Tunable orbital angular momentum in high-harmonic generation

Optical vortices are currently one of the most intensively studied topics in optics. These lightbeams, which carry orbital angular momentum (OAM), have been successfully utilized in thevisible and infrared in a wide variety of applications. Moving to shorter wavelengths mayopen up completely new research directions in the areas of optical physics and materialcharacterization. Here, we report on the generation of extreme-ultraviolet optical vorticeswith femtosecond duration carrying a controllable amount of OAM. From a basic physicsviewpoint, our results help to resolve key questions such as the conservation of angularmomentum in highly nonlinear light–matter interactions, and the disentanglement andindependent control of the intrinsic and extrinsic components of the photon’s angularmomentum at short-wavelengths. The methods developed here will allow testing some of therecently proposed concepts such as OAM-induced dichroism, magnetic switching in organicmolecules and violation of dipolar selection rules in atoms.