65 research outputs found

    Tracking ultrafast solid-state dynamics using high harmonic spectroscopy

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    We establish time-resolved high harmonic generation (tr-HHG) as a powerful spectroscopy for photoinduced dynamics in strongly correlated materials through a detailed investigation of the insulator-to-metal transitions in vanadium dioxide. We benchmark our technique by comparing our measurements to established momentum-resolved ultrafast electron diffraction, and theoretical density functional calculations. Tr-HHG allows distinguishing of individual dynamic channels, including a transition to a thermodynamically hidden phase. In addition, the HHG yield is shown to be modulated at a frequency characteristic of a coherent phonon in the equilibrium monoclinic phase over a wide range of excitation fluences. These results demonstrate that tr-HHG is capable of tracking complex dynamics in solids through its sensitivity to the band structure.Comment: 20 pages and 4 figures main text, 8 pages and 4 figures supplementary informatio

    Measurement of the charge asymmetry in top-quark pair production in the lepton-plus-jets final state in pp collision data at s=8 TeV\sqrt{s}=8\,\mathrm TeV{} with the ATLAS detector

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    ATLAS Run 1 searches for direct pair production of third-generation squarks at the Large Hadron Collider

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    Miroirs et rĂ©seaux plasmas en champs lasers ultra-intenses : gĂ©nĂ©ration d’harmoniques d’ordre Ă©levĂ© et de faisceaux d’électrons relativistes

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    When focusing an ultra-intense femtosecond laser pulse [I>10Âč⁶W/cmÂČ] onto a solid target, this target is ionized at the very beginning of the laser pulse. The resulting dense plasma then reflects the laser in the specular direction: it is a plasma mirror. The ultra-intense laser field can accelerate electrons within the plasma at relativistic speeds. Some are ejected towards the vacuum and these plasma mirrors are therefore sources of relativistic electron beams. Moreover, at each optical cycle they radiate in the form of extreme ultraviolet light, resulting in the generation of high-order harmonics of the laser frequency (HHG). The objective of this PhD is to understand laser-plasma interaction though the characterization of high-order harmonics and relativistic electron beams generated from plasma mirrors. The first part deals with harmonic beam measurement. Due to the extreme physical conditions during the interaction, detection can only be performed at macroscopic distance from target. Thus, the characterization of the harmonic beams’ angular properties (carried out as a function of interaction conditions in previous works) only provides partial information on the interaction itself. A technique of coherent diffraction imaging, named ptychography, which consists of diffracting a probe onto an object, is transposed to HHG on plasma mirrors by optically micro-structuring the plasma on a target surface. Harmonic fields are then reconstructed spatially in amplitude and phase directly in the target