Suchergebnisse

Recientemente, el gobierno nacional radicó un proyecto de reforma financiera en el que se propone flexibilizar la regulación de los Fondos de Pensiones. En particular, se propone que los agentes pueden escoger la composición del portafolio en el que están invertidos sus ahorros pensionales. Para evaluar los posibles efectos de este cambio sobre el bienestar de los agentes, este trabajo analiza las decisiones de inversión de un individuo con función de utilidad con aversión absoluta al riesgo constante (CARA) frente a la Teoría de la diversificación del portafolio. Adicionalmente, se realiza un ejercicio contrafactual con el fin de calcular cual hubiera sido el valor del activo pensional para diferentes individuos si la legislación propuesta por el gobierno hubiera aplicado para el período 1980-2008. Este ejercicio se realiza utilizando información de las Bolsas de Valores de Colombia y la Encuesta de Calidad de Vida 2003 (ECV 2003), siguiendo la metodología de Herscovich (2003) los resultados del análisis teórico sugieren que ante un mayor valor acumulado en las cuentas de pensión, los individuos disminuyen su exposición ante el riesgo en sus portafolios. Así, la composición del portafolio debe estar más concentrada en renta variable para los agentes jóvenes y más concentrada en renta fija para los agentes viejos. Por otro lado, el ejercicio contrafactual, indica que la mejor decisión habría sido invertir todo el portafolio en activos de renta variable. Este contraste en los resultados llama la atención acerca de dos problemas: Primero, es posible que la estrategia que maximiza la utilidad ex-ante de los individuos no sea la misma que maximiza el valor de su pensión. Segundo, el ejercicio presentado parte del supuesto de que no hay información adicional que permita suponer cambios de tendencia o de volatilidad en las rentabilidades de los distintos activos financieros. No obstante, es claro que los especialistas en el mercado financiero cuentan con información suficiente para predecir este tipo de eventos. Por esta razón, el estudio sugiere que el papel de la asesoría financiera a los ahorradores es fundamental para permitir un cambio al sistema multifondos, puesto que el incremento en las opciones de inversión no conduce a un incremento en el bienestar de los individuos en ausencia de información. Adicionalmente, al comparar la evolución de las cuentas de pensión con los porcentajes históricos y con el sistema multifondos, se encuentra un mejor desempeño cuando el porcentaje de inversión en activos de renta variable es mayor que el actual, lo cual sugiere un incremento en la restricción actual de inversión de activos de renta variable para mejorar el desempeño de los fondos. ; Recently, the national government settled a financial reform project in which it was suggested the possibility to loosen the regulation of the Pension Funds. In particular, it is proposed the agents can choose the composition of the portfolio in which their pension savings are invested. In order to evaluate the possible effects of this change over the agent"s well-being, this paper analyzes the investment decisions of an individual with utility function with absolute aversion to the constant risk (CARA) facing the Portfolio Diversification Theory. Additionally, a counterfactual exercise is performed with the end of calculating the value the pension active for different individuals if the legislation proposed by the government had been applied for the 1980 - 2008 period. This exercised is performed using information from the Colombian Stock Market (Bolsa de Valores de Colombia) and the Quality of Life Survey 2003 (Encuesta de Calidad de Vida 2003 - ECV 2003), following the Herscovich Methodology (2003), the theoretical analysis" results suggest that faced with a greater cumulative valued in the pension accounts, the individual lessen their exposition due to the risk in their portfolios. This way, the composition of the portfolio must be concentrated more towards the variable incomefor the younger agents y more concentrated on fixed income for the older agents. On the other hand, the counterfactual exercise indicates that the best decision would have been to invest the entire portfolio on variable income assets. This contrast in the results draws attention to two problems: First, it is possible that the strategy that increases the ex-ante utility of the individuals is not the same as the one that increases the value of their pension. Second, the presented exercise is based on the assumption that there is no additional information that lets one assume changes in tendency or the volatility in the different financial assets" profitability. However, it is clear that the specialists in the financial market have enough information to predict this type of events. For this reason, the study suggests that the financial counseling job for the saver is fundamental for allowing a change in the multi-fund system, given that the increment in the investment options do not lead to an increase in the individual"s well-being lacking the information. In addition to this, when comparing the evolution of the pension accounts with the historic percentages and with the multi-fund system, a better performance is found when the investment percentage in the variable income assets is larger than the current, which suggests an increment in the actual investment restriction of variable income assets to improve the funds" performance.

