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Electron and X-ray beams originating from compact laser-wakefield accelerators have very small source sizes that are typically on the micrometre scale. Therefore, the beam divergences are relatively high, which makes it difficult to preserve their high quality during transport to applications. To improve on this, tremendous efforts have been invested in controlling the divergence of the electron beams, but no mechanism for generating collimated X-ray beams has yet been demonstrated experimentally. Here we propose and realize a scheme where electron bunches undergoing focusing in a dense, passive plasma lens can emit X-ray pulses with divergences approaching the incoherent limit. Compared with conventional betatron emission, the divergence of this so-called plasma lens radiation is reduced by more than an order of magnitude in solid angle, while maintaining a similar number of emitted photons per electron. This X-ray source offers the possibility of producing brilliant and collimated few-femtosecond X-ray pulses for ultra-fast science, in particular for studies based on X-ray diffraction and absorption spectroscopy. X-ray pulses with low divergences are produced in a laser-wakefield accelerator by focusing electron bunches in a dense passive plasma lens.

Strong-field laser-matter interactions in nanoscale targets offer unique avenues for the generation and detailed characterization of matter under extreme conditions. Field-driven, subcycle ionizati...

We report on the detection of spatio-temporal couplings in a 700-1000 nm NOPA using an optimized characterization method. The technique is performed during normal focus observation and requires little additional hardware.

Optimization and simulation of non-collinear ultra-broadband optical parametric chirped pulse amplification setups rely on exact knowledge of the phase matching conditions. We present a method for their accurate retrieval by deterministic angular jitter and Monte-Carlo simulations.

Optical parametric amplification (OPA) is a powerful tool for the generation of ultrashort light pulses. However, under certain circumstances, it develops spatio-spectral couplings, color dependent aberrations that degrade the pulse properties. In this work, we present a spatio-spectral coupling generated by a non-collimated pump beam and resulting in the change of direction of the amplified signal with respect to the input seed. We experimentally characterize the effect, introduce a theoretical model to explain it as well as reproduce it through numerical simulations. It affects high-gain non-collinear OPA configurations and becomes especially relevant in sequential optical parametric synthesizers. In collinear configuration, however, beyond the direction change, also angular and spatial chirp is produced. We obtain with a synthesizer about 40% decrease in peak intensity in the experiments and local elongation of the pulse duration by more than 25% within the spatial full width at half maximum at the focus. Finally, we present strategies to correct or mitigate the coupling and demonstrate them in two different systems. Our work is important for the development of OPA-based systems as well as few-cycle sequential synthesizers.

Optical parametric chirped-pulse amplification (OPCPA) [1] is an established light amplification technique with many beneficial properties, like high single pass gain, scalability, large spectral bandwidth, tunability and good conversion efficiency. Different methods have been proposed for optimization of conversion [2] – [4] mainly altering the pump or the crystal properties. However, seed manipulation to increase the OPCPA conversion efficiency has been only described in a general spatiotemporal field optimization theory so far [5] . Here, we show numerical and experimental results of a novel method to improve the gain saturation in an ultra-broadband OPCPA, hence conversion efficiency, by applying an adaptive spectral filter function to the seed pulses.

Optics express 29(3), 4210 (2021). doi:10.1364/OE.415564, Optical parametric chirped-pulse amplification (OPCPA) is a light amplification technique that provides the combination of broad spectral gain bandwidth and large energy, directly supporting few-cycle pulses with multi-terawatt (TW) peak powers. Saturation in an OPCPA increases the stability and conversion efficiency of the system. However, distinct spectral components experience different gain and do not saturate under the same conditions, which reduces performance. Here, we describe a simple and robust approach to control the saturation for all spectral components. The demonstrated optimal saturation increases the overall gain, conversion efficiency and spectral bandwidth. We experimentally obtain an improvement of the pulse energy by more than 18%. This technique is easily implemented in any existing OPCPA system with a pulse shaper to maximize its output., Published by Soc., Washington, DC

We report on systematic and high-precision measurements of the evolution of electron beams in a laser-wakefield accelerator (LWFA). Utilizing shock-front injection, a technique providing stable, tunable and high-quality electron bunches, acceleration and deceleration of few-MeV quasimonoenergetic beams were measured with cutting-edge technology sub-5-fs and 8-fs laser pulses. We explain the observations with dephasing, an effect that fundamentally limits the performance of LWFAs. Typical density dependent electron energy evolution with $57--300\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ dephasing length and 6--20 MeV peak energy was observed and is well described by a parabolic fit. This is a promising electron source for time-resolved few-fs electron diffraction.

