360 research outputs found
Spatio-temporal characterisation of a 100 kHz 24 W sub-3-cycle NOPCPA laser system
In recent years, OPCPA and NOPCPA laser systems have shown the potential to supersede Ti:sapphire plus post-compression based laser systems to drive next generation attosecond light sources via direct amplification of few-cycle pulses to high pulse energies at high repetition rates. In this paper, we present a sub 3-cycle, 100 kHz, 24 W NOPA laser system and characterise its spatio-temporal properties using the SEA-F-SPIDER technique. Our results underline the importance of spatio-temporal diagnostics for these emerging laser systems
Tracing transient charges in expanding clusters
We study transient charges formed in methane clusters following ionization by intense near-infrared laser pulses. Cluster ionization by 400-fs (I=1Ă1014 W/cm2) pulses is highly efficient, resulting in the observation of a dominant C3+ ion contribution. The C4+ ion yield is very small but is strongly enhanced by applying a time-delayed weak near-infrared pulse. We conclude that most of the valence electrons are removed from their atoms during the laser-cluster interaction and that electrons from the nanoplasma recombine with ions and populate Rydberg states when the cluster expands, leading to a decrease of the average charge state of individual ions. Furthermore, we find clear bound-state signatures in the electron kinetic energy spectrum, which we attribute to Auger decay taking place in expanding clusters. Such nonradiative processes lead to an increase of the final average ion charge state that is measured in experiments. Our results suggest that it is crucial to include both recombination and nonradiative decay processes for the understanding of recorded ion charge spectra
Attosecond imaging
The natural timescale for electron dynamics reaches down to the attosecond
domain. Following the discovery of attosecond laser pulses, about a decade
ago, attosecond science has developed into a vibrant, new research field,
where the motion of single or multiple electrons and, in molecules, the
coupling of electronic and nuclear motion, can be investigated, on attosecond
to few-femtosecond timescales. Attosecond experiments require suitable
observables. This review describes how âattosecond imagingâ, basing itself on
kinetic energy and angle-resolved detection of photoelectrons and fragment
ions using a velocity map imaging (VMI) spectrometer, has been exploited in a
number of pumpâprobe experiments. The use of a VMI spectrometer in attosecond
experiments has allowed the characterization of attosecond pulse trains and
isolated attosecond pulses, the elucidation of continuum electron dynamics and
wave packet interferometry in atomic photoionization and the observation of
electron localization in dissociative molecular photoionization
Ion-photoelectron entanglement in photoionization with chirped laser pulses
The investigation of coherent dynamics induced by photoionization of atoms or molecules by extreme ultra-violet (XUV) attosecond laser pulses requires careful consideration of the degree of ion + photoelectron entanglement that results from the photoionization process. Here, we consider coherent H2+ vibrational dynamics induced by photoionization of neutral H2 by a chirped attosecond laser pulse. We show that chirping the attosecond laser pulse leads to ion + photoelectron entanglement and the transition from a pure to a mixed state. This transition is characterized by evaluating the purity, which is close to unity for a transform-limited attosecond laser pulse and which decreases to a value that is determined by the number of vibrational states populated in the photoionization process for increasing values of the chirp parameter. In the calculations, the vibrational dynamics is probed by calculating time-delayed dissociation of the H2+ cation by a short ultra-violet (UV) laser pulse. Independent of the magnitude of the chirp, the coherent vibrational dynamics can be recovered by recording the XUV-UV delay-dependent kinetic energy release in coincidence with the kinetic energy of the accompanying photoelectron
Numerical study of spatiotemporal distortions in noncollinear optical parametric chirped-pulse amplifiers
During amplification in a noncollinear optical parametric amplifier the spatial and temporal coordinates of the amplified field are inherently coupled. These couplings or distortions can limit the peak intensity, among other things. In this work, a numerical study of the spatiotemporal distortions in BBO-based noncollinear optical parametric chirped-pulse amplifiers (NOPCPAs) is presented for a wide range of parameters and for different amplification conditions. It is shown that for Gaussian pump beams, gain saturation introduces strong distortions and high conversion efficiency always comes at the price of strong spatiotemporal couplings which drastically reduce the peak intensity even when pulse fronts of the pump and the signal are matched. However, high conversion efficiencies with minimum spatiotemporal distortions can still be achieved with flat-top pump beam profiles
Ionization avalanching in clusters ignited by extreme-ultraviolet driven seed electrons
We study the ionization dynamics of Ar clusters exposed to ultrashort
near-infrared (NIR) laser pulses for intensities well below the threshold at
which tunnel ionization ignites nanoplasma formation. We find that the emission
of highly charged ions up to Ar can be switched on with unit contrast by
generating only a few seed electrons with an ultrashort extreme ultraviolet
(XUV) pulse prior to the NIR field. Molecular dynamics simulations can explain
the experimental observations and predict a generic scenario where efficient
heating via inverse bremsstrahlung and NIR avalanching are followed by resonant
collective nanoplasma heating. The temporally and spatially well-controlled
injection of the XUV seed electrons opens new routes for controlling
avalanching and heating phenomena in nanostructures and solids, with
implications for both fundamental and applied laser-matter science.Comment: 5 pages, 4 figure
Attosecond transient absorption spectroscopy without inversion symmetry
Transient absorption is a very powerful observable in attosecond experiments on atoms, molecules and solids and is frequently used in experiments employing phase-locked few-cycle infrared and XUV laser pulses derived from high harmonic generation. We show numerically and analytically that in non-centrosymmetric systems, such as many polyatomic molecules, which-way interference enabled by the lack of parity conservation leads to new spectral absorption features, which directly reveal the laser electric field. The extension of attosecond transient absorption spectroscopy (ATAS) to such targets hence becomes sensitive to global and local inversion symmetry. We anticipate that ATAS will find new applications in non-centrosymmetric systems, in which the carrier-to-envelope phase of the infrared pulse becomes a relevant parameter and in which the orientation of the sample and the electronic symmetry of the molecule can be addressed
Intense XUV pulses from a compact HHG setup using a single harmonic
We report on a compact and spectrally intense extreme-ultraviolet (XUV) source, which is based on high-harmonic generation (HHG) driven by 395 nm pulses. In order to minimize the XUV virtual source size and to maximize the XUV flux, HHG is performed several Rayleigh lengths away from the driving laser focal plane in a high-density gas jet. As a result, a high focused XUV intensity of 5 Ă 1013 W cmâ2 is achieved, using a beamline with a length of only two meters and a modest driving laser pulse energy of 3 mJ. The high XUV intensity is demonstrated by performing a nonlinear ionization experiment in argon, using an XUV spectrum that is dominated by a single harmonic at 22 eV. Ion charge states up to Ar3+ are observed, which requires the absorption of at least four XUV photons. The high XUV intensity and the narrow bandwidth are ideally suited for a variety of applications including photoelectron spectroscopy, the coherent control of resonant transitions and the imaging of nanoscale structures
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