[EN]The coherent manipulation of the electron wavefunction at the atomic spatial and temporal scales is the fundamental breakthrough underlying far-reaching ultrafast phenomena as high-order harmonic radiation and attosecond pulse generation. In this work, we present a next step in the coherent control of matter waves by translating the concept of Talbot interferometry to the subnanomenter–femtosecond realm.We study the high-harmonic emission from a periodic system irradiated by an intense mid-infrared laser beam at grazing incidence. Our calculations show that Bloch electrons, once ionized, follow a sequence of ultrafast (femtosecond) revivals associated with the temporal Talbot effect. We demonstrate that these revivals leave a distinct signature in the high-frequency harmonic spectrum, in the form of structures extending beyond the main spectral cutoff, toward the x-rays. The reinterpretation of the process of high-order harmonic generation as the temporal realization of a Talbot–Lau interferometer suggests high-harmonic spectroscopy as an appropriate scheme to develop subnanometer ultrafast Talbot interferometry. ; We acknowledge support Ministerio de Ciencia e Innovación (PID2019-106910GB-I00). This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant Agreement No. 851201). We also acknowledge support from Junta de Castilla y León FEDER funds (Project No. SA287P18). AG-C acknowledges support from Ministerio de Educación, Cultura y Deporte (FPU18/03348). CH-G acknowledges Ministerio de Ciencia, Innovación, y Universidades for a Ramón y Cajal contract (RYC-2017-22745), co-funded by the European. Social Fund.

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).

Datos de investigación en: http://hdl.handle.net/10366/148373 ; Few-electron atoms interacting with electromagnetic fields provide for privileged scenarios for disentangling elemental correlation mechanisms. We demonstrate that high-order harmonic generation (HHG) from neutral helium presents a distinctive trace of correlation back reaction in the electron dynamics prior to ionization. We identify a mechanism in which the field interacts with one of the electrons, while the second is excited to a Rydberg level through the Coulomb interaction. As back response to this correlated excitation, the first electron is knocked down to a lower level, from which it ionizes to generate high-order harmonics. Unlike other multielectron phenomena, where the electron-field interaction is modified by cross correlations with the rest of the interacting electrons, back reaction is a mechanism of autocorrelation. Our numerical simulations reveal back reaction as a relevant aspect in HHG from correlated two-electron systems, and paves the way to disentangle valence electron-electron correlation dynamics using high-harmonic spectroscopy. ; We acknowledge fruitful discussions with Tenio Popmintchev. We also acknowledge support from Junta de Castilla y León FEDER funds (Project No. SA287P18) and Ministerio de Ciencia e Innovación (FIS2016-75652-P and PID2019-106910GB-I00). 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. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant Agreement No. 851201).

[EN]The efficiency of high-harmonic generation (HHG) from a macroscopic sample is strongly linked to the proper phase matching of the contributions from the microscopic emitters. We develop a combined micro+macroscopic theoretical model that allows us to distinguish the relevance of high-order harmonic phase matching in single-layer graphene. For a Gaussian driving beam, our simulations show that the relevant HHG emission is spatially constrained to a phase-matched ring around the beam axis. This remarkable finding is a direct consequence of the non-perturbative behavior of HHG in graphene—whose harmonic efficiency scaling is similar to that already observed in gases— and bridges the gap between the microscopic and macroscopic HHG in single-layer graphene. ; This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 851201). We thankfully acknowledge the computer resources at MareNostrum and the technical support provided by Barcelona Supercomputing Center (FI-2020-3-0013). Junta de Castilla y León and FEDER (SA287P18); European Research Council (851201); Ministerio de Ciencia, Innovación y Universidades (FIS2016-75652-P, RYC-2017-22745, PID2019-106910GB-I00).

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.

In this paper, we review the development and application of coherent short wavelength light sources implemented using the high harmonic generation (HHG) process. The physics underlying HHG brought quantum physics into the domain of attosecond time-scales for the first time. The observation and manipulation of electron dynamics on such short time-scales—a capability not conceived—of just a few decades ago—is becoming both more-and-more sophisticated and useful as a route to achieve exquisite control over short wavelength light. New experimental techniques are enabling HHG light sources to provide new insights into fundamental quantum interactions in materials, making it possible for the first time to capture the fastest charge, spin, photon and phonon interactions and to achieve diffraction-limited imaging at short wavelengths. ; We gratefully acknowledge support from the National Science Foundation through the JILA Physics Frontiers Center PHY-1734006, and a Gordon and Betty Moore Foundation EPiQS Award GBMF4538, for the attosecond ARPES setups and methods. We gratefully acknowledge support from the Department of Energy Office of Basic Energy Sciences XRay Scattering Program Award DE-SC0002002 for the spin dynamics setups and methods. We gratefully acknowledge support from the Department of EnergyOffice of Basic Energy Sciences AMOS Award DE-FG02-99ER14982 (experiment) as well as an MURI Grant from the Air Force Office of Scientific Research under Award Number FA9550-16-1-0121 (theory) for the HHG light source setups and methods. The authors gratefully acknowledge support from the STROBE National Science Foundation Science and Technology Center, Grant No. DMR-1548924, for the imaging setups and methods. C H-G acknowledges support by Ministerio de Ciencia, Innovación y Universidades for projects (FIS2016-75652-P and PID2019-106910GB-I00) and a Ramon y Cajal contract (RYC-2017-22745), co-funded by the European Social Fund; Junta de Castilla y León and European Union, FEDER, (Project SA287P18); and from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant agreement No. 851201). ZT gratefully acknowledges the financial support from the National Natural Science Foundation of China (Grant No.11874121), the Shanghai Municipal Science and Technology Basic Research Project (Grant No. 19JC1410900) and the Alexander-von-Humboldt foundation.

