Near- and Extended-Edge X-Ray-Absorption Fine-Structure Spectroscopy Using Ultrafast Coherent High-Order Harmonic Supercontinua

Table-top source for x-ray absorption spectroscopy with photon energies up to 350 eV

We present a table-top setup for x-ray absorption spectroscopy (XAS) based on high harmonic generation (HHG) in noble gases. Using sub-millijoule pump pulses at a central wavelength of 1550 nm, broadband HHG in the range of 70-350 eV was demonstrated. The HHG coherence lengths of several millimeters were achieved by reaching the nonadiabatic regime of harmonic generation. Near edge x-ray absorption fine structure spectroscopy experiments on the boron K edge of a boron foil and a hexagonal boron nitride (hBN) 2D material demonstrate the capabilities of the setup. Femtosecond pulse duration makes pump-probe XAS experiments with corresponding time resolution possible.

Mapping Hydrogen Positions along the Proton Transfer Pathway in an Organic Crystal by Computational X-ray Spectra

Understanding proton transfer (PT) dynamics in condensed phases is crucial in chemistry. We computed a 2D map of N 1s X-ray photoelectron/absorption spectroscopy (XPS/XAS) for an organic donor-acceptor salt crystal against two varying N-H distances to track proton motions. Our results provide a continuous spectroscopic mapping of O-H···N↔O-··· H+-N processes via hydrogen bonds at both nitrogens, demonstrating the sensitivity of N 1s transient XPS/XAS to hydrogen positions and PT. By reducing the O-H length at N1 by only 0.2 Å, we achieved excellent theory-experiment agreement in both XPS and XAS. Our study highlights the challenge in refining proton positions in experimental crystal structures by periodic geometry optimizations and proposes an alternative scaled snapshot protocol as a more effective approach. This work provides valuable insights into X-ray spectra for correlated PT dynamics in complex crystals, benefiting future experimental studies.

Attosecond-pump attosecond-probe x-ray spectroscopy of liquid water

Attosecond-pump/attosecond-probe experiments have long been sought as the most straightforward method for observing electron dynamics in real time. Although there has been much success with overlapped near-infrared femtosecond and extreme ultraviolet attosecond pulses combined with theory, true attosecond-pump/attosecond-probe experiments have been limited. We used a synchronized attosecond x-ray pulse pair from an x-ray free-electron laser to study the electronic response to valence ionization in liquid water through all x-ray attosecond transient absorption spectroscopy (AX-ATAS). Our analysis showed that the AX-ATAS response is confined to the subfemtosecond timescale, eliminating any hydrogen atom motion and demonstrating experimentally that the 1b1 splitting in the x-ray emission spectrum is related to dynamics and is not evidence of two structural motifs in ambient liquid water.

Ultrashort large-bandwidth X-ray free-electron laser generation with a dielectric-lined waveguide

Large-bandwidth pulses produced by cutting-edge X-ray free-electron lasers (FELs) are of great importance in research fields like material science and biology. In this paper, a new method to generate high-power ultrashort FEL pulses with tunable spectral bandwidth with spectral coherence using a dielectric-lined waveguide without interfering operation of linacs is proposed. By exploiting the passive and dephasingless wakefield at terahertz frequency excited by the beam, stable energy modulation can be achieved in the electron beam and large-bandwidth high-intensity soft X-ray radiation can be generated. Three-dimensional start-to-end simulations have been carried out and the results show that coherent radiation pulses with duration of a few femtoseconds and bandwidths ranging from 1.01% to 2.16% can be achieved by changing the undulator taper profile.

Propagation effects in polarization-gated attosecond soft-X-ray pulse generation

Accurate estimation of the duration of soft-x-ray pulses from high-harmonic generation (HHG) remains challenging given their higher photon energies and broad spectral bandwidth. The carrier-envelope-phase (CEP) dependence of generated soft-x-ray spectra is indicative of attosecond pulse generation, but advanced simulations are needed to infer the pulse duration from such data. Here, we employ macroscopic propagation simulations to reproduce experimental polarization-gated CEP-dependent soft-x-ray spectra. The simulations indicate chirped pulses, which we theoretically find to be compressible in hydrogen plasmas, suggesting this as a viable compression scheme for broadband soft-x-rays from HHG.

Highly efficient and aberration-free off-plane grating spectrometer and monochromator for EUV-soft X-ray applications

We demonstrate a novel flat-field, dual-optic imaging EUV-soft X-ray spectrometer and monochromator that attains an unprecedented throughput efficiency exceeding 60% by design, along with a superb spectral resolution of λ/Δλ > 200 accomplished without employing variable line spacing gratings. Exploiting the benefits of the conical diffraction geometry, the optical system is globally optimized in multidimensional parameter space to guarantee optimal imaging performance over a broad spectral range while maintaining circular and elliptical polarization states at the first, second, and third diffraction orders. Moreover, our analysis indicates minimal temporal dispersion, with pulse broadening confined within 80 fs tail-to-tail and an FWHM value of 29 fs, which enables ultrafast spectroscopic and pump-probe studies with femtosecond accuracy. Furthermore, the spectrometer can be effortlessly transformed into a monochromator spanning the EUV-soft X-ray spectral region using a single grating with an aberration-free spatial profile. Such capability allows coherent diffractive imaging applications to be conducted with highly monochromatic light in a broad spectral range and extended to the soft X-ray region with minimal photon loss, thus facilitating state-of-the-art imaging of intricate nano- and bio-systems, with a significantly enhanced spatiotemporal resolution, down to the nanometer-femtosecond level.

