9,125 research outputs found
Quantum interference in attosecond transient absorption of laser-dressed helium atoms
We calculate the transient absorption of an isolated attosecond pulse by
helium atoms subject to a delayed infrared (\ir) laser pulse. With the central
frequency of the broad attosecond spectrum near the ionization threshold, the
absorption spectrum is strongly modulated at the sub-\ir-cycle level. Given
that the absorption spectrum results from a time-integrated measurement, we
investigate the extent to which the delay-dependence of the absorption yields
information about the attosecond dynamics of the atom-field energy exchange. We
find two configurations in which this is possible. The first involves multi
photon transitions between bound states that result in interference between
different excitation pathways. The other involves the modification of the bound
state absorption lines by the IR field, which we find can result in a sub-cycle
time dependence only when ionization limits the duration of the strong field
interaction
Influence of Phase Matching on the Cooper Minimum in Ar High Harmonic Spectra
We study the influence of phase matching on interference minima in high
harmonic spectra. We concentrate on structures in atoms due to interference of
different angular momentum channels during recombination. We use the Cooper
minimum (CM) in argon at 47 eV as a marker in the harmonic spectrum. We measure
2d harmonic spectra in argon as a function of wavelength and angular
divergence. While we identify a clear CM in the spectrum when the target gas
jet is placed after the laser focus, we find that the appearance of the CM
varies with angular divergence and can even be completely washed out when the
gas jet is placed closer to the focus. We also show that the argon CM appears
at different wavelengths in harmonic and photo-absorption spectra measured
under conditions independent of any wavelength calibration. We model the
experiment with a simulation based on coupled solutions of the time-dependent
Schr\"odinger equation and the Maxwell wave equation, including both the single
atom response and macroscopic effects of propagation. The single atom
calculations confirm that the ground state of argon can be represented by its
field free symmetry, despite the strong laser field used in high harmonic
generation. Because of this, the CM structure in the harmonic spectrum can be
described as the interference of continuum and channels, whose relative
phase jumps by at the CM energy, resulting in a minimum shifted from the
photoionization result. We also show that the full calculations reproduce the
dependence of the CM on the macroscopic conditions. We calculate simple phase
matching factors as a function of harmonic order and explain our experimental
and theoretical observation in terms of the effect of phase matching on the
shape of the harmonic spectrum. Phase matching must be taken into account to
fully understand spectral features related to HHG spectroscopy
Semi-Classical Wavefunction Perspective to High-Harmonic Generation
We introduce a semi-classical wavefunction (SCWF) model for strong-field
physics and attosecond science. When applied to high harmonic generation (HHG),
this formalism allows one to show that the natural time-domain separation of
the contribution of ionization, propagation and recollisions to the HHG process
leads to a frequency-domain factorization of the harmonic yield into these same
contributions, for any choice of atomic or molecular potential. We first derive
the factorization from the natural expression of the dipole signal in the
temporal domain by using a reference system, as in the quantitative
rescattering (QRS) formalism [J. Phys. B. 43, 122001 (2010)]. Alternatively, we
show how the trajectory component of the SCWF can be used to express the
factorization, which also allows one to attribute individual contributions to
the spectrum to the underlying trajectories
High harmonic generation from Bloch electrons in solids
We study the generation of high harmonic radiation by Bloch electrons in a
model transparent solid driven by a strong mid-infrared laser field. We solve
the single-electron time-dependent Schr\"odinger equation (TDSE) using a
velocity-gauge method [New J. Phys. 15, 013006 (2013)] that is numerically
stable as the laser intensity and number of energy bands are increased. The
resulting harmonic spectrum exhibits a primary plateau due to the coupling of
the valence band to the first conduction band, with a cutoff energy that scales
linearly with field strength and laser wavelength. We also find a weaker second
plateau due to coupling to higher-lying conduction bands, with a cutoff that is
