1,740 research outputs found
Phase-matched coherent hard x-rays from relativistic high-order harmonic generation
High-order harmonic generation (HHG) with relativistically strong laser
pulses is considered employing electron ionization-recollisions from multiply
charged ions in counterpropagating, linearly polarized attosecond pulse trains.
The propagation of the harmonics through the medium and the scaling of HHG into
the multi-kilo-electronvolt regime are investigated. We show that the phase
mismatch caused by the free electron background can be compensated by an
additional phase of the emitted harmonics specific to the considered setup
which depends on the delay time between the pulse trains. This renders feasible
the phase-matched emission of harmonics with photon energies of several tens of
kilo-electronvolt from an underdense plasma
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
Power, Energy, and Thermal Management for Clustered Manycores
Efficient and effective system-level power, energy, and thermal management are very important issues in modern computing systems, for which clustered architectures with multiple voltage islands are an expected compromise between global and per-core DVFS. In this dissertation, we focus on two of the most relevant problems for such architectures, specifically, optimizing performance under power/thermal constraints, and minimizing energy under performance constraints
Study of the spatial and temporal coherence of high order harmonics
We apply the theory of high-order harmonic generation by low-frequency laser
fields in the strong field approximation to the study of the spatial and
temporal coherence properties of the harmonics. We discuss the role of
dynamically induced phases of the atomic polarization in determining the
optimal phase matching conditions and angular distributions of harmonics. We
demonstrate that the phase matching and the spatial coherence can be controlled
by changing the focusing parameters of the fundamental laser beam. Then we
present a detailed study of the temporal and spectral properties of harmonics.
We discuss how the focusing conditions influence the individual harmonic
spectra and time profiles, and how the intensity dependence of the dynamically
induced phase leads to a chirp of the harmonic frequency. This phase modulation
can be used to control the temporal and spectral properties of the harmonic
radiation. Temporally, the harmonic chirped pulse can be recompressed to very
small durations. Spectrally, chirping of the fundamental beam may be employed
to compensate for the dynamically induced chirp and to control the individual
harmonic spectrum. Finally, we discuss the short pulse effects, in particular
nonadiabatic phenomena and the possibility of generating attosecond pulses.Comment: Latex file with 37 pages, 25 postscript figures. to appear in
Advances in Atomic, Molecular and Optical Physic
High-order harmonic generation from inhomogeneous fields
We present theoretical studies of high-order harmonic generation (HHG)
produced by non-homogeneous fields as resulting from the illumination of
plasmonic nanostructures with a short laser pulse. We show that both the
inhomogeneity of the local fields and the confinement of the electron movement
play an important role in the HHG process and lead to the generation of even
harmonics and a significantly increased cutoff, more pronounced for the longer
wavelengths cases studied. In order to understand and characterize the new HHG
features we employ two different approaches: the numerical solution of the time
dependent Schr\"odinger equation (TDSE) and the semiclassical approach known as
Strong Field Approximation (SFA). Both approaches predict comparable results
and show the new features, but using the semiclassical arguments behind the SFA
and time-frequency analysis tools, we are able to fully understand the reasons
of the cutoff extension.Comment: 25 pages, 12 figure
Intrapulse x-ray parametric amplification in high-order-harmonic generation
We demonstrate strong-field-driven impulsive XUV-x-ray parametric amplification (IXPA) processes in high-order harmonic generation at the single-atom level by using ab initio calculations. We consider the example of Li+ ions exposed simultaneously to an intense IR pulse and a weak 200-as XUV-x-ray pulse with central photon energies varying from 90 to 400 eV. We determine optimal parameter ranges and the precise delays between the IR and the XUV-x-ray pulses for IXPA to occur. The present results might be a guide to achieve exponential growth of the XUV-x-ray signal in tabletop XUV-x-ray lasers.Peer ReviewedPostprint (author's final draft
Intense keV isolated attosecond pulse generation by orthogonally polarized multicycle midinfrared two-color laser field
We theoretically investigate the generation of intense keV attosecond pulses
in an orthogonally polarized multicycle midinfrared two-color laser field. It
is demonstrated that multiple continuum-like humps, which have a spectral width
of about twenty orders of harmonics and an intensity of about one order higher
than adjacent normal harmonic peaks, are generated under proper two-color
delays, owing to the reduction of the number of electron-ion recollisions and
suppression of inter-half-cycle interference effect of multiple electron
trajectories when the long wavelength midinfrared driving field is used. Using
the semiclassical trajectory model, we have revealed the two-dimensional
manipulation of the electron-ion recollision process, which agrees well with
the time frequency analysis. By filtering these humps, intense isolated
attosecond pulses are directly generated without any phase compensation. Our
proposal provides a simple technique to generate intense isolated attosecond
pulses with various central photon energies covering the multi-keV spectral
regime by using multicycle driving pulses with high pump energy in experiment.Comment: 11 pages,5 figures, research articl
Saturated fluorescence measurements of the hydroxyl radical in laminar high-pressure flames
The efficacy of laser saturated fluorescence (LSF) for OH concentration measurements in high pressure flames was studied theoretically and experimentally. Using a numerical model describing the interaction of hydroxyl with nonuniform laser excitation, the effect of pressure on the validity of the balanced cross-rate model was studied along with the sensitivity of the depopulation of the laser-coupled levels to the ratio of rate coefficients describing: (1) electronic quenching to (sup 2) Sigma (+) (v double prime greater than 0), and (2) vibrational relaxation from v double prime greater than 0 to v double prime = 0. At sufficiently high pressures and near-saturated conditions, the total population of the laser-coupled levels reaches an asymptotic value, which is insensitive to the degree of saturation. When the ratio of electronic quenching to vibrational relaxation is small and the rate of coefficients for rotational transfer in the ground and excited electronic states are nearly the same, the balanced cross-rate model remains a good approximation for all pressures. When the above ratio is large, depopulation of the laser-coupled levels becomes significant at high pressures, and thus the balanced cross-rate model no longer holds. Under these conditions, however, knowledge of the depletion of the laser-coupled levels can be used to correct the model. A combustion facility for operation up to 20 atm was developed to allow LSF measurements of OH in high pressure flames. Using this facility, partial saturation in laminar high pressure (less than or equal to 12.3 atm) C2H6/O2/N2 flames was achieved. To evaluate the limits of the balanced cross-rate model, absorption and calibrated LSF measurements at 3.1 and 6.1 atm were compared. The fluorescence voltages were calibrated with absorption measurements in an atmospheric flame and corrected for their finite sensitivity to quenching with: (1) estimated quenching rate coefficients, and (2) an in situ measurement from a technique employing two fluorescence detection geometries
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