Toward ultrafast soft x-ray spectroscopy of organic photovoltaic devices

Novel ultrafast x-ray sources based on high harmonic generation and at x-ray free electron lasers are opening up new opportunities to resolve complex ultrafast processes in condensed phase systems with exceptional temporal resolution and atomic site specificity. In this perspective, we present techniques for resolving charge localization, transfer, and separation processes in organic semiconductors and organic photovoltaic devices with time-resolved soft x-ray spectroscopy. We review recent results in ultrafast soft x-ray spectroscopy of these systems and discuss routes to overcome the technical challenges in performing time-resolved x-ray experiments on photosensitive materials with poor thermal conductivity and low pump intensity thresholds for nonlinear effects.

Applications of X-Ray-Based Characterization in MXene Research

X-rays are a penetrating form of high-energy electromagnetic radiation with wavelengths ranging from 10 pm to 10 nm. Similar to visible light, X-rays provide a powerful tool to study the atoms and elemental information of objects. Different characterization methods based on X-rays are established, such as X-ray diffraction, small- and wide-angle X-ray scattering, and X-ray-based spectroscopies, to explore the structural and elemental information of varied materials including low-dimensional nanomaterials. This review summarizes the recent progress of using X-ray related characterization methods in MXenes, a new family of 2D nanomaterials. These methods provide key information on the nanomaterials, covering synthesis, elemental composition, and the assembly of MXene sheets and their composites. Additionally, new characterization methods are proposed as future research directions in the outlook section to enhance understanding of MXene surface and chemical properties. This review is expected to provide a guideline for characterization method selection and aid in precise interpretation of the experimental data in MXene research.

Pulse shaping in a midwave-IR OPCPA for multi-µJ few-cycle pulse generation at 12 µm via DFG

We report on dispersion management in mid-IR optical parametric chirped pulse amplifiers (OPCPA) aiming for high-energy few-cycle pulses beyond 4 µm. The available pulse shapers in this spectral region limit the feasibility of sufficient higher-order phase control. Intending the generation of high energy pulses at 12 µm via DFG driven by the signal and idler pulses of a midwave-IR OPCPA, we introduce alternative approaches for mid-IR pulse shaping, namely a germanium-prism pair and a sapphire-prism Martinez compressor. Furthermore, we explore the limits of bulk compression in Si and Ge for multi-mJ pulse energies.

Apparatus for attosecond transient-absorption spectroscopy in the water-window soft-X-ray region

We present an apparatus for attosecond transient-absorption spectroscopy (ATAS) featuring soft-X-ray (SXR) supercontinua that extend beyond 450 eV. This instrument combines an attosecond table-top high-harmonic light source with mid-infrared (mid-IR) pulses, both driven by 1.7-1.9 mJ, sub-11 fs pulses centered at 1.76 [Formula: see text]m. A remarkably low timing jitter of [Formula: see text] 20 as is achieved through active stabilization of the pump and probe arms of the instrument. A temporal resolution of better than 400 as is demonstrated through ATAS measurements at the argon L[Formula: see text]-edges. A spectral resolving power of 1490 is demonstrated through simultaneous absorption measurements at the sulfur L[Formula: see text]- and carbon K-edges of OCS. Coupled with its high SXR photon flux, this instrument paves the way to attosecond time-resolved spectroscopy of organic molecules in the gas phase or in aqueous solutions, as well as thin films of advanced materials. Such measurements will advance the studies of complex systems to the electronic time scale.

Extension of the bright high-harmonic photon energy range via nonadiabatic critical phase matching

The concept of critical ionization fraction has been essential for high-harmonic generation, because it dictates the maximum driving laser intensity while preserving the phase matching of harmonics. In this work, we reveal a second, nonadiabatic critical ionization fraction, which substantially extends the phase-matched harmonic energy, arising because of the strong reshaping of the intense laser field in a gas plasma. We validate this understanding through a systematic comparison between experiment and theory for a wide range of laser conditions. In particular, the properties of the high-harmonic spectrum versus the laser intensity undergoes three distinctive scenarios: (i) coincidence with the single-atom cutoff, (ii) strong spectral extension, and (iii) spectral energy saturation. We present an analytical model that predicts the spectral extension and reveals the increasing importance of the nonadiabatic effects for mid-infrared lasers. These findings are important for the development of high-brightness soft x-ray sources for applications in spectroscopy and imaging.

Analytical Lah-Laguerre optical formalism for perturbative chromatic dispersion

We present a generalized perturbative analytical formalism for evaluation and optimization of the chromatic dispersion of complex ultrafast optical systems. Notably, we identify polynomial and recursive relations associated with the chromatic dispersion orders that are identical to the Lah and Laguerre transforms. We explicitly outline the first ten dispersion terms and dispersion slope parameters and visualize the significance of the chromatic dispersion orders for several advanced ultrafast optical and photonic systems consisting of various optical materials and nanostructures, grating and prism-pair compressors, and hollow-core photonic anti-resonant fibers. The derived simple hypergeometric transforms are applicable for evaluation of infinitely high orders for any type of frequency-dependent phase and can facilitate the optimization of complex optical systems with controlled dispersion balance at the single-cycle waveform extreme.

