation.
22.	Varju, Katalin, et al. (author) Attosecond dynamics of electron wave packets in intense laser fields 2007 In: Ultrafast Optics V (Springer Series in Optical Sciences). - 0342-4111. - 9780387491172 ; 132, s. 15-24 Conference paper (peer-reviewed)abstract We use a train of sub-200 attosecond extreme ultraviolet (XUV) pulses with energies just above the ionization threshold in argon to create a train of temporally localized electron wave packets. We study the energy transfer from a strong infrared (IR) laser field to the ionized electrons as a function of the delay between the XUV and IR fields. When the wave packets are born at the zero crossings of the IR field, a significant amount of energy (∼20 eV) is transferred from the field to the electrons. This results in dramatically enhanced above-threshold ionization in conditions where the IR field alone does not induce any significant ionization. Because both the energy and duration of the wave packets can be varied independently of the IR laser, they are valuable tools for studying and controlling strong-field processes.
23.	Varju, Katalin, et al. (author) Experimental studies of attosecond pulse trains 2005 In: Laser Physics. - 1054-660X. ; 15:6, s. 888-898 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.
24.	Varju, Katalin, et al. (author) Frequency chirp of harmonic and attosecond pulses 2005 In: Journal of Modern Optics. - : Informa UK Limited. - 0950-0340 .- 1362-3044. ; 52:2-3, s. 379-394 Journal article (peer-reviewed)abstract We study the phase of the atomic polarization in the process of high-order harmonic generation. Its dependence on the laser intensity and the harmonic order induce a frequency variation in time (chirp) respectively of the harmonic pulses and attosecond pulses. We review the recent experimental results on the temporal characterization of the harmonic emission and show that measurements performed using very different techniques (like XFROG and RABITT), probing the phase in different parameter spaces, can be connected through the mixed phase derivatives, demonstrating the common underlying physics
25.	Varju, Katalin, et al. (author) Physics of attosecond pulses produced via high harmonic generation 2009 In: American Journal of Physics. - : American Association of Physics Teachers (AAPT). - 0002-9505 .- 1943-2909. ; 77:5, s. 389-395 Journal article (peer-reviewed)abstract The physics of extreme ultraviolet attosecond pulse trains generated during the interaction between an intense laser pulse and a gas medium is presented, including a simple model based on the solution of the classical equations of motion of an electron in an oscillating laser field. The reconstruction of attosecond beating by the interference of a two-photon transition is described and used to determine the pulse duration of the emitted attosecond pulses. This application is the basis of a laboratory practical for a graduate atomic physics course.

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