Nicolas Camus

Attosecond tracing of correlated electron-emission in non-sequential double ionization

Despite their broad implications for phenomena such as molecular bonding or chemical reactions, our knowledge of multi-electron dynamics is limited and their theoretical modelling remains a most difficult task. From the experimental side, it is highly desirable to study the dynamical evolution and interaction of the electrons over the relevant timescales, which extend into the attosecond regime. Here we use near-single-cycle laser pulses with well-defined electric field evolution to confine the double ionization of argon atoms to a single laser cycle. The measured two-electron momentum spectra, which substantially differ from spectra recorded in all previous experiments using longer pulses, allow us to trace the correlated emission of the two electrons on sub-femtosecond timescales. The experimental results, which are discussed in terms of a semiclassical model, provide strong constraints for the development of theories and lead us to revise common assumptions about the mechanism that governs double ionization.

Ionization with low-frequency fields in the tunneling regime

Strong-field ionisation surprises with richness beyond current understanding despite decade long investigations. Ionisation with mid-IR light has promptly revealed unexpected kinetic energy structures that seem related to unanticipated quantum trajectories of the electrons. We measure first 3D momentum distributions in the deep tunneling regime (γ = 0.3) and observe surprising new electron dynamics of near-zero momentum electrons and extremely low momentum structures, below the eV, despite very high quiver energies of 95 eV. Such level of high-precision measurements at only 1 meV above the threshold, despite 5 orders higher ponderomotive energies, has now become possible with a specifically developed ultrafast mid-IR light source in combination with a reaction microscope, thereby permitting a new level of investigations into mid-IR recollision physics.

Evolution of dopant-induced helium nanoplasmas

Two-component nanoplasmas generated by strong-field ionization of doped helium nanodroplets are studied in a pump–probe experiment using few-cycle laser pulses in combination with molecular dynamics simulations. High yields of helium ions and a pronounced resonance structure in the pump–probe transients which is droplet size dependent reveal the evolution of the dopant-induced helium nanoplasma with an active role for He shells in the ensuing dynamics. The pump–probe dynamics is interpreted in terms of strong inner ionization by the pump pulse and resonant heating by the probe pulse which controls the final charge states detected via the frustration of electron–ion recombination.

Kinematically complete measurements of strong field ionization with mid-IR pulses

Recent observations of three unique peaks near 1 eV, 100 meV and 1 meV in the electron spectra generated by ionization using intense mid-IR pulses have challenged the current understanding of strong-field (SF) ionization. The results came as a surprise as they could not be reproduced by the standard version of the commonly used SF approximation. We present results showing the simultaneous measurement of all three low energy ranges at high resolution. This capability is possible due to a unique experimental combination of a high repetition rate mid-IR source, which allows probing deep in the quasi-static regime at high data rates, with a reaction microscope, which allows high resolution three dimensional imaging of the electron momentum distribution.

Minimizing dispersive distortions in carrier-envelope phase sweeping with glass wedges

A systematic experimental study is performed to examine the f-2f technique for sweeping the carrier-envelope phase (CEP) of few-cycle laser pulses by changing the amount of positive dispersion in the extracavity beam path. Slightly changing the dispersion not only changes the CEP but affects the entire spectral-phase function. As a result, large discrepancies are found between the true CEP as independently measured with a stereo-above-threshold-ionization spectrometer and the CEP detected by an f-2f interferometer when sweeping the phase with glass wedges. A new CEP-stabilization scheme is proposed and experimentally shown to significantly improve the performance of CEP sweeping.

Experimental Evidence for Wigner’s Tunneling Time

Figure 1: The difference between the most probable photo-electron emission angle for argon and krypton: experiment and theories (with and without initial momentum and tunneling delay time) Tunneling of a particle through a barrier is one of the counter-intuitive properties of quantum mechanical motion. Thanks to advances in laser technology and generation of electric fields comparable to those electrons experience in atoms, new opportunities to dynamically investigate this process have been developed. For example, in the so-called attoclock measurements [1] the properties of the electron after tunneling are mapped on its emission direction after its interaction with the laser pulse. In this work we investigate the first hundred attoseconds of the electron dynamics during strong field tunneling ionization. We achieve a high sensitivity on the tunneling barrier thanks to two ameliorations to the attoclock principle. Using near-IR wavelength (1300 nm) we place firmly the ionization process in the tunneling regime and limit non-adiabatic effects. Furthermore, we compare the momentum distributions of two atomic species of slightly different atomic potentials (argon and krypton) being ionized under absolutely identical conditions. Experimentally, using a reaction microscope, we apply coincident electron-ion detection in combination with a gas-target that contains a mixture of the two species and succeed in measuring the 3D electronmomentum distributions for both targets simultaneously. Theoretically, the time resolved description of tunneling in strong-field ionization is studied using the leading quantum mechanical Wigner treatment. A detailed analysis of the most probable photoelectron emission for Ar and Kr (Fig. 1) allows testing the theoretical models and a sensitive check of the electron initial conditions at the tunnel exit. The agreement between experiment and theory provides a clear evidence for a non-zero tunneling time delay and a non-vanishing longitudinal momentum at this point [2].

Electron-Nuclear Coupling through Autoionizing States after Strong-Field Excitation of H2 Molecules

Channel-selective electron emission from strong-field photoionization of H_{2} molecules is experimentally investigated by using ultrashort laser pulses and a reaction microscope. The electron momenta and energy spectra in coincidence with bound and dissociative ionization channels are compared. Surprisingly, we observed an enhancement of the photoelectron yield in the low-energy region for the bound ionization channel. By further investigation of asymmetrical electron emission using two-color laser pulses, this enhancement is understood as the population of the autoionizing states of H_{2} molecules in which vibrational energy is transferred to electronic energy. This general mechanism provides access to the vibrational-state distribution of molecular ions produced in a strong-field interaction.

Ionization of atoms and molecules in a strong two-color field

We experimentally investigated the asymmetry of photo-ion and photoelectron momentum spectra of argon and nitrogen by ionizing an argon-nitrogen gas mixture in a strong two-color (400 nm + 800 nm) laser field. By changing the time delay between the 400 nm laser pulse and the 800 nm pulse, the fan-like stripes in low-energy electron momentum shift from negative momentum to positive for both argon and nitrogen. A clear difference of asymmetric electron emission between Ar and N2 is observed.

Control of strong-field ionization with two-color laser pulses

Experimental results and theoretical analysis of the ionization process of argon atoms interacting with linearly polarized two-color fields (λ1 = 800 nm, λ2 = 400 nm) are presented. We observe complex asymmetry patterns in the measured three-dimensional momentum distributions.

New features of strong-field ionization with low-frequency fields in the tunnelling regime

The advent of mid-IR laser sources (λ ≥ 3.0 μm) permits scrutinizing strong-field photoionization of atoms and molecules deep into the tunnelling regime and unexpected features are being revealed [1]. A low kinetic energy structure (LES) was observed in the experimental photoelectron energy distribution and it was conspicuously absent in semi-classical and Keldysh-Faisal-Reiss (KFR) calculations. Further measurements are required revealing a full 3D momentum picture of the before-mentioned effects.