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

The recent development of novel extreme ultraviolet (XUV) coherent light sources bears great potential for a better understanding of the structure and dynamics of matter 1,2 . Promising routes are advanced coherent control and nonlinear spectroscopy schemes in the XUV energy range, yielding unprecedented spatial and temporal resolution 3,4 . However, their implementation has been hampered by the experimental challenge of generating XUV pulse sequences with precisely controlled timing and phase properties. In particular, direct control and manipulation of the phase of individual pulses within a XUV pulse sequence opens exciting new possibilities for coherent control and multidimensional spectroscopy 4 , but has

It is shown that an elementary semi-quantitative approach explains essential features of the X-ray free-electron laser mechanism, in particular those of the gain and saturation lengths. Using mathematical methods and derivations simpler than complete theories, this treatment reveals the basic physics that dominates the mechanism and makes it difficult to realise free-electron lasers for short wavelengths. This approach can be specifically useful for teachers at different levels and for colleagues interested in presenting X-ray free-electron lasers to non-specialized audiences.

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We demonstrate the ability to control and shape the spectro-temporal content of extremeultraviolet (XUV) pulses produced by a seeded free-electron laser (FEL). The control over the spectro-temporal properties of XUV light was achieved by precisely manipulating the linear frequency chirp of the seed laser. Our results agree with existing theory, which allows retrieving the temporal properties (amplitude and phase) of the FEL pulse from measurements of the spectra as a function of the FEL operating parameters. Furthermore, we show the first direct evidence of the full temporal coherence of FEL light and generate Fourier limited pulses by fine-tuning the FEL temporal phase. The possibility to tailor the spectro-temporal content of intense short-wavelength pulses represents the first step towards efficient nonlinear optics in the XUV to X-ray spectral region and will enable precise manipulation of core-electron excitations using the methods of coherent quantum control. 1The development of the first high-power pulsed lasers in the 1960s marks an important milestone in the long-standing effort to actively control the temporal evolution of quantummechanical systems. However, it was not until the early 1990s, when ultrashort pulse shaping techniques became practical, that coherent control of quantum phenomena finally became a reality [1]. Tailoring the spectro-temporal content of intense laser pulses in the visible range opened up countless possibilities for manipulating the quantum state of matter, with examples ranging from control of population transfer in optical transitions [2] and currents in semiconductors [3] to control of chemical reactions [4] and energy flow in biomolecular complexes [5], and many others (see, e.g. Ref.[6] and references therein). Undoubtedly, the ability to shape the spectro-temporal content of powerful extreme-ultraviolet (XUV) and X-ray pulses would trigger widespread efforts to extend the concepts of coherent control into the short-wavelength regime, leading ultimately to the development of new methods for probing and manipulating core electrons in atoms, molecules and materials. As a more immediate application, it may find use in numerous advanced spectroscopic techniques such as resonant inelastic X-ray scattering [7] and coherent ultrafast core-hole correlation spectroscopy, offering unique capabilities for probing elementary excitations [8].In the XUV/X-ray spectral region, free-electron lasers (FELs) are currently the only devices that can deliver femtosecond laser-like pulses with peak powers in the gigawatt range [9][10][11][12][13]. However, the ability to generate fully coherent pulses and to shape their spectrotemporal content with high stability on a shot-to-shot basis is extremely challenging, due to the difficulties in precisely controlling the light generation process. In this Letter we show that the spectro-temporal content of powerful ultrashort XUV pulses can be precisely shaped using a laser-seeded FEL. By tuning the seed laser operating parameters we generate in...