Ultrafast Time-resolved X-ray Diffraction Research Articles

Macroscopic electric polarization and microscopic electron dynamics: Quantitative insight from femtosecond x-ray diffraction

Phenomena in the macroscopic world are described by classical physics while processes at the microscopic atomic scale are governed by the laws of quantum mechanics. The connection between microscopic and macroscopic physical quantities is far from being trivial and remains partly unexplained. This study presents a novel theoretical approach for calculating macroscopic polarizations from transient microscopic charge densities that are derived from ultrafast time-resolved x-ray diffraction experiments. This is important for understanding the properties of polar and ionic materials and, in particular, those of ferroelectrics with widespread technical applications.

Transient lattice deformations of crystals studied by means of ultrafast time-resolved x-ray and electron diffraction

Ultrafast lattice deformation of tens to hundreds of nanometer thick metallic crystals, after femtosecond laser excitation, was measured directly using 8.04 keV subpicosecond x-ray and 59 keV femtosecond electron pulses. Coherent phonons were generated in both single crystal and polycrystalline films. Lattice compression was observed within the first few picoseconds after laser irradiation in single crystal aluminum, which was attributed to the generation of a blast force and the propagation of elastic waves. The different time scales of lattice heating for tens and hundreds nanometer thick films are clearly distinguished by electron and x-ray pulse diffraction. The electron and lattice heating due to ultrafast deposition of photon energy was simulated using the two-temperature model and the results agreed with experimental observations. This study demonstrates that the combination of two complementary ultrafast time-resolved methods, ultrafast x-ray, and electron diffraction will provide a panoramic picture of the transient structural changes in crystals.

Local terahertz field enhancement for time-resolved x-ray diffraction

We report local field strength enhancement of single-cycle terahertz (THz) pulses in an ultrafast time-resolved x-ray diffraction experiment. We show that patterning the sample with gold microstructures increases the THz field without changing the THz pulse shape or drastically affecting the quality of the x-ray diffraction pattern. We find a five-fold increase in THz-induced x-ray diffraction intensity change in the presence of microstructures on a SrTiO3 thin-film sample.

Femtosecond laser induced structural dynamics and melting of Cu (111) single crystal. An ultrafast time-resolved x-ray diffraction study

Femtosecond, 8.04 keV x-ray pulses are used to probe the lattice dynamics of a 150 nm Cu (111) single crystal on a mica substrate irradiated with 400 nm, 100 fs laser pulses. For pump fluences below the damage and melting thresholds, we observed lattice contraction due to the formation of a blast force and coherent acoustic phonons with a period of ∼69 ps. At larger pump fluence, solid to liquid phase transition, annealing, and recrystallization were measured in real time by monitoring the intensity evolution of the probing fs x-ray rocking curves, which agreed well with theoretical simulation results. The experimental data suggest that the melting process is a purely thermal phase transition. This study provides, in real time, an ultrafast time-resolved detailed description of the significant processes that occur as a result of the interaction of a femtosecond light-pulse with the Cu (111) crystal surface.

Communication: The formation of rarefaction waves in semiconductors after ultrashort excitation probed by grazing incidence ultrafast time-resolved x-ray diffraction.

We explore the InSb-semiconductor lattice dynamics after excitation of high density electron-hole plasma with an ultrashort and intense laser pulse. By using time resolved x-ray diffraction, a sub-mÅ and sub-ps resolution was achieved. Thus, a strain of 4% was measured in a 3 nm thin surface layer 2 ps after excitation. The lattice strain was observed for the first 5 ps as exponentially decaying, changing rapidly by time and by depth. The observed phenomena can only be understood assuming nonlinear time dependent laser absorption where the absorption depth decreases by a factor of twenty compared to linear absorption.

Study on the Cross-taxonal Distribution of Mechanosensitive Components in Insect Flight Muscle by Ultrafast Time-resolved X-ray Diffraction Recordings

Study on the Cross-taxonal Distribution of Mechanosensitive Components in Insect Flight Muscle by Ultrafast Time-resolved X-ray Diffraction Recordings

Ultrafast chemical kinetics: Elementary chemical act

Predictions of the classical theory of chemical reaction rates are compared with experimental results obtained by ultrafast time-resolved X-ray diffraction techniques. Our analysis is illustrated with the reaction I+I=I2 in solution at times immediately preceding recombination. Main features of experimentally detected dynamics are discussed and are compared with what is expected according to Eyring and Kramers. It is emphasized that atomic dynamics are unexpectedly complex at very earliest times.

