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New capabilities at accelerator-based and laser-based sources are continuously being developed, being possible to nowadays generate two-color XUV/x-ray pulses with controlled time delay. In the optical regime, in which lasers have achieved a high-degree of spatio-temporal coherence several decades ago, we find many control techniques for atoms and molecules based on coherence. The novel two-color XUV/x-ray sources will allow exploring novel quantum control schemes by using the intrinsic site-specificity excitation of high-energy photons.

Optical vortices are structures of the electromagnetic field with a spiral phase ramp about a point-phase singularity, carrying orbital angular momentum (OAM). These beams are commonly generated in the optical and infrared (IR) regimes, being used in a wide range of applications: classical and quantum communications, micromanipulation, microscopy, etc. [1]. The production of OAM beams in the extreme ultraviolet (EUV)/x-ray regimes is of great interest, as it allows to extend the applications of optical vortices down to the nanometric and attosecond scales. The nonperturbative process of high harmonic generation (HHG) conveys a unique possibility of mapping properties of the driving IR field into shorter-wavelength radiation. For instance, EUV vortices have been obtained through OAM-driven HHG [2], being emitted in the form of helical attosecond beams [3, 4]. It has been theoretically [3, 5] and experimentally [6] shown that OAM is conserved in HHG driven by single-mode vortices through the simple conservation law l q = ql, i.e., each harmonic is generated with OAM of ql, being q the harmonic order and l the OAM of the driver. This simple rule follows the energy conservation of the harmonic conversion process, also found in perturbative harmonic generation.