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Synchrotrons and free-electron lasers are the most powerful sources of X-ray radiation. They constitute invaluable tools for a broad range of research 1 ; however, their dependence on largescale radiofrequency electron accelerators means that only a few of these sources exist worldwide. Laser-driven plasmawave accelerators 2-10 provide markedly increased accelerating fields and hence offer the potential to shrink the size and cost of these X-ray sources to the university-laboratory scale. Here, we demonstrate the generation of soft-X-ray undulator radiation with laser-plasma-accelerated electron beams. The well-collimated beams deliver soft-X-ray pulses with an expected pulse duration of ∼10 fs (inferred from plasma-accelerator physics). Our source draws on a 30-cmlong undulator 11 and a 1.5-cm-long accelerator delivering stable electron beams 10 with energies of ∼210 MeV. The spectrum of the generated undulator radiation typically consists of a main peak centred at a wavelength of ∼18 nm (fundamental), a second peak near ∼9 nm (second harmonic) and a highenergy cutoff at ∼7 nm. Magnetic quadrupole lenses 11 ensure efficient electron-beam transport and demonstrate an enabling technology for reproducible generation of tunable undulator radiation. The source is scalable to shorter wavelengths by increasing the electron energy. Our results open the prospect of tunable, brilliant, ultrashort-pulsed X-ray sources for small-scale laboratories.Resolving the structure and dynamics of matter on the atomic scale requires a probe with ångstrøm resolution in space and femtosecond to attosecond resolution in time. Third-generation synchrotron sources produce X-ray pulses with durations of typically a few tens of picoseconds and can achieve 100 fs by using complex beam-manipulation techniques 12,13 . They have already proven their capability of imaging static structures with atomic (spatial) resolution 1 and upcoming X-ray free-electron lasers hold promise for also extending the temporal resolution into the atomic/sub-atomic range [14][15][16][17][18] . Both of these sources consist of an electron accelerator and an undulator, which is a periodic magnetic structure that forces the electrons to oscillate and emit radiation 19 . Whereas current facilities require a kilometre-scale accelerator, new laser-plasma accelerators offer the potential for a marked reduction in size and cost as well as pulse durations of a few femtoseconds.Femtosecond-laser-driven plasma accelerators have produced quasi-monoenergetic electron beams 2-7 with energies up to 1 GeV (refs 8, 9, 20, 21) from centimetre-scale interaction lengths. The concept is based on an ultra-intense laser pulse, which ionizes atoms of a gas target and excites a plasma wave. This trails the pulse at 1 Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany, 2 Department für Physik, Ludwig-Maximilians-Universität, Am Coulombwall 1, 85748 Garching, Germany, 3 Forschungszentrum Dresden-Rossendorf, Bautzner Landstraße 128, 01328 Dresden, Germany, 4 ...

Proton and ion acceleration using high-intensity lasers is a field of rapidly growing interest. For possible applications of proton beams produced in laser-solid interactions, the generation of beams with controllable parameters such as energy spectrum, brightness, and spatial profile is crucial. Hence, the physics underlying the acceleration processes has to be well understood. After the first proof-of-principle experiments [1,2], systematical studies were carried out to examine the influence of target material and thickness [3,4]. To establish the influence of the main laser parameters such as intensity, pulse energy, and duration over a wide range, results from different laser systems have to be compared, since usually each system covers a small parameter range only. Besides these parameters, strength and duration of the prepulse due to amplified spontaneous emission (ASE) play an important role, too [3]. We report on experiments carried out to establish the influence of the laser prepulse due to ASE and the target thickness on the acceleration of protons from thin aluminum foils.The protons originate from water and hydrocarbon contaminations on the foil surfaces. We used the 6-TW ATLAS laser facility at MPQ delivering 150 fs pulses at 790 nm wave length containing up to 900 mJ of energy. The pulses are focused by an f /2.5 off-axis parabolic mirror onto aluminum foils of 0.8 . . .86 µm thickness to intensities in excess of 10 19 W/cm 2 .The duration of the ASE prepulse having a peak intensity of 8 × 10 11 W/cm 2 can be controled by means of an ultra-fast Pockels cell in the laser chain. The shortest prepulse duration is 500 ps and it can be extended to several ns. The protons accelerated from the foils are detected by a Thomson parabola positioned in normal direction of the target rear side. CR 39 plates are used as a detector. The proton pits made visible by etching the CR 39 in NaOH after the shot are counted by an optical microscope equipped with a pattern-recognition software.