We report on details of a peak-power upgrade of a sub-5-fs Optical Parametric Synthesizer towards 100 TW. System design, pump pulse delaying and relay imaging system are presented. A tailored second and third harmonic generation reaches conversion efficiencies of 80% and 60% with 80ps pump pulses.

Laser wakefield acceleration (LWFA) and its particle-driven counterpart, particle or 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 present a comprehensive study of this regime, which we may term laser-plasma wakefields. Using datasets of hundreds of shots, we demonstrate the influence of beam loading on the spectral shape of electron bunches. Similar results are obtained using both 100-TW-class and few-cycle lasers, highlighting the scale invariance of the involved physical processes. Furthermore, we probe the interplay of dual electron bunches in the same or in two subsequent plasma periods under the influence of beam loading. We show that, with decreasing laser intensity, beam loading transitions to a beam-dominated regime, where the first bunch acts as the main driver of the wakefield. This transition is evidenced experimentally by 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. Our results also present an important step in the development of LWFA using controlled injection in a shock front. The electron beams in this study reach record performance in terms of laser-to-beam energy transfer efficiency (up to 10%), spectral charge density (regularly exceeding 10 pC MeV^{−1}), and angular charge density (beyond 300 pC μsr^{-1} at 220 MeV). We provide an experimental scaling for the accelerated charge per terawatt (TW) of laser power, which approaches 2 nC at 300 TW. With the expanding availability of petawatt-class (PW) lasers, these beam parameters will become widely accessible. Thus, the physics of laser-plasma wakefields is expected to become increasingly relevant, as it provides new paths toward low-emittance beam generation for future plasma-based colliders or light sources.

High-order harmonics of laser pulses yield spectral components with shorter wavelength and duration and tighter focus than the original pulse. Precise spatiotemporal characterization of this radiation from a relativistic plasma mirror is relevant for ultrafast science.

Ultrahigh-intensity laser-plasma physics provides unique light and particle beams as well as novel physical phenomena. A recently available regime is based on the interaction between a relativistic intensity few-cycle laser pulse and a sub-wavelength-sized mass-limited plasma target. Here, we investigate the generation of electron bunches under these extreme conditions by means of particle-in-cell simulations. In a first step, up to all electrons are expelled from the nanodroplet and gain relativistic energy from time-dependent local field enhancement at the surface. After this ejection, the electrons are further accelerated as they copropagate with the laser pulse. As a result, a few, or under specific conditions isolated, pC-class relativistic attosecond electron bunches are generated with laser pulse parameters feasible at state-of-the-art laser facilities. This is particularly interesting for some applications, such as generation of attosecond x-ray pulses via Thomson backscattering.

High-field experiments are very sensitive to the exact value of the peak intensity of an optical pulse due to the nonlinearity of the underlying processes. Therefore, precise knowledge of the pulse intensity, which is mainly limited by the accuracy of the temporal characterization, is a key prerequisite for the correct interpretation of experimental data. While the detection of energy and spatial profile is well established, the unambiguous temporal characterization of intense optical pulses, another important parameter required for intensity evaluation, remains a challenge, especially at relativistic intensities and a few-cycle pulse duration. Here, we report on the progress in the temporal characterization of intense laser pulses and present the relativistic surface second harmonic generation dispersion scan (RSSHG-D-scan)—a new approach allowing direct on-target temporal characterization of high-energy, few-cycle optical pulses at relativistic intensity., Lasers: Lighting up high-intensity pulses A new approach for characterizing intense optical pulses, by directly analyzing their interaction with a target, will help researchers to advance many strong-field applications. The development of applications such as laser-based charged-particle acceleration and bright coherent X-ray generation requires accurate knowledge of the characteristics of laser light at relativistic intensity. The approach is a refined form of ‘dispersion scanning’, which monitors the effect of an intense laser pulse as it interacts with material, thereby taking advantage of the processes that occur at relativistic intensities. The technique has been developed by Vyacheslav Leshchenko and colleagues at the Max-Planck Institute for Quantum Optics in Garching, Germany. It is especially effective at tackling the difficult challenge of characterizing how very short intense pulses vary with time.

