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 × 1013 W cm−2 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

Low-Energy Structures in Strong Field Ionization Revealed by Quantum Orbits

Experiments on atoms in intense laser pulses and the corresponding exact ab initio solutions of the time-dependent Schr\"odinger equation (TDSE) yield photoelectron spectra with low-energy features that are not reproduced by the otherwise successful work horse of strong field laser physics: the "strong field approximation" (SFA). In the semi-classical limit, the SFA possesses an appealing interpretation in terms of interfering quantum trajectories. It is shown that a conceptually simple extension towards the inclusion of Coulomb effects yields very good agreement with exact TDSE results. Moreover, the Coulomb quantum orbits allow for a physically intuitive interpretation and detailed analysis of all low-energy features in the semi-classical regime, in particular the recently discovered "low-energy structure" [C.I. Blaga et al., Nature Physics 5, 335 (2009) and W. Quan et al., Phys. Rev. Lett. 103, 093001 (2009)].Comment: 4 pages, 3 figures, REVTe

Electron angular distributions in near-threshold atomic ionization

International audienceWe present angle- and energy-resolved measurements of photoelectrons produced in strongfield ionisation of Xe using a tunable femtosecond laser. An occurrence of highly oscillatory patterns in the angular distribution at low photoelectron kinetic energy is observed that correlates with channel closing/opening over a wide range of laser parameters. The correlation is investigated both experimentally and by means of systematic analysis of numerical solutions of the time-dependent Schrödinger equation (TDSE). Our experimental and numerical results are in quantitative agreement with the semi-classical model introduced by Arbó et al. (Phys. Rev. A 78, 013406 (2008)), which relates the oscillatory patterns to interference between photoelectrons produced during different cycles of the laser pulse in the course of non-resonant ionisation of the atom. We observe that an increase of the laser intensity eventually leads to qualitative invariance of the pattern, defining a limit on the applicability of the semi-classical model

Rare-Gas Clusters in Intense Extreme-Ultraviolet Pulses from a High-Order Harmonic Source

We report evidence for two previously unidentified effects in the ionization of rare-gas clusters by intense extreme-ultraviolet pulses. First, electron spectra indicate multistep photoemission with increasing isotropy for larger clusters due to electron-atom collisions. Second, very slow (meV) electrons are interpreted as the first experimental evidence for Rydberg-like atomic state formation in the nanoplasma expansion. Only small fractions of Xe2+ ions were found, in sharp contrast to previous results recorded under comparable conditions [Murphy et al., Phys. Rev. Lett. 101, 203401 (2008)]

Attosecond control of electron dynamics in carbon monoxide

Laser pulses with stable electric field waveforms establish the opportunity to achieve coherent control on attosecond timescales. We present experimental and theoretical results on the steering of electronic motion in a multi-electron system. A very high degree of light-waveform control over the directional emission of C+ and O+ fragments from the dissociative ionization of CO was observed. Ab initio based model calculations reveal contributions to the control related to the ionization and laser-induced population transfer between excited electronic states of CO+ during dissociation

Criteria for the observation of strong-field photoelectron holography

Photoelectron holography is studied experimentally and computationally using the ionization of ground-state xenon atoms by intense near-infrared radiation. A strong dependence of the occurrence of the holographic pattern on the laser wavelength and intensity is observed, and it is shown that the observation of the hologram requires that the ponderomotive energy Up is substantially larger than the photon energy. The holographic interference is therefore favored by longer wavelengths and higher laser intensities. Our results indicate that the tunneling regime is not a necessary condition for the observation of the holographic pattern, which can be observed under the conditions formally attributed to the multiphoton regime. © 2011 American Physical Society

High power, high repetition rate laser-based sources for attosecond science

Within the last two decades attosecond science has been established as a novel research field providing insights into the ultrafast electron dynamics that follows a photoexcitation or photoionization process. Enabled by technological advances in ultrafast laser amplifiers, attosecond science has been in turn, a powerful engine driving the development of novel sources of intense ultrafast laser pulses. This article focuses on the development of high repetition rate laser-based sources delivering high energy pulses with a duration of only a few optical cycles, for applications in attosecond science. In particular, a high power, high repetition rate optical parametric chirped pulse amplification system is described, which was developed to drive an attosecond pump-probe beamline targeting photoionization experiments with electron-ion coincidence detection at high acquisition rates