Strong-field physics

Typically, the length scale of electrons in atoms and molecules is one Angström (ten billion of Angström is one meter) and their time scale is the attosecond (one billion of billion of attosecond is a second). Electrons are so small and so fast that the direct measurement of their real-time motion is not feasible. By subjecting atoms or molecules to a strong and ultrashort laser pulse, this information is encoded into its output, such as the speed of the ionized electrons or the frequency of the emitted photons. It becomes possible to capture snapshots of their real-time motion during ultrafast processes, such as chemical reactions and charge migration.

Despite the inherent quantal nature of electrons inside atoms and molecules, they manifest strong classical behaviors when subjected to strong laser pulses. The objective is to identify and understand mechanisms underlying the highly nonlinear phenomena observed in experiments, such as high harmonic generation (HHG), non-sequential multiple ionization (NSMI), or above threshold ionization (ATI) - with tools from nonlinear dynamics. 

References

Figure: Atom driven by a strong laser pulse. After ionizing, the electron is driven back towards the core by the combined laser and Coulomb force, this is referred to as a recollision. Recollisions give rise to nonperturbative and highly nonlinear phenomena such as high harmonic generation (HHG), above-threshold ionization (ATI) and non-sequential double ionization (NSDI).

Research projects

Guiding centers for the electron motion in strong laser pulses

There is a fundamental obstacle to the interpretation of the electron dynamics in atoms subject to intense laser fields: The motion of an electron in combined laser and electric fields is an unsolved problem. Sufficiently far from the core, the characteristic time scales of the electron in the Coulomb field is much larger than the characteristic time scales of the electron in the laser field. This separation of time scales is used to determine the averaged motion of the electron in the laser field. The averaged motion of the electron in the laser field is interpreted as the guiding center of the electron.

The guiding-center motion of the electron can be used to interpret and understand the electron dynamics in the laser field. The objective is to understand the nonlinear phenomena observed in a atoms and molecules using the description of the electron dynamics in terms of its guiding center. For instance, the concept of guiding-center helped to predict and understand the bifurcation of the peak of the photoelectron momentum distributions (PMDs) as a function of the laser ellipticity, how the electron can return to the core and undergo a Coulomb-driven recollision after very long excursions, and how the electron can be trapped into a Rydberg state after ionizing by a laser pulse. 

Figure: Non sequential double ionization of an atom subjected to an intense laser field. The light blue and light red curves are the electron trajectories. The dark red curve is the guiding center of the light red electron. After ionization of the light red electron, it is driven back towards the core by its guiding center. Then, the electrons exchange energy and both ionize.

Publications

Envelope-driven recollisions triggered by an elliptically polarized pulse

By increasing the laser ellipticity, it is commonly believed that the recollision probability decreases. Two main arguments support this: (i) For increasing ellipticities, the sideways drift of the electron become stronger and pushes it away from the core without recolliding. (ii) The energy gained by the electron in its excursion outside the core region decreases for increasing ellipticity, and in particular, is zero for circularly polarized laser pulses. These obstacles are overcome by taking into account a crucial and eminent element, nonetheless always present in experiments: The envelope of the laser pulse. By ionizing early after the laser field is turned on (when the  laser amplitude is still small compared to its peak amplitude), (i) the sideways drift of the electron at ionization vanishes and (ii) the electron gains energy from the variations of the laser pulse envelope. This recollision channel is called envelope-driven recollisions, and can be observed for specific target atoms and laser wavelength. Where it exists, it triggers nonlinear and highly nonperturbative phenomena, such as HHG, ATI and NSDI, regardless of the laser ellipticity.

Figure: Typical envelope-driven recollisions driven by a circularly polarized laser pulse. The left panel shows the trajectory of the electron which goes very far away from the core and then returns. The right panels show at which time the electron ionizes (during the ramp-up of the laser pulse), and at which time it returns (during the plateau) and how its energy evolves. We observe that the energy gained by the electron is when the laser pulse envelope changes in time.

Publications

Numerical computation of invariant structures in phase space

Phase space, the space in which evolves dynamical variables and defining the state of a particle, is organized by invariant objects acting often as barriers on the partile motion. These invariant objects encode relevant information on the dynamics and on the long-time state of a system. However, identifying and calculating highly-dimensional invariant objects is a challenging and computationally intensive task. 

We have calculated the stable and unstable manifolds of a family of invariant tori and showed that they are responsible for the recollisions of electrons subjected to strong laser pulses in high dimensions.

Figure: The orange dot is a fixed point (0-dimensional invariant object) of the dynamical map. The black lines are invariant tori of the family of invariant tori (1-dimensional invariant objects) associated with the fixed point. The red and gray surfaces are the stable and unstable manifolds (2-dimensional invariant objects) associated with each invariant tori, respectively.

Publications

Propagation effects in high harmonic generation (HHG)

For one atom, when the electron comes back to the parent ion, it can recombine in the ground state and emit a high frequency photon, this is the high harmonic generation (HHG). The typical intensity spectrum of HHG is peaks at each even field harmonics whose amplitude is composed of a plateau and a cutoff. By filtering the signal, the emitted frequencies in the plateau are isolated and used for producing ultrashort laser pulses. Meanwhile, the intensity of the post-process ultrashort laser field from one atom is very low. In order to increase the intensity of the post-process ultrashort laser, the laser is propagated through a gas of atoms.

When the laser propagates through the gas of atoms, there is a self-coupling between the gas and the laser. In particular, nonlinear phenomena arise at the microscopic scale in the medium and at the macroscopic scale in the laser field. The objective is to determine and understand the nonlinear phenomena related to the propagation of the gas through a medium.

Figure: Propagation of a laser beam through a gas of atoms. The position of the atoms is labeled by r and correspond to the macroscopic coordinates. The position of the electrons in the atom is labeled by x and correspond to the microscopic coordinates. The laser drives the electron dynamics in the atoms, which radiates photons and change the shape of the laser field through the propagation.

Publications