Temporal-intensity contrast is crucial in intense laser-matter interaction to circumvent the undesirable expansion of steep high-density plasma prior to the interaction with the main pulse. Nonlinear elliptical polarization rotation in an argon filled hollow-core fiber is used here for cleaning pedestals/satellite pulses of a chirped-pulse-amplifier based Ti:Sapphire laser. This source provides ∼35 μJ energy and sub-4-fs duration, and the process has >50% internal efficiency, more than the most commonly used pulse cleaning methods. Further, the contrast is improved by 3 orders of magnitude when measured after amplifying the pulses to 16 TW using non-collinear optical parametric chirped pulse amplification with a prospect to even further enhancement.

The sources of isolated ultrashort light pulses are highly requested in modern ultrafast and attosecond science. Even though some experiments with isolated attosecond pulses (APs) have already been demonstrated [1], such light sources are still quite unique and not easily available while trains of APs are routinely produced in so called high-harmonics generation experiments. Regardless of the generation mechanisms deep understanding and characterisation of these pulse trains is an essential step towards routine generation of isolated attosecond pulses.

The interaction of light with nanometer-sized solids provides the means of focusing optical radiation to sub-wavelength spatial scales with associated electric field enhancements offering new opportunities for multifaceted applications. We utilize collective effects in nanoplasmas with sub-two-cycle light pulses of extreme intensity to extend the waveform-dependent electron acceleration regime into the relativistic realm, by using 106 times higher intensity than previous works to date. Through irradiation of nanometric tungsten needles, we obtain multi-MeV energy electron bunches, whose energy and direction can be steered by the combined effect of the induced near-field and the laser field. We identified a two-step mechanism for the electron acceleration: (i) ejection within a sub-half-optical-cycle into the near-field from the target at >TVm−1 acceleration fields, and (ii) subsequent acceleration in vacuum by the intense laser field. Our observations raise the prospect of isolating and controlling relativistic attosecond electron bunches, and pave the way for next generation electron and photon sources.

Relativistic surface high harmonics have been considered a unique source for the generation of intense isolated attosecond pulses in the extreme ultra-violet and x-ray spectral ranges. Their practical realization, however, is still a challenging task and requires identification of optimum experimental conditions and parameters. Here, we present measurements and particle-in-cell simulations to determine the optimum values for the most important parameters. In particular, we investigate the dependence of harmonics efficiency, divergence, and beam quality on the pre-plasma scale length as well as identify the optimum conditions for generation of isolated attosecond pulses by measuring the dependence of the harmonics spectrum on the carrier - envelope phase of the driving infrared field. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

We present laser-waveform-dependent relativistic high harmonics from plasma surfaces, and use spectral interferometry to understand its generation process. The attotrain structure as well as the field-driven plasma surface motion during the process are revealed.