How electronic superpositions drive nuclear motion following the creation of a localized hole in the glycine radical cation

In this work, we have studied the nuclear and electron dynamics in the glycine cation starting from localized hole states using the quantum Ehrenfest method. The nuclear dynamics is controlled both by the initial gradient and by the instantaneous gradient that results from the oscillatory electron dynamics (charge migration). We have used the Fourier transform (FT) of the spin densities to identify the “normal modes” of the electron dynamics. We observe an isomorphic relationship between the electron dynamics normal modes and the nuclear dynamics, seen in the vibrational normal modes. The FT spectra obtained this way show bands that are characteristic of the energy differences between the adiabatic hole states. These bands contain individual peaks that are in one-to-one correspondence with atom pair (+·) ↔ (·+) resonances, which, in turn, stimulate nuclear motion involving the atom pair. With such understanding, we anticipate “designer” coherent superpositions that can drive nuclear motion in a particular direction.

Experimental Demonstration of Efficient Harmonic Generation via Surface Plasma Compression with Lasers

The efficiency of high-order harmonic generation from a relativistic laser interacting with solid targets depends greatly on surface plasma distribution. The usual method of enhancing efficiency involves tuning the plasma scale length carefully by improving the laser contrast. Here, we experimentally demonstrate that efficient harmonics can be achieved directly by compressing large-scale surface plasma via the radiation pressure of a circularly polarized normally incident prepulse. The harmonic generation efficiency obtained by this method is comparable to that obtained with optimized plasma scale length by high-contrast lasers. Our scheme does not rely on high-contrast lasers and is robust and easy to implement. Thus, it may pave a way for the development of intense extreme ultraviolet sources and future applications with high repetition rates.

Direct observation of ultrafast exciton localization in an organic semiconductor with soft X-ray transient absorption spectroscopy

The localization dynamics of excitons in organic semiconductors influence the efficiency of charge transfer and separation in these materials. Here we apply time-resolved X-ray absorption spectroscopy to track photoinduced dynamics of a paradigmatic crystalline conjugated polymer: poly(3-hexylthiophene) (P3HT) commonly used in solar cell devices. The π→π^* transition, the first step of solar energy conversion, is pumped with a 15 fs optical pulse and the dynamics are probed by an attosecond soft X-ray pulse at the carbon K-edge. We observe X-ray spectroscopic signatures of the initially hot excitonic state, indicating that it is delocalized over multiple polymer chains. This undergoes a rapid evolution on a sub 50 fs timescale which can be directly associated with cooling and localization to form either a localized exciton or polaron pair.

A detailed understanding of ultrafast exciton dynamics is crucial for improving the efficiency of organic light-harvesting-devices. Here, the authors track exciton localization on a sub-50 fs timescale in an organic semiconductor using time resolved soft x-ray absorption spectroscopy.

Attosecond spectroscopy for the investigation of ultrafast dynamics in atomic, molecular and solid-state physics

Since the first demonstration of the generation of attosecond pulses (1 as = 10^-18s) in the extreme-ultraviolet spectral region, several measurement techniques have been introduced, at the beginning for the temporal characterization of the pulses, and immediately after for the investigation of electronic and nuclear ultrafast dynamics in atoms, molecules and solids with unprecedented temporal resolution. The attosecond spectroscopic tools established in the last two decades, together with the development of sophisticated theoretical methods for the interpretation of the experimental outcomes, allowed to unravel and investigate physical processes never observed before, such as the delay in photoemission from atoms and solids, the motion of electrons in molecules after prompt ionization which precede any notable nuclear motion, the temporal evolution of the tunneling process in dielectrics, and many others. This review focused on applications of attosecond techniques to the investigation of ultrafast processes in atoms, molecules and solids. Thanks to the introduction and ongoing developments of new spectroscopic techniques, the attosecond science is rapidly moving towards the investigation, understanding and control of coupled electron-nuclear dynamics in increasingly complex systems, with ever more accurate and complete investigation techniques. Here we will review the most common techniques presenting the latest results in atoms, molecules and solids.

Advances in laboratory-scale ptychography using high harmonic sources [Invited]

Extreme ultraviolet microscopy and wavefront sensing are key elements for next-generation ultrafast applications, such as chemically-resolved imaging, focal spot diagnostics in pump-and-probe experiments, and actinic metrology for the state-of-the-art lithography node at 13.5 nm wavelength. Ptychography offers a robust solution to the aforementioned challenges. Originally adapted by the electron and synchrotron communities, advances in the stability and brightness of high-harmonic tabletop sources have enabled the transfer of ptychography to the laboratory. This review covers the state of the art in tabletop ptychography with high harmonic generation sources. We consider hardware options such as illumination optics and detector concepts as well as algorithmic aspects in the analysis of multispectral ptychography data. Finally, we review technological application cases such as multispectral wavefront sensing, attosecond pulse characterization, and depth-resolved imaging.

Demonstration of Near Edge X-ray Absorption Fine Structure Spectroscopy of Transition Metals Using Xe/He Double Stream Gas Puff Target Soft X-ray Source

A near 1-keV photons from the Xe/He plasma produced by the interaction of laser beam with a double stream gas puff target were employed for studies of L absorption edges of period 4 transitional metals with atomic number Z from 26 to 30. The dual-channel, compact NEXAFS system was employed for the acquisition of the absorption spectra. L_1-3 absorption edges of the samples were identified in transmission mode using broadband emission from the Xe/He plasma to show the applicability of such source and measurement system to the NEXAFS studies of the transition metals, including magnetic materials.