[EN]The use of structured ultrashort pulses with coupled spatiotemporal properties is emerging as a key tool for ultrafast manipulation. Ultrafast vector beams are opening exciting opportunities in different fields such as microscopy, time-resolved imaging, nonlinear optics, particle acceleration or attosecond science. Here, we implement a technique for the full characterization of structured time-dependent polarization light waveforms with spatiotemporal resolution, using a compact twofold spectral interferometer, based on in-line bulk interferometry and fibre-optic coupler assisted interferometry. We measure structured infrared femtosecond vector beams, including radially polarized beams and complex-shaped beams exhibiting both temporal and spatial evolving polarization. Our measurements confirm that light waveforms with polarization evolving at the micrometer and femtosecond scales can be achieved through the use of structured waveplates and polarization gates. This new scale of measurement achieved will open the way to predict, check and optimize applications of structured vector beams at the femtosecond and micrometer scales. ; We acknowledge funding from Junta de Castilla y León (SA287P18) and FEDER Funds, and from Spanish Ministerio de Economía y Competitividad (FIS2016-75652-P, FIS2017-87970-R, EQC2018-004117-P). C.H.-G. acknowledges support Ministerio de Ciencia, Innovación y Universidades for a Ramón y Cajal contract (RYC-2017-22745), cofunded by the European Social Fund. B.A. acknowledges funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Individual Fellowship grant agreement No. 798264. PGK acknowledges support of ERC project ENIGMA. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 851201).

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).

The extreme nonlinear optical process of high-harmonic generation (HHG) makes it possible to map the properties of a laser beam onto a radiating electron wave function and, in turn, onto the emitted x-ray light. Bright HHG beams typically emerge from a longitudinal phased distribution of atomic-scale quantum antennae. Here, we form a transverse necklace-shaped phased array of linearly polarized HHG emitters, where orbital angular momentum conservation allows us to tune the line spacing and divergence properties of extreme ultraviolet and soft x-ray high-harmonic combs. The on-axis HHG emission has extremely low divergence, well below that obtained when using Gaussian driving beams, which further decreases with harmonic order. This work provides a new degree of freedom for the design of harmonic combs—particularly in the soft x-ray regime, where very limited options are available. Such harmonic beams can enable more sensitive probes of the fastest correlated charge and spin dynamics in molecules, nanoparticles, and materials. ; The JILA team graciously acknowledges support from the Department of Energy BES Award No. DE-FG02-99ER14982 for the experimental implementation, a MURI grant from the Air Force Office of Scientific Research under Award No. FA9550-16-1-0121 for the mid-infrared laser soft x-ray research, and a National Science Foundation Physics Frontier Center grant PHY-1734006 for theory. N.J.B. acknowledges support from National Science FoundationGraduate Research Fellowships (grant no. DGE-1650115). Q.L.D.N. acknowledges support from National Science Foundation Graduate Research Fellowships (grant no. DGE-1144083). J.S.R., L.P., and C.H.-G. acknowledge support from Ministerio de Ciencia e Innovación (FIS2016-75652-Pand PID2019-106910GB-I00). This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement no. 851201). J.S.R., L.P., and C.H.-G. also acknowledge support from Junta de Castilla y León FEDER funds (project no. SA287P18). L.R. acknowledges support from Ministerio de Educación, Cultura y Deporte (FPU16/02591). C.H.-G. acknowledges Ministerio de Ciencia, Innovación, y Universidades for Ramón y Cajal contract (RYC-2017-22745), cofunded by the European Social Fund. L.R., J.S.R., L.P., and C.H.-G. thankfully acknowledge the computer resources at MareNostrum and the technical support provided by Barcelona Supercomputing Center (FI-2020-3-0013).