When a pulsed, few-cycle electromagnetic wave is focused by optics with f-number smaller than two, the frequency components it contains are focused to different regions of space, building up a complex electromagnetic field structure. Accurate numerical computation of this structure is essential for many applications such as the analysis, diagnostics, and control of high-intensity laser-matter interactions. However, straightforward use of finite-difference methods can impose unacceptably high demands on computational resources, owing to the necessity of resolving far-field and near-field zones at sufficiently high resolution to overcome numerical dispersion effects. Here, we present a procedure for fast computation of tight focusing by mapping a spherically curved far-field region to periodic space, where the field can be advanced by a dispersion-free spectral solver. In many cases of interest, the mapping reduces both run time and memory requirements by a factor of order 10, making it possible to carry out simulations on a desktop machine or a single node of a supercomputer. We provide an open-source C++ implementation with Python bindings and demonstrate its use for a desktop machine, where the routine provides the opportunity to use the resolution sufficient for handling the pulses with spectra spanning over several octaves. The described approach can facilitate the stability analysis of theoretical proposals, the studies based on statistical inferences, as well as the overall development and analysis of experiments with tightly-focused short laser pulses., Comment: 27 pages, 8 figures

The interaction of ultra-intense laser pulses with matter opened the way to generate the shortest light pulses available nowadays in the attosecond regime. Ionized solid surfaces, also called plasma mirrors, are promising tools to enhance the potential of attosecond sources in terms of photon energy, photon number and duration especially at relativistic laser intensities. Although the production of isolated attosecond pulses and the understanding of the underlying interactions represent a fundamental step towards the realization of such sources, these are challenging and have not yet been demonstrated. Here, we present laser-waveform-dependent high-order harmonic radiation in the extreme ultraviolet spectral range supporting well-isolated attosecond pulses, and utilize spectral interferometry to understand its relativistic generation mechanism. This unique interpretation of the measured spectra provides access to unrevealed temporal and spatial properties such as spectral phase difference between attosecond pulses and field-driven plasma surface motion during the process., High-order harmonic generation is explored in gases, solids and plasmas with moderate to high intensity lasers. Here the authors show spectral interferometry of HHG from relativistic plasma and its potential as a source of intense isolated attosecond pulses.

The study of core electron dynamics through nonlinear spectroscopy requires intense isolated attosecond extreme ultraviolet or even X-ray pulses. A robust way to produce these pulses is high-harmonic generation (HHG) in a gas medium. However, the energy upscaling of the process depends on a very demanding next-generation laser technology that provides multi-terawatt (TW) laser pulses with few-optical-cycle duration and controlled electric field. Here, we revisit the HHG process driven by 16-TW sub-two-cycle laser pulses to reach high intensity in the 100-eV spectral region and beyond. We show that the combination of above barrier-suppression intensity with a long generation medium significantly enhances the isolation of attosecond pulses compared to lower intensities and/or shorter media and this way reduces the pulse duration as well as field-stability requirements on the laser driver. This novel regime facilitates the real-time observation of electron dynamics at the attosecond timescale in atoms, molecules, and solids.

We investigate the temporal contrast of the Light Wave Synthesizer 20 (LWS-20): a powerful, few-cycle source based on the optical parametric synthesizer principle. Saturation effects in the RF amplifier driving the acousto-optic programmable dispersive filter (AOPDF) were found to degrade the coherent contrast for non-monotonic group delay corrections. We subsequently present a new dispersion scheme and design a novel transmission grism-based stretcher optimized for LWS-20. The resulting temporal contrast of the amplified, compressed output pulses is improved by 2-4 orders of magnitude compared to the former design.

We investigated the interaction of nanometric tungsten targets with ultra-relativistic-intensity sub-5-fs laser pulses. Electrons accelerated to multi-MeV energy with electric fields exceeding the TV/m range and dependence on the laser waveform (carrier-envelope phase) were observed.

We investigate relativistic high-order harmonic generation from intense few-cycle laser and plasma interactions via particle-in-cell simulations. Differences of spectral and temporal structures in XUV beam resulted from few-cycle and non-few-cycle driver pulses are compared.

We demonstrate multiphoton ionization of inner-shell electrons in Xenon with 100-eV attosecond pulses. This was achieved with a novel XUV source based on high-harmonic generation in the gas phase driven with multi-TW few-cycle laser pulses.

Ultrafast pump–probe experiments open the possibility to track fundamental material behaviour, such as changes in electronic configuration, in real time. To date, most of these experiments are performed using an electron or a high-energy photon beam that is synchronized to an infrared laser pulse. Entirely new opportunities can be explored if not only a single, but multiple synchronized, ultrashort, 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 the future on a laboratory scale.

