192 research outputs found
Phase metrology with multi-cycle two-colour pulses
Strong-field phenomena driven by an intense infrared (IR) laser depend on
during what part of the field cycle they are initiated. By changing the
sub-cycle character of the laser electric field it is possible to control such
phenomena. For long pulses, sub-cycle shaping of the field can be done by
adding a relatively weak, second harmonic of the driving field to the pulse.
Through constructive and destructive interference, the combination of strong
and weak fields can be used to change the probability of a strong-field process
being initiated at any given part of the cycle. In order to control sub-cycle
phenomena with optimal accuracy, it is necessary to know the phase difference
of the strong and the weak fields precisely. If the weaker field is an even
harmonic of the driving field, electrons ionized by the field will be
asymmetrically distributed between the positive and negative directions of the
combined fields. Information about the asymmetry can yield information about
the phase difference. A technique to measure asymmetry for few-cycle pulses,
called Stereo-ATI (Above Threshold Ionization), has been developed by [Paulus G
G, et al 2003 Phys. Rev. Lett. 91]. This paper outlines an extension of this
method to measure the phase difference between a strong IR and its second
harmonic
Attosecond Control of Ionization Dynamics
Attosecond pulses can be used to initiate and control electron dynamics on a
sub-femtosecond time scale. The first step in this process occurs when an atom
absorbs an ultraviolet photon leading to the formation of an attosecond
electron wave packet (EWP). Until now, attosecond pulses have been used to
create free EWPs in the continuum, where they quickly disperse. In this paper
we use a train of attosecond pulses, synchronized to an infrared (IR) laser
field, to create a series of EWPs that are below the ionization threshold in
helium. We show that the ionization probability then becomes a function of the
delay between the IR and attosecond fields. Calculations that reproduce the
experimental results demonstrate that this ionization control results from
interference between transiently bound EWPs created by different pulses in the
train. In this way, we are able to observe, for the first time, wave packet
interference in a strongly driven atomic system.Comment: 8 pages, 4 figure
Theory of attosecond delays in laser-assisted photoionization
We study the temporal aspects of laser-assisted extreme ultraviolet (XUV)
photoionization using attosecond pulses of harmonic radiation. The aim of this
paper is to establish the general form of the phase of the relevant transition
amplitudes and to make the connection with the time-delays that have been
recently measured in experiments. We find that the overall phase contains two
distinct types of contributions: one is expressed in terms of the phase-shifts
of the photoelectron continuum wavefunction while the other is linked to
continuum--continuum transitions induced by the infrared (IR) laser probe. Our
formalism applies to both kinds of measurements reported so far, namely the
ones using attosecond pulse trains of XUV harmonics and the others based on the
use of isolated attosecond pulses (streaking). The connection between the
phases and the time-delays is established with the help of finite difference
approximations to the energy derivatives of the phases. This makes clear that
the observed time-delays is a sum of two components: a one-photon Wigner-like
delay and an universal delay that originates from the probing process itself.Comment: 15 pages, 10 figures, special issue 'Attosecond spectroscopy' Chem.
Phy
Accessing properties of electron wave packets generated by attosecond pulse trains through time-dependent calculations
We present a time-dependent method for calculating the energy-dependent atomic dipole phase that an electron acquires when it is ionized by the absorption of a single ultraviolet photon. Our approach exactly mirrors the method used to experimentally characterize a train of attosecond pulses. In both methods the total electron phase is measured (calculated) via a two-photon interference involving the absorption or emission of an additional infrared photon in the continuum. In our calculation we use a perfect (zero spectral phase) light field and so extract the atomic dipole phase directly from the electron wave packet. We calculate the atomic phase for argon, neon, and helium at low infrared intensities and compare them to previous perturbative calculations. At moderate infrared probe intensities, we find that that the dipole phase can still be reliably determined using two-photon interference, even when higher-order processes are non-negligible. We also show that a continuum structure, in this case a Cooper minimum in argon, significantly affects the probability for infrared absorption and emission over a range of energies around the minimum, even at low infrared intensities. We conclude that well-characterized attosecond pulse trains can be used to examine continuum structures in atoms and molecules. © 2005 The American Physical Society
Phase Measurement of Resonant Two-Photon Ionization in Helium
We study resonant two-color two-photon ionization of Helium via the 1s3p 1P1
state. The first color is the 15th harmonic of a tunable titanium sapphire
laser, while the second color is the fundamental laser radiation. Our method
uses phase-locked high-order harmonics to determine the {\it phase} of the
two-photon process by interferometry. The measurement of the two-photon
ionization phase variation as a function of detuning from the resonance and
intensity of the dressing field allows us to determine the intensity dependence
of the transition energy.Comment: 4 pages, 5 figures, under consideratio
Ponderomotive shearing for spectral interferometry of extreme-ultraviolet pulses
We propose a novel method for completely characterizing ultrashort pulses at extreme-ultraviolet (XUV) wavelengths by adapting the technique of spectral phase interferometry for direct electric-field reconstruction to this spectral region. Two-electron wave packets are coherently produced by photoionizing atoms with two time-delayed replicas of the XUV pulse. For one of the XUV pulses, photoionization occurs in the presence of a strong infrared pulse that ponderomotively shifts the binding energy, thereby providing the spectral shear needed for reconstruction of the spectral phase of the XUV pulse. © 2003 Optical Society of America
Probing single-photon ionization on the attosecond time scale
We study photoionization of argon atoms excited by attosecond pulses using an
interferometric measurement technique. We measure the difference in time delays
between electrons emitted from the and from the shell, at
different excitation energies ranging from 32 to 42 eV. The determination of
single photoemission time delays requires to take into account the measurement
process, involving the interaction with a probing infrared field. This
contribution can be estimated using an universal formula and is found to
account for a substantial fraction of the measured delay.Comment: 4 pages, 4 figures, under consideratio
Spectral signature of short attosecond pulse trains
We report experimental measurements of high-order harmonic spectra generated
in Ar using a carrier-envelope-offset (CEO) stabilized 12 fs, 800nm laser field
and a fraction (less than 10%) of its second harmonic. Additional spectral
peaks are observed between the harmonic peaks, which are due to interferences
between multiple pulses in the train. The position of these peaks varies with
the CEO and their number is directly related to the number of pulses in the
train. An analytical model, as well as numerical simulations, support our
interpretation
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