Mid-Infrared Few-Cycle Pulse Generation and Amplification

In the past decade, mid-infrared (MIR) few-cycle lasers have attracted remarkable research efforts for their applications in strong-field physics, MIR spectroscopy, and bio-medical research. Here we present a review of MIR few-cycle pulse generation and amplification in the wavelength range spanning from 2 to ~20 μm. In the first section, a brief introduction on the importance of MIR ultrafast lasers and the corresponding methods of MIR few-cycle pulse generation is provided. In the second section, different nonlinear crystals including emerging non-oxide crystals, such as CdSiP2, ZnGeP2, GaSe, LiGaS2, and BaGa4Se7, as well as new periodically poled crystals such as OP-GaAs and OP-GaP are reviewed. Subsequently, in the third section, the various techniques for MIR few-cycle pulse generation and amplification including optical parametric amplification, optical parametric chirped-pulse amplification, and intra-pulse difference-frequency generation with all sorts of designs, pumped by miscellaneous lasers, and with various MIR output specifications in terms of pulse energy, average power, and pulse width are reviewed. In addition, high-energy MIR single-cycle pulses are ideal tools for isolated attosecond pulse generation, electron dynamic investigation, and tunneling ionization harness. Thus, in the fourth section, examples of state-of-the-art work in the field of MIR single-cycle pulse generation are reviewed and discussed. In the last section, prospects for MIR few-cycle lasers in strong-field physics, high-fidelity molecule detection, and cold tissue ablation applications are provided.

The 2021 ultrafast spectroscopic probes of condensed matter roadmap

In the 60 years since the invention of the laser, the scientific community has developed numerous fields of research based on these bright, coherent light sources, including the areas of imaging, spectroscopy, materials processing and communications. Ultrafast spectroscopy and imaging techniques are at the forefront of research into the light-matter interaction at the shortest times accessible to experiments, ranging from a few attoseconds to nanoseconds. Light pulses provide a crucial probe of the dynamical motion of charges, spins, and atoms on picosecond, femtosecond, and down to attosecond timescales, none of which are accessible even with the fastest electronic devices. Furthermore, strong light pulses can drive materials into unusual phases, with exotic properties. In this roadmap we describe the current state-of-the-art in experimental and theoretical studies of condensed matter using ultrafast probes. In each contribution, the authors also use their extensive knowledge to highlight challenges and predict future trends.

Agile spectral tuning of high order harmonics by interference of two driving pulses

In this work, the experimental realization of a tunable high photon flux extreme ultraviolet light source is presented. This is enabled by high harmonic generation of two temporally delayed driving pulses with a wavelength of 1030 nm, resulting in a tuning range of 0.8 eV at the 19^th harmonic at 22.8 eV. The implemented approach allows for fast tuning of the spectrum, is highly flexible and is scalable towards full spectral coverage at higher photon energies.

Femtosecond Single Cycle Pulses Enhanced the Efficiency of High Order Harmonic Generation

High-order harmonic generation is a nonlinear process that converts the gained energy during light-matter interaction into high-frequency radiation, thus resulting in the generation of coherent attosecond pulses in the XUV and soft x-ray regions. Here, we propose a control scheme for enhancing the efficiency of HHG process induced by an intense near-infrared (NIR) multi-cycle laser pulse. The scheme is based on introducing an infrared (IR) single-cycle pulse and exploiting its characteristic feature that manifests by a non-zero displacement effect to generate high-photon energy. The proposed scenario is numerically implemented on the basis of the time-dependent Schrödinger equation. In particular, we show that the combined pulses allow one to produce high-energy plateaus and that the harmonic cutoff is extended by a factor of 3 compared to the case with the NIR pulse alone. The emerged high-energy plateaus is understood as a result of a vast momentum transfer from the single-cycle field to the ionized electrons while travelling in the NIR field, thus leading to high-momentum electron recollisions. We also identify the role of the IR single-cycle field for controlling the directionality of the emitted electrons via the IR-field induced electron displacement effect. We further show that the emerged plateaus can be controlled by varying the relative carrier-envelope phase between the two pulses as well as the wavelengths. Our findings pave the way for an efficient control of light-matter interaction with the use of assisting femtosecond single-cycle fields.

Enhanced soft x-ray betatron radiation from a transversely oscillating laser plasma wake

Betatron radiation from relativistic electrons accelerated by a laser plasma wakefield is a promising x-ray source. However, the betatron photon number and energy is limited by the MeV electron energy when accelerated by a sub-terawatt laser pulse. Here we have proposed a scheme of enhancing the betatron radiation of electrons accelerated by a millijoule laser pulse. By applying a moderate mid-infrared control laser field, the plasma bubble and electrons injected inside are forced to oscillate transversely at a high, tunable frequency, enhancing the betatron oscillation strength and resulting in soft x-ray radiation.

Millijoule few-cycle pulses from staged compression for strong and high field science

Intense few-cycle laser pulses have a breadth of applications in high energy density science, including particle acceleration and x-ray generation. Multi-amplifier laser system pulses have durations of tens of femtoseconds or longer. To achieve high intensities at the single-cycle limit, a robust and efficient post-compression scheme is required. We demonstrate a staged compression technique using self-phase modulation in thin dielectric media, in which few-cycle pulses can be produced. The few-cycle pulse is then used to generate extreme ultravoilet light via high harmonic generation at strong field intensities and to generate MeV electron beams via laser solid interactions at relativistic intensities.

