Towards shot-noise limited diffraction experiments with table-top femtosecond hard x-ray sources

A modular table-top setup for ultrafast x-ray diffraction

We present a table-top setup for femtosecond time-resolved x-ray diffraction based on a Cu Kα (8.05 keV) laser driven plasma x-ray source. Due to its modular design, it provides high accessibility to its individual components (e.g., x-ray optics and sample environment). The Kα-yield of the source is optimized using a pre-pulse scheme. A magnifying multilayer x-ray mirror with Montel-Helios geometry is used to collect the emitted radiation, resulting in a quasi-collimated flux of more than 105 Cu Kα photons/pulse impinging on the sample under investigation at a repetition rate of 10 Hz. A gas ionization chamber detector is placed right after the x-ray mirror and used for the normalization of the diffraction signals, enabling the measurement of relative signal changes of less than 1% even at the given low repetition rate. Time-resolved diffraction experiments on laser-excited epitaxial Bi films serve as an example to demonstrate the capabilities of the setup. The setup can also be used for Debye-Scherrer type measurements on poly-crystalline samples.

Comparison and assessment of methods for cellulose crystallinity determination

The degree of crystallinity in cellulose significantly affects the physical, mechanical, and chemical properties of cellulosic materials, their processing, and their final application. Measuring the crystalline structures of cellulose is a challenging task due to inadequate consistency among the variety of analytical techniques available and the lack of absolute crystalline and amorphous standards. Our article reviews the primary methods for estimating the crystallinity of cellulose, namely, X-ray diffraction (XRD), nuclear magnetic resonance (NMR), Raman and Fourier-transform infrared (FTIR) spectroscopy, sum-frequency generation vibrational spectroscopy (SFG), as well as differential scanning calorimetry (DSC), and evolving biochemical methods using cellulose binding molecules (CBMs). The techniques are compared to better interrogate not only the requirements of each method, but also their differences, synergies, and limitations. The article highlights fundamental principles to guide the general community to initiate studies of the crystallinity of cellulosic materials.

Phonon-Induced Relocation of Valence Charge in Boron Nitride Observed by Ultrafast X-Ray Diffraction

The impact of coherent phonon excitations on the valence charge distribution in cubic boron nitride is mapped by femtosecond x-ray powder diffraction. Zone-edge transverse acoustic (TA) two-phonon excitations generated by an impulsive Raman process induce a steplike increase of diffracted x-ray intensity. Charge density maps derived from transient diffraction patterns reveal a spatial transfer of valence charge from the interstitial region onto boron and nitrogen atoms. This transfer is modulated with a frequency of 250 GHz due to a coherent superposition of TA phonons related to the ^{10}B and ^{11}B isotopes. Nuclear and electronic degrees of freedom couple through many-body Coulomb interactions.

Underdamped longitudinal soft modes in ionic crystallites-lattice and charge motions observed by ultrafast x-ray diffraction

Soft modes in crystals are lattice vibrations with frequencies that decrease and eventually vanish as the temperature approaches a critical point, e.g., a structural change due to a phase transition. In ionic para- or ferroelectric materials, the frequency decrease is connected with a diverging electric susceptibility and, for infrared active modes, a strong increase in oscillator strength. The traditional picture describes soft modes as overdamped transverse optical phonons of a hybrid vibrational-electronic character. In this context, potassium dihydrogen phosphate (KH₂PO₄, KDP) has been studied for decades as a prototypical material with, however, inconclusive results regarding the soft modes in its para- and ferroelectric phase. There are conflicting assignments of soft-mode frequencies and damping parameters. We report the first observation of a longitudinal underdamped soft mode in paraelectric KDP. Upon impulsive femtosecond Raman excitation of coherent low-frequency phonons in the electronic ground state of KDP crystallites, transient powder diffraction patterns are recorded with femtosecond hard x-ray pulses. Electron density maps derived from the x-ray data reveal oscillatory charge relocations over interatomic distances, much larger than the sub-picometer nuclear displacements, a direct hallmark of soft-mode behavior. The strongly underdamped character of the soft mode manifests in charge oscillations persisting for more than 10 ps. The soft-mode frequency decreases from 0.55 THz at T = 295 K to 0.39 THz at T = 175 K. An analysis of the Raman excitation conditions in crystallites and the weak damping demonstrate a longitudinal character. Our results extend soft-mode physics well beyond the traditional picture and pave the way for an atomic-level characterization of soft modes.