also approximately linear in the field strength. To facilitate the analysis of
the time-frequency characteristics of the emitted harmonics, we also solve the
TDSE in a time-dependent basis set, the Houston states [Phys. Rev. B 33, 5494
(1986)], which allows us to separate inter-band and intra-band contributions to
the time-dependent current. We find that the inter-band and intra-band
contributions display very different time-frequency characteristics. We show
that solutions in these two bases are equivalent under an unitary
transformation but that, unlike the velocity gauge method, the Houston state
treatment is numerically unstable when more than a few low lying energy bands
are used
Beyond the single-atom response in absorption lineshapes: Probing a dense, laser-dressed helium gas with attosecond pulse trains
We investigate the absorption line shapes of laser-dressed atoms beyond the
single-atom response, by using extreme ultraviolet (XUV) attosecond pulse
trains to probe an optically thick helium target under the influence of a
strong infrared (IR) field. We study the interplay between the IR-induced phase
shift of the microscopic time-dependent dipole moment and the
resonant-propagation-induced reshaping of the macroscopic XUV pulse. Our
experimental and theoretical results show that as the optical depth increases,
this interplay leads initially to a broadening of the IR-modified line shape,
and subsequently to the appearance of new, narrow features in the absorption
line.Comment: 5 pages, 5 figure
Attosecond pulse shaping around a Cooper minimum
High harmonic generation (HHG) is used to measure the spectral phase of the
recombination dipole matrix element (RDM) in argon over a broad frequency range
that includes the 3p Cooper minimum (CM). The measured RDM phase agrees well
with predictions based on the scattering phases and amplitudes of the
interfering s- and d-channel contributions to the complementary photoionization
process. The reconstructed attosecond bursts that underlie the HHG process show
that the derivative of the RDM spectral phase, the group delay, does not have a
straight-forward interpretation as an emission time, in contrast to the usual
attochirp group delay. Instead, the rapid RDM phase variation caused by the CM
reshapes the attosecond bursts.Comment: 5 pages, 5 figure
Transient absorption and reshaping of ultrafast XUV light by laser-dressed helium
We present a theoretical study of transient absorption and reshaping of
extreme ultraviolet (XUV) pulses by helium atoms dressed with a moderately
strong infrared (IR) laser field. We formulate the atomic response using both
the frequency-dependent absorption cross section and a time-frequency approach
based on the time-dependent dipole induced by the light fields. The latter
approach can be used in cases when an ultrafast dressing pulse induces
transient effects, and/or when the atom exchanges energy with multiple
frequency components of the XUV field. We first characterize the dressed atom
response by calculating the frequency-dependent absorption cross section for
XUV energies between 20 and 24 eV for several dressing wavelengths between 400
and 2000 nm and intensities up to 10^12 W/cm^2. We find that for dressing
wavelengths near 1600 nm, there is an Autler-Townes splitting of the 1s ---> 2p
transition that can potentially lead to transparency for absorption of XUV
light tuned to this transition. We study the effect of this XUV transparency in
a macroscopic helium gas by incorporating the time-frequency approach into a
solution of the coupled Maxwell-Schr\"odinger equations. We find rich temporal
reshaping dynamics when a 61 fs XUV pulse resonant with the 1s ---> 2p
transition propagates through a helium gas dressed by an 11 fs, 1600 nm laser
pulse.Comment: 13 pages, 8 figures, 1 table, RevTeX4, revise
Effective Gap Equation for the Inhomogeneous LOFF Superconductive Phase
We present an approximate gap equation for different crystalline structures
of the LOFF phase of high density QCD at T=0. This equation is derived by using
an effective condensate term obtained by averaging the inhomogeneous condensate
over distances of the order of the crystal lattice size. The approximation is
expected to work better far off any second order phase transition. As a
function of the difference of the chemical potentials of the up and down
quarks, , we get that the octahedron is energetically favored from
to , where is the gap for
the homogeneous phase, while in the range the face
centered cube prevails. At a first order phase
transition to the normal phase occurs.Comment: 11 pages, 5 figure
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