plane. Thanks to this measurement in different interaction conditions, previously developed theoretical analytical models in non-relativistic regime [I10Âč⁞W/cmÂČ] are experimentally validated. The second part of the PhD is dedicated to the experimental characterization of angular and spectral properties of relativistic electron beams. A theoretical and numerical study shows that this constitutes the first clear observation of vacuum laser acceleration (VLA). Finally, a simultaneous study of harmonic and electron signals highlights a strong correlation between both processes in the relativistic regime.Lors de la focalisation d’un laser femtoseconde ultra-intense [I>10Âč⁶W/cmÂČ] sur une cible solide, dĂšs le dĂ©but de l’impulsion le champ laser est suffisant pour totalement ioniser la surface de la cible. Le reste de l’impulsion est ensuite rĂ©flĂ©chi dans la direction spĂ©culaire par le plasma dense ainsi crĂ©Ă© : c’est un miroir plasma. Le champ laser ultra-intense peut accĂ©lĂ©rer les Ă©lectrons au sein du plasma Ă  des vitesses relativistes. Certains sont Ă©jectĂ©s vers le vide et ces miroirs plasmas sont ainsi des sources de faisceaux d’électrons Ă©nergĂ©tiques. De plus, ils rayonnent dans l’extrĂȘme ultra-violet (XUV) Ă  chaque pĂ©riode laser, ce qui se traduit par de la gĂ©nĂ©ration d’harmoniques d’ordre Ă©levĂ© de la pulsation laser. L’objectif de cette thĂšse est de mieux comprendre l’interaction laser-plasma sur miroirs plasmas Ă  l’aide de la caractĂ©risation de ces deux observables physiques qui en sont issues : les faisceaux d’électrons relativistes et les faisceaux d’harmoniques d’ordre Ă©levĂ©. Une premiĂšre partie traite de la mesure des faisceaux harmoniques. Du fait des conditions physiques extrĂȘmes d’interaction, la dĂ©tection ne peut se faire qu’à une distance macroscopique de la cible. Ainsi la caractĂ©risation des propriĂ©tĂ©s angulaires de ces faisceaux (rĂ©alisĂ©e en fonction des conditions d’interaction au cours de travaux prĂ©cĂ©dents) ne fournit que des informations partielles sur l’interaction en elle-mĂȘme. La ptychographie, une technique de mesure par diffraction cohĂ©rente oĂč une sonde est diffractĂ©e par un objet, est ici transposĂ©e Ă  la gĂ©nĂ©ration d’harmoniques sur miroirs plasmas grĂące Ă  la micro-structuration optique du plasma Ă  la surface de la cible. Les champs sources harmoniques sont ainsi reconstruits en amplitude et en phase spatiales directement dans le plan cible. GrĂące Ă  ces mesures dans diffĂ©rentes conditions d’interaction, des modĂšles thĂ©oriques analytiques d’interaction en rĂ©gime non relativiste [I10Âč⁞W/cmÂČ] dĂ©veloppĂ©s prĂ©cĂ©demment sont validĂ©s expĂ©rimentalement. Une seconde partie de cette thĂšse est consacrĂ©e Ă  l’étude expĂ©rimentale des propriĂ©tĂ©s angulaires et en Ă©nergie des faisceaux d’électrons relativistes issus des miroirs plasmas. Une Ă©tude thĂ©orique et numĂ©rique, permet de prouver que ces mesures sont la premiĂšre observation claire de l’accĂ©lĂ©ration d’électrons relativistes par laser dans le vide (VLA). Enfin, l’étude simultanĂ©e des efficacitĂ©s de gĂ©nĂ©ration des faisceaux d’électrons et d’harmoniques montre une corrĂ©lation nette entre les deux processus en rĂ©gime relativiste

