An era of exploring the interactions of high-intensity, hard X-rays with matter has begun with the start-up of a hard-X-ray free-electron laser, the Linac Coherent Light Source (LCLS). Understanding how electrons in matter respond to ultra-intense X-ray radiation is essential for all applications. Here we reveal the nature of the electronic response in a free atom to unprecedented high-intensity, short-wavelength, high-fluence radiation (respectively 10(18) W cm(-2), 1.5-0.6 nm, approximately 10(5) X-ray photons per A(2)). At this fluence, the neon target inevitably changes during the course of a single femtosecond-duration X-ray pulse-by sequentially ejecting electrons-to produce fully-stripped neon through absorption of six photons. Rapid photoejection of inner-shell electrons produces 'hollow' atoms and an intensity-induced X-ray transparency. Such transparency, due to the presence of inner-shell vacancies, can be induced in all atomic, molecular and condensed matter systems at high intensity. Quantitative comparison with theory allows us to extract LCLS fluence and pulse duration. Our successful modelling of X-ray/atom interactions using a straightforward rate equation approach augurs favourably for extension to complex systems.

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We show that high fluence, high-intensity x-ray pulses from the world's first hard x-ray free-electron laser produce nonlinear phenomena that differ dramatically from the linear x-ray-matter interaction processes that are encountered at synchrotron x-ray sources. We use intense x-ray pulses of sub-10-fs duration to first reveal and subsequently drive the 1s↔2p resonance in singly ionized neon. This photon-driven cycling of an inner-shell electron modifies the Auger decay process, as evidenced by line shape modification. Our work demonstrates the propensity of high-fluence, femtosecond x-ray pulses to alter the target within a single pulse, i.e., to unveil hidden resonances, by cracking open inner shells energetically inaccessible via single-photon absorption, and to consequently trigger damaging electron cascades at unexpectedly low photon energies.

X-ray free-electron lasers (FELs) produce femtosecond x-ray pulses with unprecedented intensities that are uniquely suited for studying many phenomena in atomic, molecular, and optical (AMO) physics. A compilation of the current developments at the Linac Coherent Light Source (LCLS) and future plans for the LCLS-II and Next Generation Light Source (NGLS) are outlined. The AMO instrumentation at LCLS and its performance parameters are summarized. A few selected experiments representing the rapidly developing field of ultra-fast and peak intensity x-ray AMO sciences are discussed. These examples include fundamental aspects of intense x-ray interaction with atoms, nonlinear atomic physics in the x-ray regime, double core-hole spectroscopy, quantum control experiments with FELs and ultra-fast x-ray induced dynamics in clusters. These experiments illustrate the fundamental aspects of the interaction of intense short pulses of x-rays with atoms, molecules and clusters that are probed by electron and ion spectroscopies as well as ultra-fast x-ray scattering.

The group delay dispersion, also known as the attochirp, of high-order harmonics generated in gases has been identified as the main intrinsic limitation to the duration of Fourier-synthesized attosecond pulses. Theory implies that the attochirp, which is inversely proportional to the laser wavelength, can be decreased at longer wavelength. Here we report the first measurement of the wavelength dependence of the attochirp using an all-optical, in situ method [N. Dudovich et al., Nature Phys. 2, 781 (2006)]. We show that a 2 m driving wavelength reduces the attochirp with respect to 0:8 m at comparable intensities. DOI: 10.1103/PhysRevLett.102.093002 PACS numbers: 32.80.Fb Attosecond science, or attophysics, is the study of electron dynamics at time-scales approaching the atomic unit of time (1 a:u: % 24 as) [1]. Currently, the production of attosecond pulses and the development of attophysics are based on high harmonic generation in gases, a nonperturbative, highly nonlinear process. Using this method, light pulses close to 100 as have been demonstrated either as an attosecond pulse train (APT) with a repetition period of half the fundamental cycle [2] or as a single pulse [3,4]. So far the carrier frequency of attosecond pulses has been limited to the extreme ultraviolet (XUV) to soft-x-ray domain (30-80 eV) but significantly more applications will be enabled if shorter bursts of more energetic x rays can be created, particularly time resolved studies of corelevel and multielectron dynamics [5]. In order to evaluate the physical limitations it is important to understand the role of the driving field characteristics. The recent development of intense midinfrared femtosecond pulses with wavelength 5 times that of the commonly used titanium: sapphire laser system has opened new routes to extreme harmonic generation and attophysics [6,7]. In this report, a measurement of the driving wavelength dependence of the group delay dispersion or attochirp, a property crucial for attosecond pulse synthesis, is described.High harmonic generation is a quantum mechanical process involving the absorption of a large number of photons, for which the usual nonlinear optics treatment breaks down. In a more effective description based on classical trajectories, a field-driven electron freed by intense field ionization is accelerated for approximately half an optical cycle, and then can recombine to emit an attosecond burst of light [8,9]. This process occurs twice every optical cycle, which in the frequency domain corresponds to a comb of odd-order harmonics of the fundamental driving field. The corresponding quantum mechanical treatment is based on the strong field approximation [10] in which the time-dependent one-electron dipole is calculated by solving the Schrödinger equation neglecting the influence of the Coulomb potential on the motion of the free electron wave packet. In this approximation the main contribution to the dipole comes from the quantum paths whose quasiclassical action is stationary and therefore follow the af...