Bright, circularly polarized, extreme ultraviolet (EUV) and soft x-ray high-harmonic beams can now be produced using counter-rotating circularly polarized driving laser fields. Although the resulting circularly polarized harmonics consist of relatively simple pairs of peaks in the spectral domain, in the time domain, the field is predicted to emerge as a complex series of rotating linearly polarized bursts, varying rapidly in amplitude, frequency, and polarization. We extend attosecond metrology techniques to circularly polarized light by simultaneously irradiating a copper surface with circularly polarized high-harmonic and linearly polarized infrared laser fields. The resulting temporal modulation of the photoelectron spectra carries essential phase information about the EUV field. Utilizing the polarization selectivity of the solid surface and by rotating the circularly polarized EUV field in space, we fully retrieve the amplitude and phase of the circularly polarized harmonics, allowing us to reconstruct one of the most complex coherent light fields produced to date. ; This work was done at JILA. We gratefully acknowledge support from the NSF through the Physics Frontiers Centers Program with grant no. PHY1125844 and the Gordon and Betty Moore Foundation EPiQS (Emergent Phenomena in Quantum Systems) Initiative through Grant GBMF4538 to M.M. C.H.-G. acknowledges support from the Marie Curie International Outgoing Fellowship within the European Union Seventh Framework Programme for Research and Technological Development (2007–2013), under Research Executive Agency grant agreement no. 328334. R.K. acknowledges the Swedish Research Council (VR) for financial support. A.J.-B. was supported by grants from the U.S. NSF (grant nos. PHY-1125844 and PHY-1068706). C.H.-G. and L.P. acknowledge support from Junta de Castilla y León (project SA116U13) and MINECO (Ministerio de Econom a y Competitividad) (FIS2013-44174-P and FIS2015-71933-REDT). This work used the Janus supercomputer, which is supported by the U.S. NSF (grant no. CNS-0821794) and the University of Colorado, Boulder. P.G. acknowledges support from the Deutsche Forschungsgemeinschaft (no. GR 4234/1-1).

We demonstrate, to our knowledge, the first bright circularly polarized high-harmonic beams in the soft X-ray region of the electromagnetic spectrum, and use them to implement X-ray magnetic circular dichroism measurements in a tabletop-scale setup. Using counterrotating circularly polarized laser fields at 1.3 and 0.79 µm, we generate circularly polarized harmonics with photon energies exceeding 160 eV. The harmonic spectra emerge as a sequence of closely spaced pairs of left and right circularly polarized peaks, with energies determined by conservation of energy and spin angular momentum. We explain the single-atom and macroscopic physics by identifying the dominant electron quantum trajectories and optimal phase-matching conditions. The first advanced phase-matched propagation simulations for circularly polarized harmonics reveal the influence of the finite phase-matching temporal window on the spectrum, as well as the unique polarization-shaped attosecond pulse train. Finally, we use, to our knowledge, the first tabletop X-ray magnetic circular dichroism measurements at the N4,5 absorption edges of Gd to validate the high degree of circularity, brightness, and stability of this light source. These results demonstrate the feasibility of manipulating the polarization, spectrum, and temporal shape of high harmonics in the soft X-ray region by manipulating the driving laser waveform. ; The authors thank Wilhelm Becker and Luis Plaja for useful discussions. Support for this work was provided by the Department of Energy (DOE) Office of Basic Energy Sciences X-Ray Scattering Program and the National Science Foundation (NSF) Physics Frontier Center Program Grant PHY-1125844 (to T.F., P.G., R.K., D.D.H., D.Z., C.G., F.J.D., C.A.M., C.W.H., J.L.E., K.M.D., C.C., T.P., A.B., H.C.K. and M.M.M.); NSF Graduate Research Fellowship DGE-1144083 (to J.L.E.); Marie Curie International Outgoing Fellowship within the European Union (EU) Seventh Framework Program for Research and Technological Development (2007–2013), under REA Grant 328334 (to C.H.-G.); Junta de Castilla y León Project SA116U13, UIC016 (to C.H.-G.); MINECO Grant FIS2013-44174-P (to C.H.-G.); US NSF Grants PHY- 1125844 and PHY-1068706 (to A.A.J.-B.); Deutsche Forschungsgemeinschaft Grant GR 4234/1-1 (to P.G.); Swedish Research Council (R.K. and P.M.O.); EU Seventh Framework Programme Grant 281043, FemtoSpin (to K.C. and P.M.O.); Czech Science Foundation Grant 15-08740Y (to K.C.); IT4Innovations Centre of Excellence Project CZ.1.05/1.1.00/02.0070 funded by the European Regional Development Fund and the national budget of the Czech Republic Project Large Research, Development and Innovations Infrastructures LM2011033 (to D.L.); US DOE, Office of Science, Office of Basic Energy Sciences Contract DE-SC0001805 (to O.G.S.); and US DOE Office of Basic Energy Sciences Award DE-SC0003678 (to E.E.F.). This work used the Janus supercomputer, which is supported by US NSF Award CNS-0821794 and the University of Colorado, Boulder.