Time-resolved Bragg coherent X-ray diffraction revealing ultrafast lattice dynamics in nano-thickness crystal layer using X-ray free electron laser

Ultrafast time-resolved Bragg coherent X-ray diffraction (CXD) has been performed to investigate lattice dynamics in a thin crystal layer with a nanoscale thickness by using a SASE (Self-Amplified Spontaneous Emission)–XFEL (X-ray Free Electron Laser) facility, SACLA. Single-shot Bragg coherent diffraction patterns of a 100 nm-thick silicon crystal were measured in the asymmetric configuration with a grazing exit using an area detector. The measured coherent diffraction patterns showed fringes extending in the surface normal direction. By using an optical femtosecond laser-pump and the XFEL-probe, a transient broadening of coherent diffraction pattern profile was observed at a delay time of around a few tens of picosecond, indicating transient crystal lattice fluctuation induced by the optical laser. A perspective application of the time-resolved Bragg CXD method to investigate small sized grains composing ceramic materials is discussed.

Ultrafast time dynamics studies of periodic lattices with free electron laser radiation

It has been proposed that radiation from free electron laser (FEL) at Hamburg (FLASH) can be used for ultrafast time-resolved x-ray diffraction experiments based on the near-infrared (NIR) pump/FEL probe scheme. Here, investigation probing the ultrafast structural dynamics of periodic nano-crystalline organic matter (silver behenate) with such a scheme is reported. Excitation with a femtosecond NIR laser leads to an ultrafast lattice modification which time evolution has been studied through the scattering of vacuum ultraviolet FEL pulses. The found effect last for 6 ps and underpins the possibility for studying nanoperiodic dynamics down to the FEL source time resolution. Furthermore, the possibility of extending the use of silver behenate (AgBh) as a wavelength and temporal calibration tool for experiments with soft x-ray/FEL sources is suggested.

Ultrafast time resolved x-ray diffraction, extended x-ray absorption fine structure and x-ray absorption near edge structure

Ultrafast time resolved x-ray absorption and x-ray diffraction have made it possible to measure, in real time, transient phenomena structures and processes induced by optical femtosecond pulses. To illustrate the power of these experimental methods, we present several representative examples from the literature. (I) Time resolved measurements of photon/electron coupling, electron/phonon interaction, pressure wave formation, melting and recrystallization by means of time resolved x-ray diffraction. (II) Ultrafast x-ray absorption, EXAFS, for the direct measurement of the structures and their kinetics, evolved during electron transfer within molecules in liquid phase. (III) XANES experiments that measure directly pathway for the population of high spin states and the study of the operating mechanism of dye activated TiO2 solar cell devices. The construction and use of novel polycapillary x-ray lenses that focus and collimate hard x-rays efficiently are described.

Set-up for ultrafast time-resolved X-ray diffraction using a femtosecond laser-plasma keV X-ray source

A short-pulse 4.51 keV Ti K o femtosecond laser-plasma driven hard x-ray source has been built and characterized. This source is incorporated into a set-up for ultrafast time-resolved x-ray diffraction experiments, to study ultrafast lattice dynamics in laser-excited materials.

Non-thermal melting in semiconductors measured at femtosecond resolution.

Ultrafast time-resolved optical spectroscopy has revealed new classes of physical, chemical and biological reactions, in which directed, deterministic motions of atoms have a key role. This contrasts with the random, diffusive motion of atoms across activation barriers that typically determines kinetic rates on slower timescales. An example of these new processes is the ultrafast melting of semiconductors, which is believed to arise from a strong modification of the inter-atomic forces owing to laser-induced promotion of a large fraction (10% or more) of the valence electrons to the conduction band. The atoms immediately begin to move and rapidly gain sufficient kinetic energy to induce melting--much faster than the several picoseconds required to convert the electronic energy into thermal motions. Here we present measurements of the characteristic melting time of InSb with a recently developed technique of ultrafast time-resolved X-ray diffraction that, in contrast to optical spectroscopy, provides a direct probe of the changing atomic structure. The data establish unambiguously a loss of long-range order up to 900 A inside the crystal, with time constants as short as 350 femtoseconds. This ability to obtain the quantitative structural characterization of non-thermal processes should find widespread application in the study of ultrafast dynamics in other physical, chemical and biological systems.