Suchergebnisse

X-ray spectroscopy is an important tool for the investigation of matter. X rays primarily interact with inner-shell electrons, creating core (inner-shell) holes that will decay on the time scale of attoseconds to a few femtoseconds through electron relaxations involving the emission of a photon or an electron. The advent of femtosecond x-ray pulses expands x-ray spectroscopy to the time domain and will eventually allow the control of core-hole population on time scales comparable to core-vacancy lifetimes. For both cases, a theoretical approach that accounts for the x-ray interaction while the electron relaxations occur is required. Here we describe a time-dependent framework, based on solving the time-dependent Schrödinger equation, that is suitable for describing the induced electron and nuclear dynamics. ; A.P. is grateful to C. Bostedt for inspiring this project, scientific discussions, and his support. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division and supported by the Argonne group under Contract No. DE-AC02-06CH11357. A.P. also acknowledges support from the European Union's Horizon 2020 research and innovation program under Marie Sklodowska-Curie Grant Agreement No. 702565.

[EN]We study the high-harmonic spectrum emitted by a single-layer graphene, irradiated by an ultrashort intense infrared laser pulse. We show the emergence of the typical non-perturbative spectral features, harmonic plateau and cut-off, for mid-infrared driving fields, at fluences below the damage threshold. In contrast to previous works, using THz drivings, we demonstrate that the harmonic cut-off frequency saturates with the intensity. Our results are derived from the numerical integration of the time-dependent Schrödinger equation using a nearest neighbor tight-binding description of graphene. We also develop a saddle-point analysis that reveals a mechanism for harmonic emission in graphene different from that reported in atoms, molecules and finite gap solids. In graphene, the first step is initiated by the non-diabatic crossing of the valence band electron trajectories through the Dirac points, instead of tunneling ionization/excitation. We include a complete identification of the trajectories contributing to any particular high harmonic and reproduce the harmonic cut-off scaling with the driving intensity. ; We acknowledge fruitful discussions with I J Sola, H Crespo, E Pinsanty, J Biegert, J M Pérez-Iglesias, R Rengel, M J Martín and C Hernández García. We acknowledge support from Junta de Castilla y León (Project SA046U16) and MINECO (FIS2013-44174-P, FIS2016-75652-P) and European Union (FEDER). AP acknowledges support from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 702565

We demonstrate that the standard picture of strong-field tunnel-ionization from molecules should be reformulated. The extended nature of the molecular potential implies the separation of some of the molecular sites from the edge of the ionization barrier. We show that the dependence of the tunnel probability with the distance to the barrier is translated into the ionized wavepacket, modifying substantially the high-order harmonic emission. The introduction of the dependence of tunnel ionization with the molecular site significantly improves the theoretical description of high-order harmonic generation in molecules, which is used as a cornerstone in high-harmonic spectroscopy and attosecond imaging. ; C H-G and L P acknowledge support from Junta de Castilla y León and European Union, FEDER (Project SA287P18), Spanish MICINN (FIS2016-75652 P). C H-G acknowledges Ministerio de Ciencia, Innovación y Universidades for a Ramón y Cajal contract (RYC-2017-22745), co-funded by the European Social Fund. L R acknowledges support from Ministerio de Educación, Cultura y Deporte (FPU16/02591). A P acknowledges support from Comunidad de Madrid through the TALENTO program with Reference No. 2017-T1/IND-5432. We thankfully acknowledge the computer resources at MareNostrum and the technical support provided by Barcelona Supercomputing Center (FI-2019-1-0013).

We investigate theoretically the generation of extreme-ultraviolet (EUV) beams carrying fractional orbital angular momentum. To this end, we drive high-order harmonic generation with infrared conical refraction (CR) beams. We show that the high-order harmonic beams emitted in the EUV/soft x-ray regime preserve the characteristic signatures of the driving beam, namely ringlike transverse intensity profile and CR-like polarization distribution. As a result, through orbital and spin angular momentum conservation, harmonic beams are emitted with fractional orbital angular momentum, and they can be synthesized into structured attosecond helical beams –or "structured attosecond light springs"– with rotating linear polarization along the azimuth. Our proposal overcomes the state of the art limitations for the generation of light beams far from the visible domain carrying non-integer orbital angular momentum and could be applied in fields such as diffraction imaging, EUV lithography, particle trapping, and super-resolution imaging. ; We acknowledge Prof. Luis Plaja and Prof. Jordi Mompart for fruitful discussions. C.H.-G. acknowledges support from the Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007–2013), under REA grant Agreement No. 328334. C.H.-G., and J.S.-R. acknowledge support from Junta de Castilla y León (Projects SA116U13, SA046U16) and MINECO (FIS2013-44174-P, FIS2016-75652-P). A.T. acknowledges support from the MECD (AP2010-2310), the MINECO (FIS2011-23719), the Catalan government (SGR2014-1639), and the Max Planck Society. A.P. acknowledges support from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 702565.

Vortex light beams are structures of the electromagnetic field with a spiral phase ramp around a point-phase singularity. These vortices have many applications in the optical regime, ranging from optical trapping and quantum information to spectroscopy and microscopy. The extension of vortices into the extreme-ultraviolet (XUV)/X-ray regime constitutes a significant step forward to bring those applications to the nanometer or even atomic scale. The recent development of a new generation of X-ray sources, and the refinement of other techniques, such as harmonic generation, have boosted the interest of producing vortex beams at short wavelengths. In this manuscript, we review the recent studies in the subject, and we collect the major prospects of this emerging field. We also focus on the unique and promising applications of ultrashort XUV/X-ray vortex pulses ; A.P. acknowledges support from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 702565. C.H.-G. acknowledges support from the Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007-2013), under REA grant Agreement No 328334. We acknowledge support and from Junta de Castilla y León (Project SA046U16) and MINECO (FIS2013-44174-P, FIS2015-71933-REDT, FIS2016-75652-P).