Attosecond Dual Nature of Core Excitons

Excitons are electron-hole quasi-particles, which strongly influence the electro-optical properties of solids and hold the potential for a variety of applications [1] . Acquiring a detailed understanding of their dynamic nature is thus essential to promote their exploitation in advanced technological areas. In particular, the ultrafast processes unfolding at the femto- and attosecond domain are of primary relevance in view of the desired extension towards the petahertz regime. Here we applied attosecond transient reflectivity spectroscopy to study the ultrafast response of a MgF2 single crystal and report the first observation of sub-fs core-exciton dynamics [2] . Combined with simulations, our results allowed us to disentangle the dual atomic-solid nature of this fundamental quasi-particle.