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For many applications of extreme ultraviolet (XUV) and x-ray pulses a small focus size is crucial to reach the required intensity or spatial resolution. In this article, we present a simple way to characterize an XUV focus with a resolution of 1.85 micrometer. Furthermore, this technique is applied for the measurement and optimization of the focus of an ellipsoidal mirror for photon energies ranging from 18 to 150 eV generated by high-order harmonics. We envisage a broad range of applications of this approach with sub-micrometer resolution from high-harmonic sources via synchrotrons to free-electron lasers., Comment: 9 pages, 6 figures

Laser wakefield acceleration (LWFA) and its particle-driven counterpart, plasma wakefield acceleration (PWFA), are commonly treated as separate, though related branches of high-gradient plasma-based acceleration. However, novel proposed schemes are increasingly residing at the interface of both concepts where the understanding of their interplay becomes crucial. Here, we report on experiments covering a wide range of parameters by using nanocoulomb-class quasi-monoenergetic electron beams from LWFA with a 100-TW-class laser. Based on a controlled electron injection, these beams reach record-level performance in terms of laser-to-beam energy transfer efficiency (up to 10%), spectral charge density (regularly exceeding 10 pC/MeV) and divergence (1 mrad full width at half maximum divergence). The impact of charge fluctuations on the energy spectra of electron bunches is assessed for different laser parameters, including a few-cycle laser, followed by a presentation of results on beam loading in LWFA with two electron bunches. This scenario is particularly promising to provide high-quality electron beams by using one of the bunches to either tailor the laser wakefield via beam loading or to drive its own, beam-dominated wakefield. We present experimental evidence for the latter, showing a varying acceleration of a low-energy witness beam with respect to the charge of a high-energy drive beam in a spatially separate gas target. With the increasing availability of petawatt-class lasers the access to this new regime of laser-plasma wakefield acceleration will be further facilitated, thus providing new paths towards low-emittance beam generation for future plasma-based colliders or light sources.

The development of ultra-short intense laser sources in the visible and extreme ultraviolet (XUV) spectral range led to fascinating studies in laser-matter interactions and attosecond science. In the majority of these studies the system under investigation interacts with a focused laser beam, which ionizes the system. The ionization products are usually measured by devices, which spatiotemporally integrate the ionization signal originating from the entire focal area, discarding in this way valuable information about the ionization dynamics that take place in the interaction volume. Here, we review a recently developed approach in measuring the spatially resolved photoionization yields resulting from the interaction of infrared (IR)/XUV ultra-short laser pulses in gas phase media. We show how this approach enables a) the in-situ focus diagnostic, b) quantitative studies of linear and non-linear ionization processes in the IR/XUV regime, c) single-shot XUV-pump-XUV-probe studies and d) single-shot 2nd-order XUV autocorrelation measurements. The article has been published in J. Opt. 20, 024018 (2018) (doi.org/10.1088/2040-8986/aaa326)., Comment: 32 pages, 13 figures

Relativistic surface high harmonics have been considered a unique source for the generation of intense isolated attosecond pulses in the extreme ultra-violet (XUV) and X-ray spectral range. However, its experimental realization is still a challenging task requiring identification of the optimum conditions for the generation of isolated attosecond pulses as well as their temporal characterization. Here, we demonstrate measurements in both directions. Particularly, we have made a first step towards the temporal characterization of the emitted XUV radiation by adapting the attosecond streak camera concept to identify the time domain characteristics of relativistic surface high harmonics. The results, supported by PIC simulations, set the upper limit for the averaged (over many shots) XUV duration to <6 fs, even when driven by not CEP controlled relativistic few-cycle optical pulses. Moreover, by measuring the dependence of the spectrum of the relativistic surface high harmonics on the carrier envelope phase (CEP) of the driving infrared laser field, we experimentally determined the optimum conditions for the generation of intense isolated attosecond pulses.

Ultrafast pump-probe experiments open the possibility to track fundamental material behaviour like changes in its electronic configuration in real time. To date, most of these experiments are performed using an electron or a high-energy photon beam, which is synchronized to an infrared laser pulse. Entirely new opportunities can be explored if not only a single, but multiple synchronized, ultra-short, high-energy beams are used. However, this requires advanced radiation sources that are capable of producing dual-energy electron beams, for example. Here, we demonstrate simultaneous generation of twin-electron beams from a single compact laser wakefield accelerator. The energy of each beam can be individually adjusted over a wide range and our analysis shows that the bunch lengths and their delay inherently amount to femtoseconds. Our proof-of-concept results demonstrate an elegant way to perform multi-beam experiments in future on a laboratory scale.

We report on systematic and high-precision measurements of dephasing, an effect that fundamentally limits the performance of laser wakefield accelerators. Utilizing shock-front injection, a technique providing stable, tunable and high-quality electron bunches, acceleration and deceleration of few-MeV quasi-monoenergetic beams were measured with sub-5-fs and 8-fs laser pulses. Typical density dependent electron energy evolution with 65-300 micrometers dephasing length and 6-20 MeV peak energy was observed and is well described with a simple model., Comment: 5 pages, 5 figures Inset in Fig.2 corrected

Brilliant X-ray sources are of great interest for many research fields from biology via medicine to material research. The quest for a cost-effective, brilliant source with unprecedented temporal resolution has led to the recent realization of various high-intensity-laser-driven X-ray beam sources. Here we demonstrate the first all-laser-driven, energy-tunable and quasi-monochromatic X-ray source based on Thomson backscattering. This is a decisive step beyond previous results, where the emitted radiation exhibited an uncontrolled broad energy distribution. In the experiment, one part of the laser beam was used to drive a few-fs bunch of quasi-monoenergetic electrons from a Laser-Wakefield Accelerator (LWFA), while the remainder was scattered off the bunch in a near-counter-propagating geometry. When the electron energy was tuned from 10-50 MeV, narrow-bandwidth X-ray spectra peaking at 5-35keV were directly measured, limited in photon energy by the sensitivity curve of our X-ray detector. Due to the ultrashort LWFA electron bunches, these beams exhibit few-fs pulse duration., Comment: Submitted to Nature Photonics on 12.09.2013

The tunnelling of an electron through a suppressed atomic potential followed by its motion in the continuum, is the fundamental mechanism underlying strong-field laser-atom/molecule interactions. Due to its quantum nature, the interaction is governed by the phase of the released electron wave-packets. Thus, detailed mapping of the electron wave-packet interferences provides essential insight into the physics underlying the interaction. A process which is providing access to the intricacies of the interaction is the generation of high order harmonics of the laser frequency. The phase-amplitude distribution of the emitted extreme-ultraviolet (EUV), carries the complete information about the harmonic generation process and vice-versa. Thus, the visualization of the EUV-spatial-amplitude-distribution, as it results from interfering electron wave-packet contributions, is of crucial importance. Restrictions to this accomplishment are due to the spatially integrating measurement approaches applied so far, that average out the phase effects in the generation process. In this work, we demonstrate a method which overcomes this obstacle. An EUV-spatial-amplitude-distribution-image is induced from the imprint on the measured spatial distribution of ions, produced through EUV-photon ionization of atoms. Interference extremes in the image carry phase information about the interfering electron wave-packets. The present approach provides detailed insight on the strong-field laser-atom interaction mechanism, while establishing the era of phase selective interaction studies. Furthermore, it paves the way for substantial enhancement of the spectral and temporal precision of measurements by in-situ controlling the phase distribution of the emitted radiation and/or spatially selecting the EUV-radiation-atom interaction products., Comment: 13 pages main text with 4 figures, 7 pages supporting material with 3 figures