Nonlinear light-matter interactions in the extreme ultraviolet (XUV) are a prerequisite to perform XUV-pump/XUV-probe spectroscopy of core electrons. Such interactions are now routinely investigate ...

Cluster size distributions were investigated in case of different nozzle geometries in argon and xenon using Rayleigh scattering diagnostics. Different nozzle geometries result in different behaviour, therefore both spatial- and temporal cluster size distributions were studied to obtain a well-characterized cluster target. It is shown that the generally used Hagena scaling can result in a significant deviation from the observed data and the behaviour cannot be described by a single material condensation parameter. The results along with the nanoplasma model applied to the data of previous high harmonic generation experiments allow the independent measurement of cluster size and cluster density., 7 pages, 6 figures

Nonlinear photoionization with energetic FEL pulses has opened up new horizons for the investigation of inner-shell electron dynamics in atomic and molecular systems [1]. So far, however, the limited temporal resolution (typically a few tens of femtoseconds) achievable with FELs has hampered the time resolution of these dynamics.

We have investigated the polarization of attosecond light pulses generated from relativistic few-cycle laser pulse interaction with the surface of overdense plasmas using particle-in-cell simulation. Under suitable conditions, a desired polarization state of the generated attosecond pulse can be achieved by controlling the polarization of the incident laser. In particular, an elliptically polarized laser pulse of suitable ellipticity can generate an almost circularly polarized attosecond pulse without compromising the harmonic generation efficiency. The process is thus applicable as a new tabletop circularly-polarized XUV radiation source for probing attosecond phenomena with high temporal resolution.

We report on the generation of high-order harmonics in gas media using a sub-5 fs multi-ten-terawatt laser system. The application of this source towards XUV-pump/XUV-probe experiments with photon energies around 100 eV is discussed.

Few-fs electron bunches from laser wakefield acceleration (LWFA) can efficiently drive plasma wakefields (PWFs), as shown by their propagation through underdense plasma in two experiments. A strong and density-insensitive deceleration of the bunches has been observed in 2 mm of 10^{18} cm^{-3} density plasma with 5.1 GV/m average gradient, which is attributed to a self-driven PWF. This observation implies that the physics of PWFs, usually relying on large-scale rf accelerators as drivers, can be studied by tabletop LWFA electron sources.

When a high-intensity laser pulse interacts with a plasma it generates immense fields that can accelerate charged particles. Combining high-speed polarimetry and plasma shadowgraph enables the detailed evolution of this process to be imaged in real time. Electron acceleration by laser-driven plasma waves1,2 is capableof producing ultra-relativistic, quasi-monoenergetic electron bunches3,4,5 with orders of magnitude higher accelerating gradients and much shorter electron pulses than state-of-the-art radio-frequency accelerators. Recent developments have shown peak energies reaching into the GeV range6 and improved stability and control over the energy spectrum and charge7. Future applications, such as the development of laboratory X-ray sources with unprecedented peak brilliance8,9 or ultrafast time-resolved measurements10 critically rely on a temporal characterization of the acceleration process and the electron bunch. Here, we report the first real-time observation of the accelerated electron pulse and the accelerating plasma wave. Our time-resolved study allows a single-shot measurement of the 5.8−2.1+1.9 fs electron bunch full-width at half-maximum (2.5−0.9+0.8 fs root mean square) as well as the plasma wave with a density-dependent period of 12–22 fs and reveals the evolution of the bunch, its position in the surrounding plasma wave and the wake dynamics. The results afford promise for brilliant, sub-angstrom-wavelength ultrafast electron and photon sources for diffraction imaging with atomic resolution in space and time11.