Investigation of spatiotemporal output beam profile instabilities from differentially pumped capillaries

Differentially pumped capillaries, i.e., capillaries operated in a pressure gradient environment, are widely used for nonlinear pulse compression. In this work, we show that strong pressure gradients and high gas throughputs can cause spatiotemporal instabilities of the output beam profile. The instabilities occur with a sudden onset as the flow evolves from laminar to turbulent. Based on the experimental and numerical results, we derive guidelines to predict the onset of those instabilities and discuss possible applications in the context of nonlinear flow dynamics.

Bright, high-repetition-rate water window soft X-ray source enabled by nonlinear pulse self-compression in an antiresonant hollow-core fibre

Bright, coherent soft X-ray radiation is essential to a variety of applications in fundamental research and life sciences. To date, a high photon flux in this spectral region can only be delivered by synchrotrons, free-electron lasers or high-order harmonic generation sources, which are driven by kHz-class repetition rate lasers with very high peak powers. Here, we establish a novel route toward powerful and easy-to-use SXR sources by presenting a compact experiment in which nonlinear pulse self-compression to the few-cycle regime is combined with phase-matched high-order harmonic generation in a single, helium-filled antiresonant hollow-core fibre. This enables the first 100 kHz-class repetition rate, table-top soft X-ray source that delivers an application-relevant flux of 2.8 × 10⁶ photon s^-1 eV^-1 around 300 eV. The fibre integration of temporal pulse self-compression (leading to the formation of the necessary strong-field waveforms) and pressure-controlled phase matching will allow compact, high-repetition-rate laser technology, including commercially available systems, to drive simple and cost-effective, coherent high-flux soft X-ray sources.

Laboratory quick near edge x-ray absorption fine structure spectroscopy in the soft x-ray range with 100 Hz frame rate using CMOS technology

In laboratory based x-ray absorption fine structure (XAFS) spectroscopy, the slow readout speed of conventional CCD cameras can prolong the measuring times by multiple orders of magnitude. Using pulsed sources, e.g., laser-based x-ray sources, the pulse repetition rate often exceeds the frame rate of the CCD camera. We report the use of a scientific CMOS (sCMOS) camera for XAFS spectroscopy with a laser-produced plasma source facilitating measurements at 100 Hz. With this technological improvement, a new class of experiments becomes possible, starting from the time consuming analysis of samples with small absorption to pump-probe investigations. Furthermore, laboratory quick soft x-ray absorption fine structure (QXAFS) measurements with 10 ms time resolution are rendered feasible. We present the characterization of the sCMOS camera concerning noise characteristics and a comparison to conventional CCD camera performance. The feasibility of time resolved QXAFS measurements is shown by analyzing the statistical uncertainty of single shot spectra. Finally, XAFS spectroscopy on a complex sandwich structure with minute amounts of NiO exemplifies the additional merits of fast detectors.

Spin dynamics at interfaces on femtosecond timescales

The excitation of magnetically ordered materials with ultrashort laser pulses results in magnetization dynamics on femto- to picosecond timescales. These non-equilibrium spin dynamics have emerged as a rapidly developing research field in recent years. Unraveling the fundamental microscopic processes in the interaction of ultrashort optical pulses with the charge, spin, orbital, and lattice degrees of freedom in magnetic materials shows the potential for controlling spin dynamics on their intrinsic timescales and thereby bring spintronics applications into the femtosecond range. In particular, femtosecond spin currents offer fascinating new possibilities to manipulate magnetization in an ultrafast and non-local manner, via spin injection and spin transfer torque at the interfaces of ferromagnetic layered structures. This topical review covers recent progress on spin dynamics at interfaces on femtosecond time scales. The development of the field of ultrafast spin dynamics in ferromagnetic heterostructures will be reviewed, starting from spin currents propagating on nanometer length scales through layered structures before focusing on femtosecond spin transfer at interfaces. The properties of these ultrafast spin-dependent charge currents will be discussed, as well as the materials dependence of femtosecond spin injection, the role of the interface properties, and competing microscopic processes leading to a loss of spin polarization on sub-picosecond timescales.

Biological Applications of Short Wavelength Microscopy Based on Compact, Laser-Produced Gas-Puff Plasma Source

Over the last decades, remarkable efforts have been made to improve the resolution in photon-based microscopes. The employment of compact sources based on table-top laser-produced soft X-ray (SXR) in the “water window” spectral range (λ = 2.3–4.4 nm) and extreme ultraviolet (EUV) plasma allowed to overcome the limitations imposed by large facilities, such as synchrotrons and X-ray free electron lasers (XFEL), because of their high complexity, costs, and limited user access. A laser-plasma double stream gas-puff target source represents a powerful tool for microscopy operating in transmission mode, significantly improving the spatial resolution into the nanometric scale, comparing to the traditional visible light (optical) microscopes. Such an approach allows generating the plasma efficiently, without debris, providing a high flux of EUV and SXR photons. In this review, we present the development and optimization of desktop imaging systems: a EUV and an SXR full field microscope, allowing to achieve a sub-50 nm spatial resolution with short exposure time and an SXR contact microscope, capable to resolve internal structures in a thin layer of sensitive photoresist. Details about the source, as well as imaging results for biological applications, will be presented and discussed.

