Carla Figueira de Morisson Faria (University College, London)

Unravelling the extreme: Matter in intense laser fields and the physics of attosecond time scales

Strong-field and attosecond physics deals with matter in extreme conditions. Not only does this involve fields with typical intensities of 1013W/cm2 or higher, but also some of the shortest time scales in nature: hundreds of attoseconds (10-18s). This is roughly the time it takes for an electron to travel through atomic distances, so that one may resolve and control electron dynamics in real time. In this context, a widely used physical picture for describing strong-field phenomena is an electron undergoing a laser-induced collision with its parent ion. As, quantum mechanically, there is more than one pathway that the electron may follow, the associated transition amplitudes will interfere. Thus, quantum interference may be explored to probe the ultrafast dynamics of matter and it is not clear whether usual decoherence mechanisms will have time to develop. In this talk, I will exemplify this using the theoretical work by my group at UCL. This includes high-order harmonic generation applied to molecular imaging, quantum effects in laser-induced double ionization, approaches brought from other areas to attoscience and a novel path integral method applied to ultrafast electron holography. I intend to delve not photoelectron holography in more and NSDI in more depth.

In photoelectron holography, different types of interfering orbits are used as a “probe” and “reference” in order to reconstruct a specific target. Most models interpreting the patterns obtained, however, are oversimplified and neglect the residual potentials. By developing a novel semi-analytic approach that fully accounts for the laser field and of the binding potential, we not only explained in far more detail well-known holographic structures, but also unveiled a myriad of previously overlooked patterns, that only now have been identified experimentally (for a review see C. Figueira de Morisson Faria, A. S. Maxwell, Rep. Prog. Phys. 83 (3), 034401 (2020)).

Laser-induced nonsequential double ionization (NSDI) is the quintessential example of a correlated strong-field process. For over two decades, it was widely believed that this correlation is essentially classical. We call this assumption into question in the below-threshold regime, in which the returning electron excites a second electron, by showing that quantum interference is more robust than anticipated. We identify two interference types, associated with electron exchange and different excitation channels, and show that it survives focal averaging and integration over several degrees of freedom (see, e.g., A. S. Maxwell and C. Figueira de Morisson Faria, , Phys. Rev. Lett. 116, 143001 (2016)).