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Temporal probing of a number of fundamental dynamical processes requires intense pulses at femtosecond or even attosecond (1 as = 10(-18) s) timescales. A frequency 'comb' of extreme-ultraviolet odd harmonics can easily be generated in the interaction of subpicosecond laser pulses with rare gases: if the spectral components within this comb possess an appropriate phase relationship to one another, their Fourier synthesis results in an attosecond pulse train. Laser pulses spanning many optical cycles have been used for the production of such light bunching, but in the limit of few-cycle pulses the same process produces isolated attosecond bursts. If these bursts are intense enough to induce a nonlinear process in a target system, they can be used for subfemtosecond pump-probe studies of ultrafast processes. To date, all methods for the quantitative investigation of attosecond light localization and ultrafast dynamics rely on modelling of the cross-correlation process between the extreme-ultraviolet pulses and the fundamental laser field used in their generation. Here we report the direct determination of the temporal characteristics of pulses in the subfemtosecond regime, by measuring the second-order autocorrelation trace of a train of attosecond pulses. The method exhibits distinct capabilities for the characterization and utilization of attosecond pulses for a host of applications in attoscience.

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...

Laser-driven coherent extreme-ultraviolet (XUV) sources provide pulses lasting a few hundred attoseconds 1,2 , enabling real-time access to dynamic changes of the electronic structure of matter 3,4 , the fastest processes outside the atomic nucleus. These pulses, however, are typically rather weak. Exploiting the ultrahigh brilliance of accelerator-based XUV sources 5 and the unique time structure of their laser-based counterparts would open intriguing opportunities in ultrafast X-ray and high-field science, extending powerful nonlinear optical and pump-probe techniques towards X-ray frequencies, and paving the way towards unequalled radiation intensities. Relativistic laser-plasma interactions have been identified as a promising approach to achieve this goal 6-13 . Recent experiments confirmed that relativistically driven overdense plasmas are able to convert infrared laser light into harmonic XUV radiation with unparalleled efficiency, and demonstrated the scalability of the generation technique towards hard X-rays 14-19 . Here we show that the phases of the XUV harmonics emanating from the interaction processes are synchronized, and therefore enable attosecond temporal bunching. Along with the previous findings concerning energy conversion and recent advances in high-power laser technology, our experiment demonstrates the feasibility of confining unprecedented amounts of light energy to within less than one femtosecond.The nonlinear response of matter exposed to intense femtosecond laser pulses gives rise to the emission of highfrequency radiation at harmonics of the laser oscillation frequency. If the harmonics are phase-locked, their superposition results in a train of attosecond bursts 20 . The concept has been so far successfully implemented in atomic gases 21 , and culminated in isolated attosecond pulses by using few-cycle laser drivers 1,2 . The low generation efficiency of harmonic radiation from atoms has motivated research into alternative concepts. Dense, femtosecond-laser-produced plasmas hold promise of converting laser light into coherent harmonics with much higher efficiency and of exploiting much higher laser intensities, because the plasma medium-in contrast to the atomic emitters-imposes no restriction on the strength of the laser field driving the harmonics [6][7][8][9][10][11][12][13] . Recent experimental studies of harmonics produced from overdense plasmas impressively corroborate several theoretical predictions: the high conversion efficiency 19 , the favourable scalability of the generation technique towards high photon energies 14,16,19 and excellent divergence due to the spatial coherence of the generated harmonics 19,22 . Whether the high-order harmonics that are produced in overdense plasmas • gold-coated off-axis parabolic mirror with the same focal length as the laser focusing parabola. The recollimating mirror is mounted on a flipper stage for easy withdrawal, thus enabling the spectral characterization of the emitted XUV light. Thin metal filters (typically 150 nm Al, In or Sn)...

When a pulse of light reflects from a mirror that is travelling close to the speed of light, Einstein's theory of relativity predicts that it will be up-shifted to a substantially higher frequency and compressed to a much shorter duration. This scenario is realized by the relativistically oscillating plasma surface generated by an ultraintense laser focused onto a solid target. Until now, it has been unclear whether the conditions necessary to exploit such phenomena can survive such an extreme interaction with increasing laser intensity. Here, we provide the first quantitative evidence to suggest that they can. We show that the occurrence of surface smoothing on the scale of the wavelength of the generated harmonics, and plasma denting of the irradiated surface, enables the production of high-quality X-ray beams focused down to the diffraction limit. These results improve the outlook for generating extreme X-ray fields, which could in principle extend to the Schwinger limit