The versatile X-ray beamline of the Munich Compact Light Source: design, instrumentation and applications

Inverse Compton scattering provides means to generate low-divergence partially coherent quasi-monochromatic, i.e. synchrotron-like, X-ray radiation on a laboratory scale. This enables the transfer of synchrotron techniques into university or industrial environments. Here, the Munich Compact Light Source is presented, which is such a compact synchrotron radiation facility based on an inverse Compton X-ray source (ICS). The recent improvements of the ICS are reported first and then the various experimental techniques which are most suited to the ICS installed at the Technical University of Munich are reviewed. For the latter, a multipurpose X-ray application beamline with two end-stations was designed. The beamline's design and geometry are presented in detail including the different set-ups as well as the available detector options. Application examples of the classes of experiments that can be performed are summarized afterwards. Among them are dynamic in vivo respiratory imaging, propagation-based phase-contrast imaging, grating-based phase-contrast imaging, X-ray microtomography, K-edge subtraction imaging and X-ray spectroscopy. Finally, plans to upgrade the beamline in order to enhance its capabilities are discussed.

Energy-Dispersive X-ray Absorption Spectroscopy with an Inverse Compton Source

Novel compact x-ray sources based on inverse Compton scattering can generate brilliant hard x-rays in a laboratory setting. Their collimated intense beams with tunable well-defined x-ray energies make them well suited for x-ray spectroscopy techniques, which are typically carried out at large facilities. Here, we demonstrate a first x-ray absorption spectroscopy proof-of-principle experiment using an inverse Compton x-ray source with a flux of >10¹⁰ photons/s in <5% bandwidth. We measured x-ray absorption near edge structure and extended x-ray absorption fine structure at the silver K-edge (~25.5 keV) for a series of silver samples. We propose an energy-dispersive geometry specifically adapted to the x-ray beam properties of inverse Compton x-ray sources together with a fast concentration correction method that corrects sample inhomogeneities very effectively. The combination of our setup with the inverse Compton source generates x-ray absorption spectra with high energy resolution in exposure times down to one minute. Our results unravel the great benefit of inverse Compton scattering sources for x-ray absorption techniques in a laboratory environment, especially in the hard x-ray regime, which allows to probe absorption edges of high Z materials.

Efficient table-top dual-wavelength beamline for ultrafast transient absorption spectroscopy in the soft X-ray region

We present a table-top beamline providing a soft X-ray supercontinuum extending up to 370 eV from high-order harmonic generation with sub-13 fs 1300 nm driving pulses and simultaneous production of sub-5 fs pulses centered at 800 nm. Optimization of high harmonic generation in a long and dense gas medium yields a photon flux of  ~ 1.4 × 106 photons/s/1% bandwidth at 300 eV. The temporal resolution of X-ray transient absorption experiments with this beamline is measured to be 11 fs for 800 nm excitation. This dual-wavelength approach, combined with high flux and high spectral and temporal resolution soft X-ray absorption spectroscopy, is a new route to the study of ultrafast electronic dynamics in carbon-containing molecules and materials at the carbon K-edge.

Femtosecond Soft-X-ray Absorption Spectroscopy of Liquids with a Water-Window High-Harmonic Source

Femtosecond X-ray absorption spectroscopy (XAS) is a powerful method to investigate the dynamical behavior of a system after photoabsorption in real time. So far, the application of this technique has remained limited to large-scale facilities, such as femtosliced synchrotrons and free-electron lasers (FEL). In this work, we demonstrate femtosecond time-resolved soft-X-ray absorption spectroscopy of liquid samples by combining a sub-micrometer-thin flat liquid jet with a high-harmonic tabletop source covering the entire water-window range (284-538 eV). Our work represents the first extension of tabletop XAS to the oxygen edge of a chemical sample in the liquid phase. In the time domain, our measurements resolve the gradual appearance of absorption features below the carbon K-edge of ethanol and methanol during strong-field ionization and trace the valence-shell ionization dynamics of the liquid alcohols with a temporal resolution of ∼30 fs. This technique opens unique opportunities to study molecular dynamics of chemical systems in the liquid phase with elemental, orbital, and site sensitivity.

27 W 2.1 µm OPCPA system for coherent soft X-ray generation operating at 10 kHz

We developed a high power optical parametric chirped-pulse amplification (OPCPA) system at 2.1 µm harnessing a 500 W Yb:YAG thin disk laser as the only pump and signal generation source. The OPCPA system operates at 10 kHz with a single pulse energy of up to 2.7 mJ and pulse duration of 30 fs. The maximum average output power of 27 W sets a new record for an OPCPA system in the 2 µm wavelength region. The soft X-ray continuum generated through high harmonic generation with this driver laser can extend to around 0.55 keV, thus covering the entire water window (284 eV - 543 eV). With a repetition rate still enabling pump-probe experiments on solid samples, the system can be used for many applications.

Novel method for the angular chirp compensation of passively CEP-stable few-cycle pulses

We demonstrate a novel, energy-efficient, cost-effective simple method for seeding CEP-stable OPCPAs. We couple the CEP-stable idler of a broadband OPCPA into a hollow core Kagome fiber thus compensating for the angular chirp. We obtain either relatively narrow bandwidths with ∼36% coupling efficiency or quarter-octave spanning bandwidths with ∼2.2% coupling efficiency. We demonstrate spectral compressibility, good beam quality and CEP stability. Our source is an ideal seed for high-energy, high-average power, CEP-stable few-cycle OPCPA pulses around 2 µm, which can drive the generation of coherent soft X-ray radiation in the water window spectral region via HHG.

