Hadas Soifer

Resolving the time when an electron exits a tunnelling barrier

The tunnelling of a particle through a barrier is one of the most fundamental and ubiquitous quantum processes. When induced by an intense laser field, electron tunnelling from atoms and molecules initiates a broad range of phenomena such as the generation of attosecond pulses, laser-induced electron diffraction and holography. These processes evolve on the attosecond timescale (1 attosecond ≡ 1 as = 10−18 seconds) and are well suited to the investigation of a general issue much debated since the early days of quantum mechanics—the link between the tunnelling of an electron through a barrier and its dynamics outside the barrier. Previous experiments have measured tunnelling rates with attosecond time resolution and tunnelling delay times. Here we study laser-induced tunnelling by using a weak probe field to steer the tunnelled electron in the lateral direction and then monitor the effect on the attosecond light bursts emitted when the liberated electron re-encounters the parent ion. We show that this approach allows us to measure the time at which the electron exits from the tunnelling barrier. We demonstrate the high sensitivity of the measurement by detecting subtle delays in ionization times from two orbitals of a carbon dioxide molecule. Measurement of the tunnelling process is essential for all attosecond experiments where strong-field ionization initiates ultrafast dynamics. Our approach provides a general tool for time-resolving multi-electron rearrangements in atoms and molecules—one of the key challenges in ultrafast science.

Near-threshold high-order harmonic spectroscopy with aligned molecules.

We study high-order harmonic generation in aligned molecules close to the ionization threshold. Two distinct contributions to the harmonic signal are observed, which show very different responses to molecular alignment and ellipticity of the driving field. We perform a classical electron trajectory analysis, taking into account the significant influence of the Coulomb potential on the strong-field-driven electron dynamics. The two contributions are related to primary ionization and excitation processes, offering a deeper understanding of the origin of high harmonics near the ionization threshold. This Letter shows that high-harmonic spectroscopy can be extended to the near-threshold spectral range, which is in general spectroscopically rich.

Multi-channel electronic and vibrational dynamics in polyatomic resonant high-order harmonic generation

High-order harmonic generation in polyatomic molecules generally involves multiple channels of ionization. Their relative contribution can be strongly influenced by the presence of resonances, whose assignment remains a major challenge for high-harmonic spectroscopy. Here we present a multi-modal approach for the investigation of unaligned polyatomic molecules, using SF6 as an example. We combine methods from extreme-ultraviolet spectroscopy, above-threshold ionization and attosecond metrology. Fragment-resolved above-threshold ionization measurements reveal that strong-field ionization opens at least three channels. A shape resonance in one of them is found to dominate the signal in the 20–26 eV range. This resonance induces a phase jump in the harmonic emission, a switch in the polarization state and different dynamical responses to molecular vibrations. This study demonstrates a method for extending high-harmonic spectroscopy to polyatomic molecules, where complex attosecond dynamics are expected.

Role of the ionic potential in high harmonic generation.

Recollision processes provide direct insight into the structure and dynamics of electronic wave functions. However, the strength of the process sets its basic limitations--the interaction couples numerous degrees of freedom. In this Letter we decouple the basic steps of the process and resolve the role of the ionic potential which is at the heart of a broad range of strong field phenomena. Specifically, we measure high harmonic generation from argon atoms. By manipulating the polarization of the laser field we resolve the vectorial properties of the interaction. Our study shows that the ionic core plays a significant role in all steps of the interaction. In particular, Coulomb focusing induces an angular deflection of the electrons before recombination. A complete spatiospectral analysis reveals the influence of the potential on the spatiotemporal properties of the emitted light.

Multidimensional high harmonic spectroscopy

High harmonic generation (HHG) has opened up a new frontier in ultrafast science where attosecond time resolution and Angstrom spatial resolution are accessible in a single measurement. However, reconstructing the dynamics under study is limited by the multiple degrees of freedom involved in strong field interactions. In this paper we describe a new class of measurement schemes for resolving attosecond dynamics, integrating perturbative nonlinear optics with strong-field physics. These approaches serve as a basis for multidimensional high harmonic spectroscopy. Specifically, we show that multidimensional high harmonic spectroscopy can measure tunnel ionization dynamics with high precision, and resolves the interference between multiple ionization channels. In addition, we show how multidimensional HHG can function as a type of lock-in amplifier measurement. Similar to multi-dimensional approaches in nonlinear optical spectroscopy that have resolved correlated femtosecond dynamics, multi-dimensional high harmonic spectroscopy reveals the underlying complex dynamics behind attosecond scale phenomena.

Two-Dimensional Frequency Resolved Optomolecular Gating of High-Order Harmonic Generation.

Probing electronic wave functions of polyatomic molecules is one of the major challenges in high-harmonic spectroscopy. The extremely nonlinear nature of the laser-molecule interaction couples the multiple degrees of freedom of the probed system. We combine two-dimensional control of the electron trajectories and vibrational control of the molecules to disentangle the two main steps in high-harmonic generation-ionization and recombination. We introduce a new measurement scheme, frequency-resolved optomolecular gating, which resolves the temporal amplitude and phase of the harmonic emission from excited molecules. Focusing on the study of vibrational motion in N_{2}O_{4}, we show that such advanced schemes provide a unique insight into the structural and dynamical properties of the underlying mechanism.

Probe of Multielectron Dynamics in Xenon by Caustics in High-Order Harmonic Generation

We investigated the giant resonance in xenon by high-order harmonic generation spectroscopy driven by a two-color field. The addition of a nonperturbative second harmonic component parallel to the driving field breaks the symmetry between neighboring subcycles resulting in the appearance of spectral caustics at two distinct cutoff energies. By controlling the phase delay between the two color components it is possible to tailor the harmonic emission in order to amplify and isolate the spectral feature of interest. In this Letter we demonstrate how this control scheme can be used to investigate the role of electron correlations that give birth to the giant resonance in xenon. The collective excitations of the giant dipole resonance in xenon combined with the spectral manipulation associated with the two-color driving field allow us to see features that are normally not accessible and to obtain a good agreement between the experimental results and the theoretical predictions.

Studying the universality of field induced tunnel ionization times via high-order harmonic spectroscopy

High-harmonic generation spectroscopy is a promising tool for resolving electron dynamics and structure in atomic and molecular systems. This scheme, commonly described by the strong field approximation, requires a deep insight into the basic mechanism that leads to the harmonic generation. Recently, we have demonstrated the ability to resolve the first stage of the process—field induced tunnel ionization—by adding a weak perturbation to the strong fundamental field. Here we generalize this approach and show that the assumptions behind the strong field approximation are valid over a wide range of tunnel ionization conditions. Performing a systematic study—modifying the fundamental wavelength, intensity and atomic system—we observed a good agreement with quantum path analysis over a range of Keldysh parameters. The generality of this scheme opens new perspectives in high harmonics spectroscopy, holding the potential of probing large, complex molecular systems.

Two-color HHG Spectroscopy of the Xenon Giant Resonance

We probed the electronic structure of xenon by two-color high-order harmonic spectroscopy. We were able to identify the contribution of electron correlations in the spectral region corresponding to the giant resonance.

Resolving Ultrafast Molecular Dynamics via Multidimensional High Harmonic Spectroscopy

Extending the dimensionality of high harmonic generation (HHG) measurements is essential for revealing complex attosecond dynamics. We identify structural and dynamical features in molecular HHG spectra, and resolve the contributions of multiple ionization channels.