Ionization of atoms or molecules in strong laser fields is often viewed as optical tunnelling of the departing electron through the potential barrier created by the combined action of the binding potential and the voltage applied by the laser electric field. Optical tunnelling launches a variety of strong-field processes triggered and controlled by the laser field, such as high harmonic generation, laser-induced electron diffraction and holography.
In this topic, we focus on the roles of the orbital momentum and spin of the departing electron, both in optical tunnelling and in the processes that follow. The spin and the orbital momentum of the tunneling electron are correlated to the spin and the orbital momentum of the hole it leaves behind, and the latter are coupled by spin-orbit interaction in the core. As a result, the strong sensitivity of optical tunnelling to the electron’s orbital angular momentum, together with the spin-orbit interaction, can lead to the generation of highly spin-polarized electrons during optical tunnelling. Controlling the motion of attosecond spin-polarized electron bunches with the laser field after tunnelling opens an exciting opportunity to study spin-resolved processes, such as spin-orbit hole dynamics, elastic and inelastic spin-resolved scattering, and strong-field induced holography with spin-polarized electrons, with attosecond temporal resolution.