149 research outputs found
Space-time description of strong-field ionization and high-order-harmonic generation
We develop the spatiotemporal description of matter-field interaction within the strong-field approximation. We show that the space-time form of the ionized wave function has analogies with the diffraction phenomenon, allowing for the definition of two different regimes: Fresnel and Fraunhofer. We demonstrate that the standard saddle-point analysis corresponds to the paraxial approximation of the Fraunhofer case. The Fresnel number therefore appears as a useful parameter to characterize the validity of the saddle-point approach. We give a closed formula for the ionized wave function beyond the standard saddle-point analysis that takes the form of a chirped Volkov wave. We apply our results to the study of high-order-harmonic generation, demonstrating that the saddle-point approximation breaks down for extended systems, i.e., when the Fresnel number approaches or is above the unity. As a simple example, we analyze the harmonic generation of dissociating H2+ and demonstrate the Fresnel number as a useful parameter to determine the accuracy of the semiclassical saddle-point approach.We acknowledge support from Spanish MINECO through the research project FIS2009-09522; Junta de Castilla y León, through the research project SA116U13; and a grant from the European Research Council (ERC-2011-AdG-291561-HELIOS)
Dynamics of the Formation of Bright Solitary Waves of Bose-Einstein Condensates in Optical Lattices
We present a detailed description of the formation of bright solitary waves
in optical lattices. To this end, we have considered a ring lattice geometry
with large radius. In this case, the ring shape does not have a relevant effect
in the local dynamics of the condensate, while offering a realistic set up to
implement experiments with conditions usually not available with linear
lattices (in particular, to study collisions). Our numerical results suggest
that the condensate radiation is the relevant dissipative process in the
relaxation towards a self-trapped solution. We show that the source of
dissipation can be attributed to the presence of higher order dispersion terms
in the effective mass approach. In addition, we demonstrate that the stability
of the solitary solutions is linked with particular values of the width of the
wavepacket in the reciprocal space. Our study suggests that these critical
widths for stability depend on the geometry of the energy band, but are
independent of the condensate parameters (momentum, atom number, etc.).
Finally, the non-solitonic nature of the solitary waves is evidenced showing
their instability under collisions.Comment: 7 pages, 7 figures, to appear in PR
Attosecond x-ray transient absorption in condensed-matter: a core-state-resolved Bloch model
Attosecond transient absorption is an ultrafast technique that has opened the possibility to study electron dynamics in condensed matter systems at its natural timescale. The extension to the x-ray regime permits one to use this powerful technique in combination with the characteristic element specificity of x-ray spectroscopy. At these timescales, the coherent effects of the electron transport are essential and have a relevant signature on the absorption spectrum. Typically, the complex light-driven dynamics requires a theoretical modeling for shedding light on the time-dependent changes in the spectrum. Here we construct a semiconductor Bloch equation model for resolving the light-induced and core-electron dynamics simultaneously, which enables to easily disentangle the interband and intraband contributions. By using the Bloch model, we demonstrate a universal feature on attosecond x-ray transient absorption spectra that emerges from the light-induced coherent intraband dynamics. This feature is linked to previous studies of light-induced Fano resonances in atomic systemsThis project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Grant Agreement No. 702565 as well as from Comunidad de Madrid through the TALENTO program with ref. 2017-T1/IND-5432. LP acknowledges support from Junta de Castilla y León (Project SA046U16) and MINECO (FIS2016-75652-P). JB acknowledges financial support from the Spanish Ministry of Economy and Competitiveness (MINECO), through the Severo Ochoa Programme for Centres of Excellence in R&D (SEV- 2015-0522) Fundació Cellex Barcelona and the CERCA Programme / Generalitat de Catalunya, the European Research Council for ERC Advanced Grant TRANSFORMER (788218), MINECO for Plan Nacional FIS2017-89536-P; AGAUR for 2017 SGR 1639 and Laserlab-Europe (EU-H2020 654148
Continuous spectra in high-harmonic generation driven by multicycle laser pulses
We present observations of the emission of XUV continua in the 20-37 eV
region by high harmonic generation (HHG) with - pulses
focused onto a Kr gas jet. The underlying mechanism relies on coherent control
of the relative delays and phases between individually generated attosecond