We report on investigations of high-order harmonic generation from solid surfaces in the coherent wake emission regime with relativistically intense few-cycle (8 fs) laser pulses. Significant spectral broadening compared to previous experiments with many-cycle pulses and the appearance of substructures on the harmonics are observed that strongly fluctuate from shot-to-shot. Measurements in which the linear polarization was rotated or ellipticity of the laser pulse was varied exhibit a strong dependence of the harmonic emission on the polarization state of the incident pulse. We show that the observed spectral features are ultimately connected to the sub-cycle electron dynamics in the laser-solid interaction and thus proof of the few-cycle nature of the observed harmonic emission. Using a simple model we have investigated the factors that play an important role in the shape of the emitted spectrum.

Journal of optics 20(2), 024018 (2018). doi:10.1088/2040-8986/aaa326, The development of ultra-short intense laser sources in the visible and extreme ultraviolet (XUV) spectral range has 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 light 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 light 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., Published by IOP Publ., Bristol

We report on laser-driven electron acceleration with 8 fs, i.e. three optical cycles, pulse duration and 40 mJ energy. Theory and numerical simulations predict that this experimentally unexplored parameter range is relevant for laser wake-field acceleration. The electron spectra produced are monoenergetic with a peak up to 50 MeV and free of low-energy electrons with thermal spectrum. The electron beam has a typical divergence of 5–10 mrad. The accelerator is routinely operated at 10 Hz and correspondingly it is a promising source for several applications. To cite this article: L. Veisz et al., C. R. Physique 10 (2009).

An experiment demonstrating the generation of subfemtosecond pulses of light through the interaction of laser light with a solid target underlines the potential of this approach to lead to a new generation of intense sources of attosecond pulses.

In the present work, an innovative optical element developed for the multiterawatt few-cycle light wave synthesizer based on optical parametric amplification is demonstrated. The synthesizer currently produces sub-5-fs, 80-mJ, 18-TW pulses. The required element had to be deposited on 15 mm substrate having 75 mm in diameter; an average value of the GDD of +75 fs 2 and reflectance exceeding 99% in the spectral wavelength region from 570 to 1030 nm. Mechanical stresses of the designed dispersive mirror (DM) caused the substrate bending that distorts wave front quality resulting in the distorting of the beam waveform of laser system. In order to compensate this mechanical stress, the substrate back side was covered by an antireflection coating providing the lowest possible reflection in the working spectral range, having the total physical thickness close to DM thickness and total thicknesses of Nb 2 O 5 /SiO 2 layers close to the corresponding total thicknesses of Nb 2 O 5 /SiO 2 in DM. The produced DM-AR optical element exhibits excellent spectral properties.

We demonstrate a tool for quantitative measurements in the extreme ultraviolet (EUV) spectral region measuring spatially resolved atomic ionization products at the focus of an EUV beam. The ionizing radiation is a comb of the 11th–15th harmonics of a Ti:Sapphire femtosecond laser beam produced in a Xenon gas jet. The spatial ion distribution at the focus of the harmonics is recorded using an ion microscope. Spatially resolved single- and two-photon ionization products of Argon and Helium are observed. From such ion distributions single- and two-photon generalized cross sections can be extracted by a self-calibrating method. The observation of spatially resolved two-EUV-photon ionization constitutes an initial step towards future single-shot temporal characterization of attosecond pulses.

Strong-field ultrafast photoemission was studied by propagating surface plasmons generated on gold metal layer in Kretschmann configuration at 3.1 microns wavelength. Tunneling photoemission and electron acceleration was demonstrated at an unprecedently low laser intensity (1–5 GW/cm2).

We report on simultaneous efficiency and divergence measurements for harmonics from solid targets generated by the relativistic oscillating mirror mechanism. For a value of the normalized vector potential of \(a_{\mathrm{L}}\simeq 1.5\), we demonstrate the generation of 30 μJ high-harmonic radiation in a \(17\pm 3\) mrad divergence cone. This corresponds to a conversion efficiency of \(\gtrsim\)10−4 in the 17–80 nm range into a well-confined beam. Presuming phase-locked harmonics, our results predict unprecedented levels of average power for a single attosecond pulse in the generated pulse train. Results of PIC simulations raise the prospect of attaining efficiencies of a few percent at higher laser intensities.