[en] Complete test of publication follows. The advent of attosecond pulse generation has had a tremendous impact on the temporal measurement technology. It has provided the means of temporally resolving dynamic processes evolving at atomic time scales. The technique for the attosecond pulse generation relies on the production of phase-locked harmonics in a non-linear medium using short laser pulses. A number of attosecond spectroscopic measurements have been performed using attosecond sources based on high-harmonic generation in atomic gases. The scope of experiments that can be performed with this source is rather limited though because of the low number of photons available. Since the first observation of harmonic generation from solid targets using a tabletop laser system it became apparent that the interaction of an intense laser pulse with an overdense plasma constitutes an alternative route for the efficient production of phase-locked harmonics. Given the rapid technological advancements in laser technology, tabletop lasers based on the Optical Parametric Chirp Pulse Amplification (OPCPA) technique delivering several tens of TW power with kHz repetition rate appear to be within our reach. Motivated by these prospects, we have performed a series of simulations using the 1-D particle-in-cell (PIC) code LPIC. The results indicated that it is quite feasible that surface harmonics generated at laser intensities of 10²⁰ W/cm² can produce a train of or even single attosecond pulses in the 20-70 eV spectral range with duration of ∼ 80 as and efficiency of a few percent. More elaborate simulations using a 3D-PIC code not only corroborate these findings but also show that the XUV light reflected from the few-cycle-driven relativistic surface possess an excellent spatial coherence. The prospects of developing a source of intense attosecond XUV pulses using this method will be discussed on the basis of simulation and experimental results. The availability of such a source would allow the extension of the pump-probe femtosecond techniques to the extreme ultra-violet (XUV) and soft x-ray (SXR) regime, thus opening the way to real-time observation of a wide range of fast evolving phenomena in atomic, molecular and plasma physics.

[en] The coherent high-order harmonic generation from the interaction of ultra-intense femtosecond laser pulses with solid density plasmas holds promise for tabletop sources of extreme ultraviolet (XUV) and soft x-ray radiation with attosecond duration and unprecedented intensities. Together with the generation of mono-energetic electron beams from gas jets and capillaries and the generation of mono-energetic ions from thin foils, this offers a unique tool box of tabletop-laser-generated radiation sources for a wide range of applications previously only accessible with large-scale accelerator and synchrotron-radiation facilities. Especially, the generation of high harmonics from laser plasmas has the potential of being applied to a wide range of experiments from plasma physics to molecular dynamics. So far the studies addressing the generation of high harmonics from laser-generated overcritical plasma surfaces have concentrated mainly on the characterization of the harmonic beams themselves not considering how, in a next step, these beams could be applied to experiments. In this paper we discuss the generation of surface harmonics with the ATLAS (800 mJ, 40 fs) laser system with the emphasis on the transport, spectral shaping refocusing of the harmonic beams, all of these being absolute prerequisites for multi-shot experiments. We also present considerations for future improvements and possible future experiments exploiting the full potential of high harmonic radiation from solid targets.

[en] Complete test of publication follows. In the quest for a way to generate ultrashort, high-power, few-cycle laser pulses the discovery of optical parametric amplification (OPA) has opened up to the path towards a completely new regime, well beyond that of conventional laser amplification technology. The main advantage of this parametric amplification process is that it allows for an extremely broad amplification bandwidth compared to any known laser amplifier medium. When combined with the chirped-pulse amplification (CPA) principle (i.e. OPCPA), on one hand pulses of just 10 fs duration and 8 mJ pulse energy have been demonstrated. On the other hand, pulse energies of up to 30 J were also achieved on a different OPCPA system; the pulse duration in this case, however, was 100 fs. In order to combine ultrashort pulse durations (i.e. pulses in the few-cycle regime) with high pulse energies (i.e. in the Joule range) we propose tu pump on OPCPA chain with TW-scale short pulses (100 fs - 1 ps instead of > 100 ps of previous OPCPA systems) delivered by a conventional CPA system. This approach inherently improves the conditions for generating high-power ultrashort pulses using OPCPA in the following ways. Firstly, the short pump pulse duration reduces the necessary stretching factor for the seed pulse, thereby increasing stretching and compression fidelity. Secondly, also due to the shortened pump pulse duration, a much higher contrast is achieved. Finally, the significantly increased pump power makes the use of thinner OPCPA crystals possible, which implies an even broader amplification bandwidth, thereby allowing for even shorter pulses. We carried out theoretical investigations to show the feasibility of such a set-up. Alongside these studies we will also present preliminary experimental results of an OPCPA system pumped by the output of our Ti:Sapphire ATLAS laser, currently delivering 350 mJ in 43 fs. An insight into the planned scaling of this technique to petawatt levels (i.e. the Petawatt-Field-Synthesizer project at the MPQ) will also be given.

