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...

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Nonlinear light-matter interactions in the extreme ultraviolet (XUV) are a prerequisite to perform XUV-pump/XUVprobe spectroscopy of core electrons. Such interactions are now routinely investigated at free-electron laser (FEL) facilities. Yet, electron dynamics are often too fast to be captured with the femtosecond resolution of state-of-theart FELs. Attosecond pulses from laser-driven XUV-sources offer the necessary temporal resolution. However, intense attosecond pulses supporting nonlinear processes have only been available for photon energy below 50 eV, precluding XUV-pump/XUV-probe investigation of typical inner-shell processes. Here, we surpass this limitation by demonstrating two-photon absorption from inner electronic shells of xenon at photon energies around 93 eV and 115 eV. This advance opens the door for attosecond real-time observation of nonlinear electron dynamics deep inside atoms.

A simple method for the determination of saturation intensities and in some cases generalized cross sections in multiphoton ionization is presented. It utilizes the dependence of the ponderomotive shift on the laser intensity above the saturation limit. An application to He and Ar interacting with 500 fs pulses at 248 nm is demonstrated. Experimental results are compared with theoretical calculations.