The angular momentum of light in High Harmonic Generation

Program Description

Angular momentum is an ubiquitous concept in physics: it is commonly used to describe atoms, fluids or solids. Light itself is known to have mechanical properties such as a linear momentum, and also angular momentum. Just like for atoms, the angular momentum of the electromagnetic field can be decomposed in two parts: the orbital and spin part. Both are very important quantities, often principal in many applications. For instance, the orbital angular momentum (OAM) of light has been used in microscopy, quantum information or microparticule manipulation. As for the spin angular momentum (SAM), which is macroscopically seen as a circular polarization, it plays a key role in the interaction with chiral objects.

Until a few years ago, the study of these properties was restricted to the visible or infrared spectral range. High Harmonic Generation (HHG) gives us the opportunity to transfer these quantities from an infrared laser to the extreme ultraviolet (XUV) domain. Additionally to these short wavelengths, HHG is able to produce pulses of light of attosecond duration.

 In this talk, we will present how both the OAM and SAM of light can be transferred through HHG in gases. We will discuss its conservation and experimentally demonstrate the production of attosecond pulses having controlled values of both types of angular momenta. Then we will show how such unique light sources can be used in light-matter interactions. Namely, XUV light carrying SAM interacting with chiral samples has allowed us to measure an asymmetry effect called photo-electron circular dichroism (PECD), which is of great interest for spectroscopy. As for XUV light carrying OAM, it was used in a two-color ionization experiment, revealing an interesting spatio-temporal profile. The future uses of attosecond XUV pulses carrying angular momentum will finally be discussed.