    Plasma mirrors and gratings under ultra-intense laser illumination : generation of high-order harmonic and relativistic electron beams

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    Lors de la focalisation d’un laser femtoseconde ultra-intense [I>10Âč⁶W/cmÂČ] sur une cible solide, dĂšs le dĂ©but de l’impulsion le champ laser est suffisant pour totalement ioniser la surface de la cible. Le reste de l’impulsion est ensuite rĂ©flĂ©chi dans la direction spĂ©culaire par le plasma dense ainsi crĂ©Ă© : c’est un miroir plasma. Le champ laser ultra-intense peut accĂ©lĂ©rer les Ă©lectrons au sein du plasma Ă  des vitesses relativistes. Certains sont Ă©jectĂ©s vers le vide et ces miroirs plasmas sont ainsi des sources de faisceaux d’électrons Ă©nergĂ©tiques. De plus, ils rayonnent dans l’extrĂȘme ultra-violet (XUV) Ă  chaque pĂ©riode laser, ce qui se traduit par de la gĂ©nĂ©ration d’harmoniques d’ordre Ă©levĂ© de la pulsation laser. L’objectif de cette thĂšse est de mieux comprendre l’interaction laser-plasma sur miroirs plasmas Ă  l’aide de la caractĂ©risation de ces deux observables physiques qui en sont issues : les faisceaux d’électrons relativistes et les faisceaux d’harmoniques d’ordre Ă©levĂ©. Une premiĂšre partie traite de la mesure des faisceaux harmoniques. Du fait des conditions physiques extrĂȘmes d’interaction, la dĂ©tection ne peut se faire qu’à une distance macroscopique de la cible. Ainsi la caractĂ©risation des propriĂ©tĂ©s angulaires de ces faisceaux (rĂ©alisĂ©e en fonction des conditions d’interaction au cours de travaux prĂ©cĂ©dents) ne fournit que des informations partielles sur l’interaction en elle-mĂȘme. La ptychographie, une technique de mesure par diffraction cohĂ©rente oĂč une sonde est diffractĂ©e par un objet, est ici transposĂ©e Ă  la gĂ©nĂ©ration d’harmoniques sur miroirs plasmas grĂące Ă  la micro-structuration optique du plasma Ă  la surface de la cible. Les champs sources harmoniques sont ainsi reconstruits en amplitude et en phase spatiales directement dans le plan cible. GrĂące Ă  ces mesures dans diffĂ©rentes conditions d’interaction, des modĂšles thĂ©oriques analytiques d’interaction en rĂ©gime non relativiste [I10Âč⁞W/cmÂČ] dĂ©veloppĂ©s prĂ©cĂ©demment sont validĂ©s expĂ©rimentalement. Une seconde partie de cette thĂšse est consacrĂ©e Ă  l’étude expĂ©rimentale des propriĂ©tĂ©s angulaires et en Ă©nergie des faisceaux d’électrons relativistes issus des miroirs plasmas. Une Ă©tude thĂ©orique et numĂ©rique, permet de prouver que ces mesures sont la premiĂšre observation claire de l’accĂ©lĂ©ration d’électrons relativistes par laser dans le vide (VLA). Enfin, l’étude simultanĂ©e des efficacitĂ©s de gĂ©nĂ©ration des faisceaux d’électrons et d’harmoniques montre une corrĂ©lation nette entre les deux processus en rĂ©gime relativiste.When focusing an ultra-intense femtosecond laser pulse [I>10Âč⁶W/cmÂČ] onto a solid target, this target is ionized at the very beginning of the laser pulse. The resulting dense plasma then reflects the laser in the specular direction: it is a plasma mirror. The ultra-intense laser field can accelerate electrons within the plasma at relativistic speeds. Some are ejected towards the vacuum and these plasma mirrors are therefore sources of relativistic electron beams. Moreover, at each optical cycle they radiate in the form of extreme ultraviolet light, resulting in the generation of high-order harmonics of the laser frequency (HHG). The objective of this PhD is to understand laser-plasma interaction though the characterization of high-order harmonics and relativistic electron beams generated from plasma mirrors. The first part deals with harmonic beam measurement. Due to the extreme physical conditions during the interaction, detection can only be performed at macroscopic distance from target. Thus, the characterization of the harmonic beams’ angular properties (carried out as a function of interaction conditions in previous works) only provides partial information on the interaction itself. A technique of coherent diffraction imaging, named ptychography, which consists of diffracting a probe onto an object, is transposed to HHG on plasma mirrors by optically micro-structuring the plasma on a target surface. Harmonic fields are then reconstructed spatially in amplitude and phase directly in the target plane. Thanks to this measurement in different interaction conditions, previously developed theoretical analytical models in non-relativistic regime [I10Âč⁞W/cmÂČ] are experimentally validated. The second part of the PhD is dedicated to the experimental characterization of angular and spectral properties of relativistic electron beams. A theoretical and numerical study shows that this constitutes the first clear observation of vacuum laser acceleration (VLA). Finally, a simultaneous study of harmonic and electron signals highlights a strong correlation between both processes in the relativistic regime

    Spatio-temporal characterization of attosecond pulses from plasma mirrors

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    International audienceReaching light intensities above 1025 W cm−2 and up to the Schwinger limit of order 1029 W cm−2 would enable the testing of fundamental predictions of quantum electrodynamics. A promising—yet challenging—approach to achieve such extreme fields consists in reflecting a high-power femtosecond laser pulse off a curved relativistic mirror. This enhances the intensity of the reflected beam by simultaneously compressing it in time down to the attosecond range, and focusing it to submicrometre focal spots. Here we show that such curved relativistic mirrors can be produced when an ultra-intense laser pulse ionizes a solid target and creates a dense plasma that specularly reflects the incident light. This is evidenced by measuring the temporal and spatial effects induced on the reflected beam by this so-called plasma mirror. The all-optical measurement technique demonstrated here will be instrumental for the use of relativistic plasma mirrors with the upcoming generation of petawatt lasers that recently reached intensities of 5 × 1022 W cm−2, and therefore constitutes a viable experimental path to the Schwinger limit
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