Ultrafast-dynamics studies and femtosecond-pulse metrology both rely on the nonlinear processes induced solely by an incident light pulse. Extending these approaches to the extreme-ultraviolet (XUV) spectral region would open up a new route to attosecond-scale dynamics. However, this has been hindered by the limited intensities available in coherent XUV continua. In the present work, we realized conditions at which simultaneous ejection of two bound electrons by two-XUVphoton absorption becomes more efficient than their removal one-by-one. In this regime we have succeeded in tracing atomic coherences evolving at the 1-fs scale with simultaneous determination of the average XUV-pulse duration. The rich and dense structure of the autoionizing manifold demonstrates the applicability of the approach to complex systems. This initiates the era of XUV-pump-XUV-probe experiments at the boundary between femto-and attosecond scales.A large variety of ultrafast phenomena, including electronic motion in atoms, molecules, condensed matter and plasmas, dynamic electron-electron correlations, charge migration, ultrafast dissociation and reaction processes, occur on the few-femtosecond to attosecond temporal scale. Attosecond (as) pulses 1 provide access to these temporal regimes in different states of matter [2][3][4][5][6] . Nonlinear (NL) XUV processes constitute the ideal tool for the study of such dynamics. Attosecond pulse trains 7-9 have reached intensities sufficient to induce two-XUV-photon processes [10][11][12][13][14] . However, isolated attosecond pulses, requisite for XUV-pump-XUV-probe experiments, have not yet attained the required parameters for an observable two-XUV-photon process. As a consequence, attosecond pulse metrology and time-domain applications have been widely based on infrared (IR)-XUV cross-correlation approaches, which entail assumptions for the analysis 15 .The present work succeeds for the first time in observing two-XUV-photon processes induced by energetic XUV continua, in part temporally confined in isolated pulses with durations on the order of 1 fs. These processes are in turn exploited in XUVpump-XUV-probe ultrafast evolving atomic coherences, as well as in determining the duration of the XUV bursts. A structured part of the single continuum of the xenon atom is excited by the first pulse, forming an electronic wave packet that undergoes rapid and complex motion before it decays. This evolution can be traced, thanks to the XUV parameters reached, at which a second pulse ejects a second electron before the first one leaves the atom carrying with it all the information on the temporal evolution of the system (coherence decay). Unconventionally, the two electrons leave the atom together and, thus, the doubly ionized Xe yield as a function of the delay between the two pulses carries the fingerprint of the wave packet motion and the XUV pulse duration. As the pulse duration and the decay The intense XUV radiation is generated by frequency upconversion of many-cycle high-peak-power laser fields...

This review presents the technological infrastructure that will be available at the Extreme Light Infrastructure Attosecond Light Pulse Source (ELI-ALPS) international facility. ELI-ALPS will offer to the international scientific community ultrashort pulses in the femtosecond and attosecond domain for time-resolved investigations with unprecedented levels of high quality characteristics. The laser sources and the attosecond beamlines available at the facility will make attosecond technology accessible for scientists lacking access to these novel tools. Time-resolved

Continuing efforts in ultrashort pulse engineering have recently led to the breakthroughs of the generation of attosecond (10−18 s) pulse trains 1-7 and isolated pulses [8][9][10][11] . Although trains of multiple pulses can be generated through the interaction of many-optical-cycle pulses with gases-a process that has led to intense extreme-ultraviolet emission 3-5 -the generation of isolated high-intensity pulses, which requires few-cycle driving pulses, remains a challenge. Here, we report a vital step towards the generation of such pulses, the production of broad continuum extreme-ultraviolet emission using a high-intensity, many-cycle, infrared pulsed laser, through the interferometric modulation of the ellipticity of 50-fs-long driving pulses. The increasing availability of high-power many-cycle lasers and their potential use in the construction of intense attosecond radiation-with either gas or solid-surface targets 12 -offer exciting opportunities for multiphoton extreme-ultravioletpump-extreme-ultraviolet-probe studies of laser-matter and laser-plasma interactions.The generation of attosecond pulse trains through the synthesis of a comb of harmonics of an infrared many-cycle femtosecond laser pulse is well established [1][2][3][4][5][6][7] , and is essentially understood in the framework of the three-step model 13,14 . According to this model, an electron is ejected in the continuum after tunnelling through the atomic potential barrier formed by the instantaneous laser field. Subsequently, the electron accelerates away from the core until the field changes sign. Within a fraction of half the laser period, the electron may revisit the parent ion to recombine and emit a burst of continuum extreme-ultraviolet radiation. As the process is repeated twice per laser cycle, the emitted spectrum consists of a superposition of coherent continua, which in the time domain is equivalent to a train of sub-femtosecond pulses. Using high-power many-cycle laser pulses, intense attosecond pulse trains have been generated and already used for the study of nonlinear phenomena in the extreme-ultraviolet spectral region [15][16][17][18][19][20][21][22] . They thus afford the means for the temporal characterization of attosecond pulses on the basis of second-order autocorrelation techniques in the extreme-ultraviolet spectral region [3][4][5]18,19 , and open the road towards extreme-ultraviolet pump-extremeultraviolet probe experiments.In the spirit of the three-step model, if the process is confined to a single revisit of the core by the driven electron, one single continuum is emitted in the form of an isolated pulse. In mathematical terms, the Fourier synthesis of a broad continuum corresponds in the time domain to a single temporal occurrence, whereas a discrete spectrum leads to a repetitive process. Thus, the emission of a single coherent continuum is an essential prerequisite for single attosecond pulse generation [8][9][10] . Indeed, the generation of isolated single attosecond pulses is based on the synthesis of a co...

We report the observation of multiple ionization of argon through multi-XUV-photon absorption induced by an unprecedentedly powerful laser driven high-order harmonic generation source. Comparing the measured intensity dependence of the yield of the different argon charge states with numerical calculations we can infer the different channels-direct and sequential-underlying the interaction. While such studies were feasible so far only with free electron laser (FEL) sources, this paper connects highly nonlinear XUV processes with the ultrashort time scales inherent to the harmonic pulses and highlights the advanced perspectives of emerging large scale laser research infrastructures.