Femtosecond multi-10-mJ pulses at 2 µm wavelength by compression in a hollow-core fiber

High-energy few-ps pulses from a Ho:YLF chirped pulse amplifier operating at a 1 kHz repetition rate are compressed in a two-stage arrangement to sub-90-fs duration. The energy of the compressed pulses is more than 20 mJ at an average power of 20 W. In the first stage, the duration of the 2.8 ps, 40 mJ pulses at 2.05 µm wavelength was reduced to 1.4 ps by using nonlinear propagation in air. Subsequently, the pulses were further compressed to 86 fs after spectral broadening in a 3-m-long Kr-filled stretched flexible hollow-core fiber. The high photon flux, peak power, and excellent beam quality and stability make this light source highly attractive for fs pulse generation in the extreme ultraviolet (XUV) to x-ray spectral range for time-resolved XUV spectroscopy or measurements of structural dynamics in solids.

Validation of a laser driven plasma X-ray microfocus source for high resolution radiography imaging

Hard X-ray radiation with high brightness and high fluxes is nowadays available on the fourth generation of synchrotrons and X-FELs, but the large size and complexity of these sources makes its use difficult for widespread applications. New table top X-ray sources driven by ultrashort high power lasers offer a compelling route to expand the availability of hard X-ray sources. They can be used for advanced imaging techniques, due to its small source size and spatial coherence. We present in this paper the validation of a compact laser-driven X-ray microfocus source for high-resolution radiography imaging. This novel device was built at the Laser Laboratory for Acceleration and Applications (L2A2) at the University of Santiago de Compostela. This paper describes the laser-plasma X-ray source with improved stability and characterize some of its properties. We demonstrate the high-contrast and resolution of the images obtained with this source by using masks with well known geometries, and detailed analysis by using the modulation transfer function. Finally, we discuss the properties of this source in comparison to other compact microfocus X-ray sources.

Compact high-flux hard X-ray source driven by femtosecond mid-infrared pulses at a 1 kHz repetition rate

A novel, to the best of our knowledge, table-top hard X-ray source driven by femtosecond mid-infrared pulses provides 8 keV pulses at a 1 kHz repetition rate with an unprecedented flux of up to 1.5×10¹² X-ray photons/s. Sub-100 fs pulses at a center wavelength of 5 µm and multi-millijoule energy are generated in a four-stage optical parametric chirped-pulse amplifier and focused onto a thin Cu tape target. Electrons are extracted from the target and accelerated in a vacuum up to 100 keV kinetic energy during the optical cycle; the electrons generate a highly stable K α photon flux from the target in a transmission geometry.

Time-resolved diffraction with an optimized short pulse laser plasma X-ray source

We present a setup for time-resolved X-ray diffraction based on a short pulse, laser-driven plasma X-ray source. The employed modular design provides high flexibility to adapt the setup to the specific requirements (e.g., X-ray optics and sample environment) of particular applications. The configuration discussed here has been optimized toward high angular/momentum resolution and uses K α -radiation (4.51 keV) from a Ti wire-target in combination with a toroidally bent crystal for collection, monochromatization, and focusing of the emitted radiation. 2 × 10 5 Ti-K α1 photons per pulse with10 - 4 relative bandwidth are delivered to the sample at a repetition rate of 10 Hz. This allows for the high dynamic range (104) measurements of transient changes in the rocking curves of materials as for example induced by laser-triggered strain waves.

Advances and opportunities in ultrafast X-ray crystallography and ultrafast structural optical crystallography of nuclear and electronic protein dynamics

Both nuclear and electronic dynamics contribute to protein function and need multiple and complementary techniques to reveal their ultrafast structural dynamics response. Real-space information obtained from the measurement of electron density dynamics by X-ray crystallography provides aspects of both, while the molecular physics of coherence parameters and frequency-frequency correlation needs spectroscopy methods. Ultrafast pump-probe applications of protein dynamics in crystals provide real-space information through direct X-ray crystallographic structure analysis or through structural optical crystallographic analysis. A discussion of methods of analysis using ultrafast macromolecular X-ray crystallography and ultrafast nonlinear structural optical crystallography is presented. The current and future high repetition rate capabilities provided by X-ray free electron lasers for ultrafast diffraction studies provide opportunities for optical control and optical selection of nuclear coherence which may develop to access higher frequency dynamics through improvements of sensitivity and time resolution to reveal coherence directly. Specific selection of electronic coherence requires optical probes, which can provide real-space structural information through photoselection of oriented samples and specifically in birefringent crystals. Ultrafast structural optical crystallography of photosynthetic energy transfer has been demonstrated, and the theory of two-dimensional structural optical crystallography has shown a method for accessing the structural selection of electronic coherence.