High-quality high-order harmonic generation through preplasma truncation

By introducing preplasma truncation to cases with an initial preplasma scale length larger than 0.2λ, the efficiency of high-order harmonics generated from relativistic laser-solid interactions can be enhanced by more than one order of magnitude and the angular spread can be confined into near-diffraction-limited divergence. Numerical simulations show that density truncation results in more compact oscillation of the surface electron sheet and the curvature of the reflection surface for the driving laser is greatly reduced. This leads to an overall improvement in the harmonic beam quality. More importantly, density truncation makes the harmonic generation weakly dependent on the preplasma scale length, which provides a way to relax the extremely high requirement on the temporal contrast of the driving laser pulse. A feasible scheme to realize the required preplasma truncation is also proposed and demonstrated by numerical simulations.

Attosecond Pulse Amplification in a Plasma-Based X-Ray Laser Dressed by an Infrared Laser Field

High-harmonic generation (HHG) of laser radiation has led to attosecond pulse formation which offers unprecedented temporal resolution in observing and controlling electron and nuclear dynamics. But the energy of attosecond pulses remains quite small, especially for photon energies exceeding 100 eV, which limits their practical applications. We propose a method for amplification of attosecond pulses in the active medium of a plasma-based x-ray laser dressed by a replica of the laser field used for HHG. The experimental implementation is suggested in hydrogenlike C5+ x-ray laser at 3.4 nm wavelength in the "water window" range.

Towards Poisson noise limited optical pump soft X-ray probe NEXAFS spectroscopy using a laser-produced plasma source

We present a laboratory setup for near edge X-ray absorption spectroscopy (NEXAFS) in the soft X-ray regime between 284 eV to 1303 eV with a resolving power of up to 1370. Based on a laser-produced plasma source, a pair of identical reflection zone plates and an X-ray CCD camera, the setup is intended for optical pump X-ray probe NEXAFS measurements with a detectable change in absorption of the excited sample down to 10^-4 and 500 ps time resolution. Because of the high stability of the source the statistical error only depends on the detector response and the number of photons detected and can reach the detector noise limit after a couple of thousands single shots. Thus, structure-function relationship investigations of bio-molecules are rendered feasible in the laboratory.

Symmetry of molecular Rydberg states revealed by XUV transient absorption spectroscopy

Transient absorption spectroscopy is utilized extensively for measurements of bound- and quasibound-state dynamics of atoms and molecules. The extension of this technique into the extreme ultraviolet (XUV) region with attosecond pulses has the potential to attain unprecedented time resolution. Here we apply this technique to aligned-in-space molecules. The XUV pulses are much shorter than the time during which the molecules remain aligned, typically \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$<$$\end{document}<100 fs. However, transient absorption is not an instantaneous probe, because long-lived coherences re-emit for picoseconds to nanoseconds. Due to dephasing of the rotational wavepacket, it is not clear if these coherences will be evident in the absorption spectrum, and whether the properties of the initial excitations will be preserved. We studied Rydberg states of N\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{2}$$\end{document}2 and O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{2}$$\end{document}2 from 12 to 23 eV. We were able to determine the polarization direction of the electronic transitions, and hence identify the symmetry of the final states.

Transient absorption spectroscopy is used to identify the structural characteristics of the atoms and molecules. Here the authors used extreme ultraviolet transient absorption spectroscopy to identify the Rydberg state symmetry of aligned molecules.

Decomposing electronic and lattice contributions in optical pump - X-ray probe transient inner-shell absorption spectroscopy of CuO

Electronic and lattice contributions to picosecond time-resolved X-ray absorption spectra (trXAS) of CuO at the oxygen K-edge are analyzed by comparing trXAS spectra, recorded using excitation wavelengths of 355 nm and 532 nm, to steady-state, temperature-dependent XAS measurements. The trXAS spectra at pump-probe time-delays ≥150 ps are dominated by lattice heating effects. Insight into the temporal evolution of lattice temperature profiles on timescales up to 100s of nanoseconds after laser excitation are reported, on an absolute temperature scale, with a temporal sensitivity and a spatial selectivity on the order of 10s of picoseconds and 10s of nanometers, respectively, effectively establishing an "ultrafast thermometer". In particular, for the 532 nm experiment at ∼5 mJ cm-2 fluence, both the initial sample temperature and its dynamic evolution are well captured by a one-dimensional thermal energy deposition and diffusion model. The thermal conductivity k = (1.3 ± 0.4) W m-1 K-1 derived from this model is in good agreement with the literature value for CuO powder, kpowder = 1.013 W m-1 K-1. For 355 nm excitation, a quantitative analysis of the experiments is hampered by the large temperature gradients within the probed sample volume owing to the small UV penetration depth. The impact of the findings on mitigating or utilizing photoinduced lattice temperature changes in future X-ray free electron laser (XFEL) experiments is discussed.

Ultrafast X-Ray Absorption Spectroscopy of Strongly Correlated Systems: Core Hole Effect

In recent years, ultrafast pump-probe spectroscopy has provided insightful information about the nonequilibrium dynamics of excitations in materials. In a typical experiment of time-resolved x-ray absorption spectroscopy, the systems are excited by a femtosecond laser pulse (pump pulse) followed by an x-ray probe pulse after a time delay to measure the absorption spectra of the photoexcited systems. We present a theory for nonequilibrium x-ray absorption spectroscopy in one-dimensional strongly correlated systems. The core hole created by the x ray is modeled as an additional effective potential of the core hole site, which changes the spectrum qualitatively. In equilibrium, the spectrum reveals the charge gap at half-filling and the metal-insulator transition in the presence of the core hole effect. Furthermore, a pump-probe scheme is introduced to drive the system out of equilibrium before the x-ray probe. The effects of the pump pulse with varying frequencies, shapes, and fluences are discussed for the dynamics of strongly correlated systems in and out of resonance. The spectrum indicates that the driven insulating state has a metallic droplet around the core hole. The rich structures of the nonequilibrium x-ray absorption spectrum give more insight into the dynamics of electronic structures.

