Résumé. Récemment nous avons débuté des séries d'expériences sur l'interaction d'impulsions intenses VUV avec les solides luminescents. Le but principal en est l'étude des mécanismes d'interaction et de transfert d'énergie entre les excitations électroniques dans les solides à large bande interdite. L'application directe de ces études sera le développement de matériaux et de méthodes de métrologie pour les faisceaux UVX intenses. Nous présentons les résultats obtenus pour CdWO 4 qui montrent que ce cristal pourra servir comme un scintillateur pour le rayonnement VUV intense.
High-order harmonic generation (HHG) from crystals is emerging as a new ultrashort source of coherent extreme ultraviolet (XUV) light. Doping the crystal structure can offer a new way to control the source properties. Here, we present a study of HHG enhancement in the XUV spectral region from an ionic crystal, using dopant-induced vacancy defects, driven by a laser centered at a wavelength of 1.55 μm. Our numerical simulations based on solutions of the semiconductor Bloch equations and density-functional theory are supported by our experimental observations and demonstrate an increase in the XUV high harmonic yield from doped bulk magnesium oxide (MgO) compared to undoped MgO, even at a low defect concentration. The anisotropy of the harmonic emission as a function of the laser polarization shows that the pristine crystal's symmetry is preserved. Our study paves the way toward the control of HHG in solids with complex defects caused by transition-metal doping.
Résumé. Nous présentons les résultats d'études de luminescence résolue en temps émise par des cristaux sous excitation par des photons VUV issus de la génération d'harmoniques d'ordre élevé. L'utilisation d'un laser Ti-Sa PLFA à Saclay, qui délivre des impulsions d'une énergie de 13 mJ et d'une durée de 35 fs à 1 KHz, pour la génération des harmoniques dans l'Argon et la microfocalisation du faisceau VUV nous ont permis de faire varier la densité du rayonnement sur la cible dans une large gamme. Nous avons étudié l'effet de la densité d'excitation VUV sur la luminescence intrinsèque de cristaux à large bande interdite (CdWO 4 et BaF 2 ). En fonction de la densité d'excitation VUV nous avons observé les modifications des spectres d'émission et de la cinétique de la luminescence excitonique. Quand la densité d'excitation VUV croît, le rendement de la luminescence diminue et apparaissent dans la cinétique de luminescence des composantes rapides. Le profil de la courbe de déclin devient alors complexe avec une forte déviation de la loi exponentielle dans sa partie initiale.
High harmonic generation (HHG) in crystals has revealed a wealth of perspectives such as all-optical mapping of the electronic band structure, ultrafast quantum information, and the creation of all-solid-state attosecond sources. Significant efforts have been made to understand the microscopic aspects of HHG in crystals, whereas the macroscopic effects, such as non-linear propagation of the driving pulse and its impact on the HHG process, are often overlooked. In this work, we study macroscopic effects by comparing two materials with distinct optical properties, silicon (Si) and zinc oxide (ZnO). By scanning the focal position of 85 fs duration and 2.123 μm wavelength pulses inside the crystals, (Z-scan) we reveal spectral shifts in the generated harmonics. We interpret the overall blueshift of the emitted harmonic spectrum as an imprint of the spectral modulation of the driving field on the high harmonics. This process is supported with numerical simulations. This study demonstrates that through manipulation of the fundamental driving field through non-linear propagation effects, precise control of the emitted HHG spectrum in solids can be realized. This method could offer a robust way to tailor HHG spectra for a range of applications.
International audienceWe present a study on the improvement of the spatial quality of a laser beam, called modal filtering,suitable to high-energy lasers. The method is theoretically compared with the classical pinhole filtering technique in the case of an astigmatic Gaussian beam, illustrating, in this particular case, its efficiency for filtering low spatial frequencies. Experimental study of the modal filtering of a temporally chirped beam from a Ti:Sapphire chirped-pulseamplification system is presented. Beam profile, wavefront and pulse duration after compression were measured, showing a dramatic improvement of beam quality and no modifications of the temporal distribution. High-order harmonic generation in a rare gas, a highly nonlinear process which is phase-matching dependent, was used to test the effect of the filter and showed a clear enhancement of the generation
Résumé : La génération d'harmoniques d'ordre élevé dans les gaz est une source de rayonnement UVX aux propriétés uniques notamment de cohérence spatiale et de courte durée. L'étude de la cohérence temporelle de l'émission est intéressante pour les applications, mais également du point de vue fondamental car elle donne des informations sur les mécanismes de la génération. Nous montrons que la phase intrinsèque du dipôle atomique induit un « chirp » de l'émission harmonique plus ou moins important suivant les conditions de génération.
Degenerate plasmas, in which quantum effects dictate the behavior of free electrons, are ubiquitous on earth and throughout space. Transitions between bound and free electron states determine basic plasma properties, yet the effects of degeneracy on these transitions have only been theorized. Here, we use an x-ray free electron laser to create and characterize a degenerate plasma. We observe a core electron fluorescence spectrum that cannot be reproduced by models that ignore free electron degeneracy. We show that degeneracy acts to restrict the available electron energy states, thereby slowing the rate of transitions to and from the continuum. We couple degeneracy and bound electron dynamics in an existing collisional-radiative code, which agrees well with observations. The impact of the shape of the cross section, and hence the magnitude of the correction due to degeneracy, is also discussed. This study shows that degeneracy in plasmas can significantly influence experimental observables such as the emission spectra, and that these effects can be included parametrically in well-established atomic physics codes. This work narrows the gap in understanding between the condensed-matter and plasma phases, which coexist in myriad scenarios.
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