pulse, achievable by adjusting the chirp of the driving pulses and the
interaction geometry. Under adequate negative chirp and phase matching
conditions, the resulting interpulse interference yields a continuum XUV
spectrum, which is due to both microscopic and macroscopic (propagation)
contributions. This technique opens the route for modifying the phase of
individual attosecond pulses and for the coherent synthesis of XUV continua
from multicycle driving laser pulses without the need of an isolated attosecond
burst.Comment: 14 pages, 5 figures. Submitted to Physical Review
Dipole spectrum structure of non-resonant non-pertubative driven two-level atoms
We analize the dipole spectrum of a two-level atom excited by a non-resonant
intense monochromatic field, under the electric dipole approximation and beyond
the rotating wave approximation. We show that the apparently complex spectral
structure can be completely described by two families: harmonic frequencies of
the driving field and field-induced nonlinear fluorescence. Our formulation of
the problem provides quantitative laws for the most relevant spectral features:
harmonic ratios and phases, non-perturbative Stark shift, and frequency limits
of the harmonic plateau. In particular, we demonstrate the locking of the
harmonic phases at the wings of the plateau opening the possibility of
ultra-short pulse generation through harmonic filtering
Auger-induced charge migration
Novel perspectives of controlling molecular systems have recently arisen from the possibility of generating attosecond pulses in the ultraviolet regime and tailoring electron dynamics in its natural time scale. The cornerstone mechanism is the so-called charge migration, he production of a coherent charge transfer with subfemtosecond oscillations across a molecule. Typically, charge migration is induced by the ionization of valence molecular orbitals. However, recent technological developments allow the generation of attosecond pulses in the x-ray regime. In this case, the absorption of photons creates core-hole states. In light elements, core-hole states mainly decay by Auger processes that, driven by electron correlations, involve valence orbitals. We theoretically demonstrate in a fluoroacetylene molecule a double-hole charge migration triggered by attosecond core-electron photoionization, followed by Auger electron relaxations. This opens a new route for inducing with x rays charge transfer processes in the subfemtosecond time scaleThis project has received funding from the European Union’s Horizon 2020 research and innovation programme under Marie Sklodowska-Curie Grant Agreement No. 702565, from Comunidad de Madrid through the TALENTO program with Reference No. 2017-T1/IND-5432, and from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences through Argonne National
Laboratory under Contract No. DE-AC02-06CH11357. We acknowledge support from Junta de Castilla y León (Project No. SA046U16) and MINECO (Grant No. FIS2016-
75652-P). C.H.-G. acknowledges support from a 2017 Leonardo Grant for Researchers and Cultural Creators, BBVA Foundatio
Relativistic Doppler effect: universal spectra and zeptosecond pulses
We report on a numerical observation of the train of zeptosecond pulses
produced by reflection of a relativistically intense femtosecond laser pulse
from the oscillating boundary of an overdense plasma because of the Doppler
effect. These pulses promise to become a unique experimental and technological
tool since their length is of the order of the Bohr radius and the intensity is
extremely high W/cm. We present the physical mechanism,
analytical theory, and direct particle-in-cell simulations. We show that the
harmonic spectrum is universal: the intensity of th harmonic scales as
for , where is the largest --factor
of the electron fluid boundary, and for the broadband and
quasimonochromatic laser pulses respectively.Comment: 4 figure
Theory of high harmonic generation in relativistic laser interaction with overdense plasma
High harmonic generation due to the interaction of a short ultra relativistic
laser pulse with overdense plasma is studied analytically and numerically. On
the basis of the ultra relativistic similarity theory we show that the high
harmonic spectrum is universal, i.e. it does not depend on the interaction
details. The spectrum includes the power law part for
, followed by exponential decay. Here
is the largest relativistic -factor of the plasma
surface and is the second derivative of the surface velocity at this
moment. The high harmonic cutoff at is parametrically
larger than the predicted by the ``oscillating mirror''
model based on the Doppler effect. The cornerstone of our theory is the new
physical phenomenon: spikes in the relativistic -factor of the plasma
surface. These spikes define the high harmonic spectrum and lead to attosecond
pulses in the reflected radiation.Comment: 12 pages, 9 figure
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