Transient absorption spectroscopy using high harmonic generation: a review of ultrafast X-ray dynamics in molecules and solids

Attosecond science opened the door to observing nuclear and electronic dynamics in real time and has begun to expand beyond its traditional grounds. Among several spectroscopic techniques, X-ray transient absorption spectroscopy has become key in understanding matter on ultrafast time scales. In this review, we illustrate the capabilities of this unique tool through a number of iconic experiments. We outline how coherent broadband X-ray radiation, emitted in high-harmonic generation, can be used to follow dynamics in increasingly complex systems. Experiments performed in both molecules and solids are discussed at length, on time scales ranging from attoseconds to picoseconds, and in perturbative or strong-field excitation regimes. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

Attosecond soft X-ray high harmonic generation

High harmonic generation (HHG) of an intense laser pulse is a highly nonlinear optical phenomenon that provides the only proven source of tabletop attosecond pulses, and it is the key technology in attosecond science. Recent developments in high-intensity infrared lasers have extended HHG beyond its traditional domain of the XUV spectral range (10-150 eV) into the soft X-ray regime (150 eV to 3 keV), allowing the compactness, stability and sub-femtosecond duration of HHG to be combined with the atomic site specificity and electronic/structural sensitivity of X-ray spectroscopy. HHG in the soft X-ray spectral region has significant differences from HHG in the XUV, which necessitate new approaches to generating and characterizing attosecond pulses. Here, we examine the challenges and opportunities of soft X-ray HHG, and we use simulations to examine the optimal generating conditions for the development of high-flux, attosecond-duration pulses in the soft X-ray spectral range. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

3.5-mJ 150-fs Fe:ZnSe hybrid mid-IR femtosecond laser at 4.4 μm for driving extreme nonlinear optics

We report on entering a new era of mid-IR femtosecond lasers based on amplification in a relatively new gain chalcogenide medium, Fe:ZnSe. Our hybrid all-solid-state laser system is based on direct pulse amplification of femtosecond seed from three-stage AGS-based-optical parametric amplification (OPA) in a Fe:ZnSe laser crystal optically pumped by a Cr:Yb:Ho:YSGG Q-switched nanosecond laser. The development of the pump source with output energy up to 90 mJ operating at a 10 Hz repetition rate regime and highly efficient grating compressor (80%) provides 3.5-mJ 150-fs femtosecond pulses centered at 4.4 μm. Diode-pumped Er:YAG/Er:YLF lasers make it possible to increase the beam quality and repetition rate of the proposed laser system up to 100 Hz. Focusing such a laser radiation into the ∼3λ beam diameter allows us to reach a focus laser intensity up to 10¹⁶  W/cm² which is only an order of magnitude lower than a relativistic intensity of 10¹⁷  W/cm² and enough to drive strong nonlinear optics in mid-IR. We show as a proof-of-principle experiment the generation of four-octave spanning (from 350 nm up to 5.5 μm) supercontinuum in xenon.

The Potential of EuPRAXIA@SPARC_LAB for Radiation Based Techniques

A proposal for building a Free Electron Laser, EuPRAXIA@SPARC_LAB, at the Laboratori Nazionali di Frascati, is at present under consideration. This FEL facility will provide a unique combination of a high brightness GeV-range electron beam generated in a X-band RF linac, a 0.5 PW-class laser system and the first FEL source driven by a plasma accelerator. The FEL will produce ultra-bright pulses, with up to 10 12 photons/pulse, femtosecond timescale and wavelength down to 3 nm, which lies in the so called “water window”. The experimental activity will be focused on the realization of a plasma driven short wavelength FEL able to provide high-quality photons for a user beamline. In this paper, we describe the main classes of experiments that will be performed at the facility, including coherent diffraction imaging, soft X-ray absorption spectroscopy, Raman spectroscopy, Resonant Inelastic X-ray Scattering and photofragmentation measurements. These techniques will allow studying a variety of samples, both biological and inorganic, providing information about their structure and dynamical behavior. In this context, the possibility of inducing changes in samples via pump pulses leading to the stimulation of chemical reactions or the generation of coherent excitations would tremendously benefit from pulses in the soft X-ray region. High power synchronized optical lasers and a TeraHertz radiation source will indeed be made available for THz and pump–probe experiments and a split-and-delay station will allow performing XUV-XUV pump–probe experiments.

Soft X-ray Absorption Spectroscopy of Aqueous Solutions Using a Table-Top Femtosecond Soft X-ray Source

We demonstrate the feasibility of soft X-ray absorption spectroscopy in the water window using a table-top laser-based approach with organic molecules and inorganic salts in aqueous solution. A high-order harmonic source delivers femtosecond pulses of short wavelength radiation in the photon energy range from 220 to 450 eV. We report static soft X-ray absorption measurements in transmission on the solvated compounds O=C(NH₂)₂, CaCl₂, and NaNO₃ using flatjet technology. We monitor the absorption of the molecular samples between the carbon (∼280 eV) and nitrogen (∼400 eV) K-edges and compare our results with previous measurements performed at the BESSYII facility. We discuss the roles of pulse stability and photon flux in the outcome of our experiments. Our work paves the way toward table-top femtosecond, solution-phase soft X-ray absorption spectroscopy in the water window.