15 research outputs found
Probing laser-driven structure formation at extreme scales in space and time
Irradiation of solid surfaces with high intensity, ultrashort laser pulses
triggers a variety of secondary processes that can lead to the formation of
transient and permanent structures over large range of length scales from mm
down to the nano-range. One of the most prominent examples are LIPSS - Laser
Induced Periodic Surface Structures. While LIPSS have been a scientific
evergreen for of almost 60 years, experimental methods that combine ultrafast
temporal with the required nm spatial resolution have become available only
recently with the advent of short pulse, short wavelength free electron lasers.
Here we discuss the current status and future perspectives in this field by
exploiting the unique possibilities of these 4th-generation light sources to
address by time-domain experimental techniques the fundamental LIPSS-question,
namely why and how laser-irradiation can initiate the transition of a "chaotic"
(rough) surface from an aperiodic into a periodic structure.Comment: 13 pages incl. 5 figure
Time-resolved diffraction with an optimized short pulse laser plasma X-ray source
We present a set-up 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 set-up to the specific requirements (e.g. X-ray
optics, sample environment) of particular applications. The configuration
discussed here has been optimized towards high angular/momentum resolution and
uses K-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. Ti-K photons per pulse
with relative bandwidth are delivered to the sample at 10 Hz
repetition rate. This allows for high dynamic range () measurements of
transient changes of the rocking curves of materials as for example induced by
laser-triggered strain waves.Comment: 29 pages, 8 figure
Structural pathways for ultrafast melting of optically excited thin polycrystalline Palladium films
Due to its extremely short timescale, the non-equilibrium melting of metals
is exceptionally difficult to probe experimentally. The knowledge of melting
mechanisms is thus based mainly on the results of theoretical predictions. This
work reports on the investigation of ultrafast melting of thin polycrystalline
Pd films studied by optical laser pump - X-ray free-electron laser probe
experiments and molecular-dynamics simulations. By acquiring X-ray diffraction
snapshots with sub-picosecond resolution, we capture the sample's atomic
structure during its transition from the crystalline to the liquid state.
Bridging the timescales of experiments and simulations allows us to formulate a
realistic microscopic picture of melting. We demonstrate that the existing
models of strongly non-equilibrium melting, developed for systems with
relatively weak electron-phonon coupling, remain valid even for ultrafast
heating rates achieved in femtosecond laser-excited Pd. Furthermore, we
highlight the role of pre-existing and transiently generated crystal defects in
the transition to the liquid state.Comment: main manuscript 33 pages, 9 figures; supplemental material 19 pages,
13 figures - all in one fil
Structural changes across thermodynamic maxima in supercooled liquid tellurium: a water-like scenario
Liquid polymorphism is an intriguing phenomenon which has been found in a few
single-component systems, the most famous being water. By supercooling liquid
Te to more than 130 K below its melting point and performing simultaneous
small-angle and wide-angle X-ray scattering measurements, we observe clear
maxima in its thermodynamic response functions around 615 K, suggesting the
possible existence of liquid polymorphism. A close look at the underlying
structural evolution shows the development of intermediate-range order upon
cooling, most strongly around the thermodynamic maxima, which we attribute to
bond-orientational ordering. The striking similarities between our results and
those of water, despite the lack of hydrogen-bonding and tetrahedrality in
tellurium, indicate that water-like anomalies may be a general phenomenon among
liquid systems with competing bond- and density-ordering.Comment: Main text: 10 pages, 5 figures; supplementary materials: 14 pages, 13
figure
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Real-Time Visualization of Nanocrystal Solid–Solid Transformation Pathways
Ultrafast single-shot diffraction imaging of nanoscale dynamics
The transient nanoscale dynamics of materials on femtosecond to picosecond timescales is of great interest in the study of condensed phase dynamics such as crack formation, phase separation and nucleation, and rapid fluctuations in the liquid state or in biologically relevant environments. The ability to take images in a single shot is the key to studying non-repetitive behaviour mechanisms, a capability that is of great importance in many of these problems. Using coherent diffraction imaging with femtosecond X-ray free-electron-laser pulses we capture time-series snapshots of a solid as it evolves on the ultrafast timescale. Artificial structures imprinted on a Si 3 N 4 window are excited with an optical laser and undergo laser ablation, which is imaged with a spatial resolution of 50nm and a temporal resolution of 10ps. By using the shortest available free-electron-laser wavelengths and proven synchronization methods this technique could be extended to spatial resolutions of a few nanometres and temporal resolutions of a few tens of femtoseconds. This experiment opens the door to a new regime of time-resolved experiments in mesoscopic dynamics. © 2008 Macmillan Publishers Limited. All rights reserved