Two-color, single-shot time-of-flight electron spectroscopy of atomic neon was employed at the Linac Coherent Light Source (LCLS) to measure laser-assisted Auger decay in the x-ray regime. This x-ray-optical cross-correlation technique provides a straightforward, non-invasive and online means of determining the duration of femtosecond (>40 fs) x-ray pulses.In combination with a theoretical model of the process based on the softphoton approximation, we were able to obtain the LCLS pulse duration and to extract a mean value of the temporal jitter between the optical pulses from a synchronized Ti-sapphire laser and x-ray pulses from the LCLS. We find that the experimentally determined values are systematically smaller than the length of the electron bunches. Nominal electron pulse durations of 175 and 75 fs, as provided by the LCLS control system, yield x-ray pulse shapes of 120 ± 20 fs full-width at half-maximum (FWHM) and an upper limit of 40 ± 20 fs FWHM, respectively. Simulations of the free-electron laser agree well with the experimental results. Contents

We report the compression of intense, carrier-envelope phase stable mid-IR pulses down to few-cycle duration using an optical filament. A filament in xenon gas is formed by using self-phase stabilized 330 J 55 fs pulses at 2 m produced via difference-frequency generation in a Ti:sapphire-pumped optical parametric amplifier. The ultrabroadband 2 m carrier-wavelength output is self-compressed below 3 optical cycles and has a 270 J pulse energy. The self-locked phase offset of the 2 m difference-frequency field is preserved after filamentation. This is to our knowledge the first experimental realization of pulse compression in optical filaments at mid-IR wavelengths ͑Ͼ0.8 m͒. © 2007 Optical Society of America OCIS codes: 190.5530, 320.5520. Progress in strong-field physics has been accelerated by the development of lasers operating near the 0.8 m wavelength that feature high peak power, few-cycle duration, and reliable control over the carrier-envelope phase 1 (CEP). Furthermore, the fundamental scaling laws 2,3 governing the intense laseratom interaction suggest that the advancement of longer-wavelength mid-IR laser sources capable of similar optical quality will have a major impact in strong-field physics. The most compelling examples include the generation of shorter attosecond x-ray bursts and the rescattering of electrons at kilovolt energies. [3][4][5] A recently demonstrated 80 J, 2 m prototype system 6 based on optical parametric chirped-pulse amplification via difference-frequency generation defines a standard for future development of longwavelength drivers. However, the optical parametric chirped-pulse amplification architecture is faced with important technical challenges, 7 such as the need for specific pump laser design and unwanted generation of parasitic fluorescence underlying the primary pulse for high parametric gain configurations. 6 Currently, femtosecond optical parametric amplifiers (OPAs) pumped by multimillijoule Ti:sapphire chirped-pulse amplification systems can deliver multicycle pulses in the mid-IR with sufficient peak power to investigate the efficacy of the nonlinear pulse compression techniques developed at shorter wavelengths. In particular, optical filaments formed in a noble gas by intense 0.8 m pulses have demonstrated pulse compression down to the few-cycle regime with excellent beam stability and spatial mode quality. 8This Letter demonstrates, for the first time to our knowledge, the self-compression in an optical filament of high-peak-power mid-IR pulses derived by difference-frequency generation in a Ti:sapphire pumped OPA. This efficient scheme produces fluorescence-free, sub-3 optical cycle pulses near the 2 m wavelength with 270 J energy at a 1 kHz repetition rate. The intense 2 m field carries a constant CEP offset, thus making it an attractive longwavelength driver for benchmark strong-field experiments.A schematic of the experimental setup is shown in Fig. 1. High-peak-power multicycle mid-IR pulses are produced in a slightly modified traveling-wave OPA (TOPAS, L...

Short-wavelength free-electron lasers are now well established as essential and unrivalled sources of ultrabright coherent X-ray radiation. One of the key characteristics of these intense X-ray pulses is their expected few-femtosecond duration. No measurement has succeeded so far in directly determining the temporal structure or even the duration of these ultrashort pulses in the few-femtosecond range. Here, by deploying the so-called streaking spectroscopy technique at the Linac Coherent Light Source, we demonstrate a non-invasive scheme for temporal characterization of X-ray pulses with sub-femtosecond resolution. This method is independent of photon energy, decoupled from machine parameters, and provides an upper bound on the X-ray pulse duration. We measured the duration of the shortest X-ray pulses currently available to be on average no longer than 4.4 fs. Analysing the pulse substructure indicates a small percentage of the free-electron laser pulses consisting of individual high-intensity spikes to be on the order of hundreds of attoseconds