[en] Complete text of publication follows. We report on recent results in the generation, characterization and applications of energetic attosecond pulse trains and ultra-broad coherent XUV continua: 1) Generation: 1a) We report experimental results confirming contribution of both long and short trajectories in on-axis harmonic generation before, at and after an atomic gas jet, i.e. under three different phase matching conditions. The contribution of both trajectories is manifested through their interference leading to a modulated harmonic (and side band) yield as a function of the driving intensity. 1b) We report the generation of sub-fs pulse trains at the 40 μJ pulse energy level from laser surface plasma, measured through 2^nd order intensity volume autocorrelation (2^nd order IVAC). 2) Characterization: We present comparative studies between RABITT and 2^nd order IVAC in on axis harmonic generation before, at and after an atomic gas jet. We find that the two techniques give fairly different results that are compatible with the differently weighted but unavoidable presence of the long and short trajectory in the generation process in all three phase matching conditions. We show that the relative contributions of the two trajectories can be estimated through RABITT measurements, while spatiotemporal mean pulse durations can be extracted from 2^nd order IVAC traces. 3) Applications: 3a) We present time resolved VUV spectroscopy of ultrafast dynamics in molecular ethylene. 3b) We present time resolved XUV spectroscopy at the 1 fs temporal scale and ultra-broad band XUV Fourier Transform Spectroscopy in a manifold of doubly excited autoionizing and inner-shell Auger decaying states excited simultaneously through a coherent broadband XUV continuum. Acknowledgments. This work is supported in part by the European Community's Human Potential Program under contract MTKD-CT-2004-517145 (X-HOMES), the Ultraviolet Laser Facility (ULF) operating at FORTH-IESL (contract PHRI-CT-2001-00139), the ELI research infrastructure preparatory phase program, the FASTQUAST ITN, and the FLUX program of the 7^th FP.

[en] In upcoming experiments with ultra-high fields of high-power short-pulse lasers we will experimentally study the radiation from electrons under extreme fields. Aiming at the detection of radiation from the Unruh effect, first we will encounter the copious classical Larmor radiation. The characterization of (linear) Larmor radiation has never been experimentally carried out, thus this amounts to a first study of physics at extreme acceleration. Moreover, we can study radiation damping effects. Furthermore, the experiment should be able to confirm or disprove whether the radiation components may be enhanced by collective effects, if a tightly clumped cluster of electrons is accelerated. The technique of laser driven dense electron sheet formation by irradiating a thin DLC foil target should provide such a coherent electron cluster with a very high density. If and when such relativistic electron sheets are realized, a counterpropagating second laser can interact with them coherently. Under these conditions enhanced Larmor and Unruh radiation signals may be observed.

[en] Efficient production of coherent harmonic radiation from solid targets relies critically on the formation of smooth, short density scalelength plasmas. Recent experimental results (Dromey et al 2009 Nat. Phys. 5 146) suggest, however, that the target roughness on the scale of the emitted harmonic wavelength does not result in diffuse reflection-in apparent contradiction to the Rayleigh criterion for coherent reflection. In this paper we show, for the first time, using analytic theory and 2D PIC simulations, that the interaction of relativistically strong laser pulses with corrugated target surfaces results in a highly effective smoothing of the interaction surface and consequently the generation of highly collimated and temporally confined XUV pulses from rough targets, in excellent agreement with experimental observations.

[en] Quasi-monoenergetic, laser-driven electron beams of up to ∼200 MeV in energy have been generated from steady-state-flow gas cells [1]. These beams are emitted within a low-divergence cone of 2.1±0.5 mrad FWHM and feature unparalleled shot-to-shot stability in energy (2.5% rms), pointing direction (1.4 mrad rms) and charge (16% rms) owing to a highly reproducible plasma-density profile within the laser-plasma-interaction volume. Laser-wakefield acceleration (LWFA) in gas cells of this type constitutes a simple and reliable source of relativistic electrons with well defined properties, which should allow for applications such as the production of extreme-ultraviolet undulator radiation in the near future.

[en] Relevant to laser based electron/ion accelerations, a single shot second harmonic generation frequency resolved optical gating (FROG) system has been developed to characterize laser pulses (80 J, ∼600 fs) incident on and transmitted through nanofoil targets, employing relay imaging, spatial filter, and partially coated glass substrates to reduce spatial nonuniformity and B-integral. The device can be completely aligned without using a pulsed laser source. Variations of incident pulse shape were measured from durations of 613 fs (nearly symmetric shape) to 571 fs (asymmetric shape with pre- or postpulse). The FROG measurements are consistent with independent spectral and autocorrelation measurements.

[en] Recently, the use of plasma optics to improve temporal pulse contrast has had a remarkable impact on the field of high-power laser-solid density interaction physics. Opening an avenue to previously unachievable plasma density gradients in the high intensity focus, this advance has enabled researchers to investigate new regimes of harmonic generation and ion acceleration. Until now, however, plasma optics for fundamental laser reflection have been used in the sub-relativistic intensity regime (10¹⁵-10¹⁶ W cm^-2) showing high reflectivity (∼70%) and good focusability. Therefore, the question remains as to whether plasma optics can be used for such applications in the relativistic intensity regime (>10¹⁸ W cm^-2). Previous studies of plasma mirrors (PMs) indicate that, for 40 fs laser pulses, the reflectivity fluctuates by an order of magnitude and that focusability of the beam is lost as the intensity is increased above 5x10¹⁶ W cm^-2. However, these experiments were performed using laser pulses with a contrast ratio of ∼10⁷ to generate the reflecting surface. Here, we present results for PM operation using high contrast laser pulses resulting in a new regime of operation-the high contrast plasma mirror (HCPM). In this regime, pulses with contrast ratio >10¹⁰ are used to form the PM surface at >10¹⁹ W cm^-2, displaying excellent spatial filtering, reflected near-field beam profile of the fundamental beam and reflectivities of 60±5%. Efficient second harmonic generation is also observed with exceptional beam quality suggesting that this may be a route to achieving the highest focusable harmonic intensities. Plasma optics therefore offer the opportunity to manipulate ultra-intense laser beams both spatially and temporally. They also allow for ultrafast frequency up-shifting without detrimental effects due to group velocity dispersion (GVD) or reduced focusability which frequently occur when nonlinear crystals are used for frequency conversion

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