104 research outputs found
Synthesis and characterization of attosecond light vortices in the extreme ultraviolet
Infrared and visible light beams carrying orbital angular momentum (OAM) are
currently thoroughly studied for their extremely broad applicative prospects,
among which are quantum information, micromachining and diagnostic tools. Here
we extend these prospects, presenting a comprehensive study for the synthesis
and full characterization of optical vortices carrying OAM in the extreme
ultraviolet (XUV) domain. We confirm the upconversion rules of a femtosecond
infrared helically phased beam into its high-order harmonics, showing that each
harmonic order carries the total number of OAM units absorbed in the process up
to very high orders (57). This allows us to synthesize and characterize
helically shaped XUV trains of attosecond pulses. To demonstrate a typical use
of these new XUV light beams, we show our ability to generate and control,
through photoionization, attosecond electron beams carrying OAM. These
breakthroughs pave the route for the study of a series of fundamental phenomena
and the development of new ultrafast diagnosis tools using either photonic or
electronic vortices
Macroscopic effects in attosecond pulse generation
We examine how the generation and propagation of high-order harmonics in a
partly ionized gas medium affect their strength and synchronization. The
temporal properties of the resulting attosecond pulses generated in long gas
targets can be significantly influenced by macroscopic effects, in particular
by the intensity in the medium and the degree of ionization. Under some
conditions, the use of gas targets longer than the absorption length can lead
to the generation of self-compressed attosecond pulses. We show this effect
experimentally, using long argon-filled gas cells as generating medium.Comment: 5 pages 4 figure
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
Intensity Dependence of Laser-Assisted Attosecond Photoionization Spectra
We study experimentally the influence of the intensity of the infrared (IR)
probe field on attosecond pulse train (APT) phase measurements performed with
the RABITT method (Reconstruction of Attosecond Beating by Interference in
Two-Photon Transitions). We find that if a strong IR field is applied, the
attosecond pulses will appear to have lower-than-actual chirp rates. We also
observe the onset of the streaking regime in the breakdown of the weak-field
RABITT conditions. We perform a Fourier-analysis of harmonic and sideband
continuum states and show that the mutual phase relation of the harmonics can
be extracted from higher Fourier components.Comment: preprint to article in Laser Physics Nikolai B. Delone memorial issue
11 pages, 9 figures, published online 15 July 200
Molecular orbital tomography from multi-channel harmonic emission in N2
International audienceHigh-order harmonic generation in aligned molecules can be used as an ultrafast probe of molecular structure and dynamics. By characterizing the emitted signal , one can retrieve information about electronic and nuclear dynamics occurring in the molecule at the attosecond timescale. In this paper , we discuss the theoretical and experimental aspects of molecular orbital tomography in N 2 and investigate the influence of multi-channel ionization on the orbital imaging. By analyzing the spectral phase of the harmonic emission as a function of the driving laser intensity , we address two distinct cases , which in principle allow the orbital reconstruction. First , the contributions from two molecular orbitals could be disentangled in the real and imaginary parts of the measured dipole , making it possible to reconstruct both orbitals. Second , by decreasing the driving laser intensity , the transition from a multi-channel to a single-channel ionization regime is shown. The highest occupied molecular orbital may then be selected as the only one contributing efficiently to the harmonic emission. The latter approach paves the way towards the generalization of tomography to more complex systems
Coherent Electron Scattering Captured by an Attosecond Quantum Stroboscope
The basic properties of atoms, molecules and solids are governed by electron
dynamics which take place on extremely short time scales. To measure and
control these dynamics therefore requires ultrafast sources of radiation
combined with efficient detection techniques. The generation of extreme
ultraviolet (XUV) attosecond (1 as = 10-18 s) pulses has, for the first time,
made direct measurements of electron dynamics possible. Nevertheless, while
various applications of attosecond pulses have been demonstrated
experimentally, no one has yet captured or controlled the full three
dimensional motion of an electron on an attosecond time scale. Here we
demonstrate an attosecond quantum stroboscope capable of guiding and imaging
electron motion on a sub-femtosecond (1 fs = 10-15 s) time scale. It is based
on a sequence of identical attosecond pulses which are synchronized with a
guiding laser field. The pulse to pulse separation in the train is tailored to
exactly match an optical cycle of the laser field and the electron momentum
distributions are detected with a velocity map imaging spectrometer (VMIS).
This technique has enabled us to guide ionized electrons back to their parent
ion and image the scattering event. We envision that coherent electron
scattering from atoms, molecules and surfaces captured by the attosecond
quantum stroboscope will complement more traditional scattering techniques
since it provides high temporal as well as spatial resolution.Comment: 6 pages, 4 figure
Spectrally resolved multi-channel contributions to the harmonic emission in N 2
International audienceWhen generated in molecules, high-order harmonics can be emitted through different ionization channels. The coherent and ultrafast electron dynamics occurring in the ion during the generation process is directly imprinted in the harmonic signal, i.e. in its amplitude and spectral phase. In aligned N2 molecules, we find evidence for a fast variation of this phase as a function of the harmonic order when varying the driving laser intensity. Basing our analysis on a three-step model, we find that this phase variation is a signature of transitions from a single- to a multi-channel regime. In particular, we show that significant nuclear dynamics may occur in the ionization channels on the attosecond timescale, affecting both the amplitude and the phase of the harmonic signal
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