Subcycle Dynamics in the Laser Ionization of Molecules

The time and momentum distributions of electron emission from a molecule during a single laser cycle are calculated by solving a two-dimensional time-dependent Schr{umlt o}dinger equation. The momentum distributions strongly depend on the orbital symmetry and orientation of the molecular axis. Field-induced internal dynamics of the molecule can shift electron emission and recollision times through a large part of the laser cycle, which leads to corresponding variations of high-harmonic emission times and to the appearance of even harmonics

Attosecond chirp-encoded dynamics of light nuclei Attosecond chirp-encoded dynamics of light nuclei

International audienceWe study the spectral phase of high-order harmonic emission as an observable for probing ultrafast nuclear dynamics after the ionization of a molecule. Using a strong-field approximation theory that includes nuclear dynamics, we relate the harmonic phase to the phase of the overlap integral of the nuclear wavefunctions of the initial neutral molecule and the molecular ion after an attosecond probe delay. We determine experimentally the group delay of the high harmonic emission from D 2 and H 2 molecules, which allows us to verify the relation between harmonic frequency and the attosecond delay. The small difference in the harmonic phase between H 2 and D 2 calculated theoretically is consistent with our experimental results

Enhanced high-harmonic generation from chromium-doped magnesium oxide

High-order harmonic generation (HHG) from crystals offers a new source of coherent extreme ultraviolet (XUV) attosecond radiation.Comment: Significant change of conten

Attosecond imaging of molecular electronic wavepackets

International audienceA strong laser field may tunnel ionize a molecule from several orbitals simultaneously, forming an attosecond electron–hole wavepacket. Both temporal and spatial information on this wavepacket can be obtained through the coherent soft X-ray emission resulting from the laser-driven recollision of the liberated electron with the core. By characterizing the emission from aligned N 2 molecules, we demonstrate the attosecond contributions of the two highest occupied molecular orbitals. We determine conditions where they are disentangled in the real and imaginary parts of the emission dipole moment. This allows us to carry out a tomographic reconstruction of both orbitals with angstrom spatial resolution. Their coherent superposition provides experimental images of the attosecond wavepacket created in the ionization process. Our results open the prospect of imaging ultrafast intramolecular dynamics combining attosecond and angstrom resolutions

Macroscopic control of high-order harmonics quantum-path components for the generation of attosecond pulses

International audienceWe present measurements of the different quantum path contributions to the high-order harmonic emission. Through spatial and spectral filtering, we evidence the strong correlation between the spatial and spectral distributions, which allows us to quantify the contribution of each quantum path. A systematic analysis as a function of the generating parameters has been done to identify the conditions for efficient generation and selection of a single quantum path. We show that combining phase matching and spatial filtering allows maximizing and selecting the short quantum path contribution, condition for the generation of "clean" and intense attosecond pulses

Measuring the complex recombination dipole of aligned CO2 molecules in high-order harmonic generation

We generate high order harmonics in aligned CO2 molecules and measure the spectral amplitudes and phases as a function of the alignment angle. We control the quantum interference occurring in the recombination of an ultrafast laser-driven electron wavepacket with the highest occupied molecular orbital of CO2. By comparing to the emission of the reference system krypton, we obtain an experimental measure of the complex recombination dipole matrix element between the CO2 HOMO and a continuum state. The measured phase jump contains signatures of Coulombic wavepacket distortion

Impact of noise in holography with extended references in the low signal regime

International audienceSignal-to-noise ratio is a key factor in lensless imaging, particularly for low diffraction signal experiments in the single shot regime.We present our recent study of the noise impact on holography with extended references. Experimental data have been measured in single shot acquisition using an intense coherent soft X-ray high harmonic source. The impact of hardware and software noise under various detection conditions is discussed. A final comparison between single shot and multi-shot regimes is given

Polarization-resolved pump-probe spectroscopy with high harmonics

High harmonic generation in gases can be used as a probe of the electronic structure of the emitting medium, with attosecond temporal resolution and angstrom spatial resolution. The prospect of measuring molecular dynamics by pump-probe spectroscopy with such precision is attracting a lot of interest. An important issue in pump-probe spectroscopy lies in the ability to detect small signals: the detected signal can be easily dominated by the contributions from non-excited molecules or from a carrier gas. In this paper, we demonstrate that polarization-resolved pump-probe spectroscopy can be used to overcome this issue. We study high harmonic generation from rotationally excited molecules. We show that by measuring the harmonic field that is generated orthogonally to the driving laser field, the contrast in the detection of alignment revivals in nitrogen can be increased by a factor 4. We use this configuration to measure alignment revivals in an argon-nitrogen mixture, in which the total harmonic signal is dominated by the contributions from argon