Recent advances in ultrafast X-ray sources

Over more than a century, X-rays have transformed our understanding of the fundamental structure of matter and have been an indispensable tool for chemistry, physics, biology, materials science and related fields. Recent advances in ultrafast X-ray sources operating in the femtosecond to attosecond regimes have opened an important new frontier in X-ray science. These advances now enable: (i) sensitive probing of structural dynamics in matter on the fundamental timescales of atomic motion, (ii) element-specific probing of electronic structure and charge dynamics on fundamental timescales of electronic motion, and (iii) powerful new approaches for unravelling the coupling between electronic and atomic structural dynamics that underpin the properties and function of matter. Most notable is the recent realization of X-ray free-electron lasers (XFELs) with numerous new XFEL facilities in operation or under development worldwide. Advances in XFELs are complemented by advances in synchrotron-based and table-top laser-plasma X-ray sources now operating in the femtosecond regime, and laser-based high-order harmonic XUV sources operating in the attosecond regime. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

Recent Advances in Ultrafast Structural Techniques

A review that summarizes the most recent technological developments in the field of ultrafast structural dynamics with focus on the use of ultrashort X-ray and electron pulses follows. Atomistic views of chemical processes and phase transformations have long been the exclusive domain of computer simulators. The advent of femtosecond (fs) hard X-ray and fs-electron diffraction techniques made it possible to bring such a level of scrutiny to the experimental area. The following review article provides a summary of the main ultrafast techniques that enabled the generation of atomically resolved movies utilizing ultrashort X-ray and electron pulses. Recent advances are discussed with emphasis on synchrotron-based methods, tabletop fs-X-ray plasma sources, ultrabright fs-electron diffractometers, and timing techniques developed to further improve the temporal resolution and fully exploit the use of intense and ultrashort X-ray free electron laser (XFEL) pulses.

Phonon driven charge dynamics in polycrystalline acetylsalicylic acid mapped by ultrafast x-ray diffraction

The coupled lattice and charge dynamics induced by phonon excitation in polycrystalline acetylsalicylic acid (aspirin) are mapped by femtosecond x-ray powder diffraction. The hybrid-mode character of the 0.9 ± 0.1 THz methyl rotation in the aspirin molecules is evident from collective charge relocations over distances of some 100 pm, much larger than the sub-picometer nuclear displacements. Oscillatory charge relocations around the methyl group generate a torque on the latter, thus coupling electronic and nuclear motions.

Hard X-ray Generation from ZnO Nanowire Targets in a Non-Relativistic Regime of Laser-Solid Interactions

We present a detailed investigation of X-ray emission from both flat and nanowire zinc oxide targets irradiated by 60 fs 5 × 1016 W/cm2 intensity laser pulses at a 0.8 µm wavelength. It is shown that the fluence of the emitted hard X-ray radiation in the spectral range 150–800 keV is enhanced by at least one order of magnitude for nanowire targets compared to the emission from a flat surface, whereas the characteristic Kα line emission (8.64 keV) is insensitive to the target morphology. Furthermore, we provide evidence for a dramatic increase of the fast electron flux from the front side of the nanostructured targets. We suggest that targets with nanowire morphology may advance development of compact ultrafast X-ray sources with an enhanced flux of hard X-ray emission that could find wide applications in highenergy density (HED) physics.

High photon flux Kα Mo x-ray source driven by a multi-terawatt femtosecond laser at 100 Hz

We develop a pulsed hard x-ray K_α source at 17.4 keV produced by the interaction of a multi-terawatt peak power infrared femtosecond laser pulse with a thick molybdenum (Mo) target at a 100 Hz repetition rate. We measure the highest Mo K_α photon production reported to date corresponding to a K_α photon flux of 1×10¹¹  ph/(sr·s) and an estimated peak brightness of ∼2.5×10¹⁷  ph/(s·mm²·mrad²(0.1% bandwidth)) at ∼5×10¹⁸  W/cm² driving laser intensity.

Soft-mode driven polarity reversal in ferroelectrics mapped by ultrafast x-ray diffraction

Quantum theory has linked microscopic currents and macroscopic polarizations of ferroelectrics, but the interplay of lattice excitations and charge dynamics on atomic length and time scales is an open problem. Upon phonon excitation in the prototypical ferroelectric ammonium sulfate [(NH₄)₂SO₄], we determine transient charge density maps by femtosecond x-ray diffraction. A newly discovered low frequency-mode with a 3 ps period and sub-picometer amplitudes induces periodic charge relocations over some 100 pm, a hallmark of soft-mode behavior. The transient charge density allows for deriving the macroscopic polarization, showing a periodic reversal of polarity.

Dynamic position shifts of X-ray emission from a water film induced by a pair of time-delayed femtosecond laser pulses

Femtosecond double-pulsed laser excitation of a water film in air showed enhancements of X-ray intensity as compared with single pulse irradiation. The position of the highest yield of X-rays strongly depends on temporal separation between the pre-pulse and the main-pulse (energy ratios where ∼ 1 : 10). The strongest X-ray emission was observed at 10-15 ns delay of the main-pulse. Nanoscale roughening of water surface can account for the observation.