Transfer of Orbital Angular Momentum in High Harmonic Generation using two Driving Beams

Light beams may carry both a spin and an orbital angular momentum (OAM). While the former is associated to their polarization state, the latter stems from the geometrical properties of their wavefront. In their prototypical form, beams with OAM have "donuts-like" intensity profile and helicoidal wavefront, carrying integral multiples of h as angular momenta. Since their "rediscovery" in the late 90's, beams with OAM of visible wavelengths have found innumerable applications in quantum optics, microscopy or information transfer. A major recent development was the generation of such beams with much smaller wavelengths - in the extreme ultraviolet (XUV) - using synchrotron sources, free electron lasers as well as high harmonic sources (HHG). In this latter case, it creates ultrashort XUV sources of beams with OAM, suited for time-resolved applications at femtosecond and attosecond time scales. Remarkably, we showed that even though HHG is a non-perturbative process, the OAM transfer from the driving beam to the harmonics was purely parametric; i.e., the q-th harmonic - which can be thought as the upconversion of q infrared photons - carries q times the OAM of the driver [3, 4]. However, this rule limits severely the flexibility in choosing the OAM of the XUV emission.