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.

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.

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.