[Show abstract][Hide abstract] ABSTRACT: Time-resolved x-ray diffraction with ultrashort x-ray pulses from a laser-produced plasma is used to study the lattice response of FeRh during a femtosecond laser-induced antiferromagnetic (AFM) to ferromagnetic (FM) phase transition. Pump-probe measurements at initial sample temperatures below as well as above the AFM-to-FM transition temperature and for different laser pump fluences allowed to disentangle the various contributions driving lattice expansion. In particular, the data reveal that the structural changes associated with the magnetic phase transition occur on a time scale of a hundred picoseconds.
[Show abstract][Hide abstract] ABSTRACT: We apply time-resolved x-ray diffraction using ultrashort x-ray pulses from a laser-produced plasma to probe the picosecond acoustic response of a thin laser-heated gold film. Measurements of the temporal changes in the angular distribution of diffracted x-rays provide direct quantitative information on the transient evolution of lattice strain. This allows to disentangle electronic and thermal pressure contributions driving lattice expansion after impulsive laser excitation. The electron-lattice energy equilibration time τE = (5±0.3) ps as well as the electronic Grüneisen parameter γe = (1.48±0.3) have been determined.
Full-text · Article · May 2011 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: Time-resolved X-ray diffraction with ultrashort X-ray pulses from a laser-produced plasma is used to study the lattice response of a thin FeRh-film during a fs laser-driven anti- ferromagnetic to ferromagnetic phase transition. OCIS codes: (320.7130) Ultrafast processes in condensed matter; (260.7120) Ultrafast phenomena
[Show abstract][Hide abstract] ABSTRACT: The structural dynamics of short‐pulse laser irradiated
surfaces and nano‐structures has been studied with nm spatial and ultrafast temporal resolution by means of single‐shot coherent XUV‐scattering techniques. The experiments allowed us to time‐resolve the formation of laser‐induced periodic surface structures, and to follow the expansion and disintegration of nano‐objects during laser ablation.
[Show abstract][Hide abstract] ABSTRACT: We apply ultrafast time‐resolved X‐ray diffraction to directly study coherent acoustic and optical phonons in laser‐excited materials. In Au the ps acoustic response has been investigated with the particular goal to clarify the interplay of electronic and thermal pressure contributions. In Bi the extreme softening of the coherently excited A1g optical phonon presents strong indication that upon intense laser‐excitation the Peierls‐transition which determines the equilibrium structure of Bi can be reversed. In FeRh we studied the lattice response after a fast laser‐induced anti‐ferromagnetic to ferromagnetic phase transition.
[Show abstract][Hide abstract] ABSTRACT: Time-resolved X-ray diffraction is used to study the lattice response of FeRh during a laser-driven anti-ferromagnetic to ferromagnetic phase transition. The experiments reveal a fast and a slow component in the induced expansion dynamics.
[Show abstract][Hide abstract] ABSTRACT: Laser-excited coherent optical phonons in Bismuth were investigated using time-resolved X-ray diffraction. The observed extreme softening of the excited A1g-mode presents strong indication that the Peierls-distortion defining the equilibrium structure of Bismuth is transiently reversed.
[Show abstract][Hide abstract] ABSTRACT: We investigated single shot damage of Mo/Si multilayer coatings exposed to the intense fs XUV radiation at the Free-electron LASer facility in Hamburg - FLASH. The interaction process was studied in situ by XUV reflectometry, time resolved optical microscopy, and "post-mortem" by interference-polarizing optical microscopy (with Nomarski contrast), atomic force microscopy, and scanning transmission electron microcopy. An ultrafast molybdenum silicide formation due to enhanced atomic diffusion in melted silicon has been determined to be the key process in the damage mechanism. The influence of the energy diffusion on the damage process was estimated. The results are of significance for the design of multilayer optics for a new generation of pulsed (from atto- to nanosecond) XUV sources.
[Show abstract][Hide abstract] ABSTRACT: An elliptical glass capillary has been used to focus ultrashort Cu K alpha x-ray pulses emitted from a femtosecond laser-produced plasma. Due to its high magnification (7x), the optic transforms the divergent x-ray emission of the plasma into a quasicollimated x-ray beam with a divergence of only 0.18 degrees. As an application we demonstrate the possibility to perform Debye-Scherrer diffraction experiments with the simultaneous detection of several diffraction orders. This will allow one to extend time-resolved x-ray diffraction with femtosecond laser-plasma x-ray sources to a much wider range of materials, which are not easily available as single crystals.
No preview · Article · Sep 2009 · The Review of scientific instruments
[Show abstract][Hide abstract] ABSTRACT: We investigate the generation of ultrashort Kalpha pulses from plasmas produced by intense femtosecond p-polarized laser pulses on Copper and Titanium targets. Particular attention is given to the interplay between the angle of incidence of the laser beam on the target and a controlled prepulse. It is observed experimentally that the Kalpha yield can be optimized for correspondingly different prepulse and plasma scale-length conditions. For steep electron-density gradients, maximum yields can be achieved at larger angles. For somewhat expanded plasmas expected in the case of laser pulses with a relatively poor contrast, the Kalpha yield can be enhanced by using a near-normal-incidence geometry. For a certain scale-length range (between 0.1 and 1 times a laser wavelength) the optimized yield is scale-length independent. Physically this situation arises because of the strong dependence of collisionless absorption mechanisms-in particular resonance absorption-on the angle of incidence and the plasma scale length, giving scope to optimize absorption and hence the Kalpha yield. This qualitative description is supported by calculations based on the classical resonance absorption mechanism and by particle-in-cell simulations. Finally, the latter simulations also show that even for initially steep gradients, a rapid profile expansion occurs at oblique angles in which ions are pulled back toward the laser by hot electrons circulating at the front of the target. The corresponding enhancement in Kalpha yield under these conditions seen in the present experiment represents strong evidence for this suprathermal shelf formation effect.
No preview · Article · Aug 2009 · Physical Review E
[Show abstract][Hide abstract] ABSTRACT: Recent theoretical and numerical studies of laser-driven femtosecond K alpha sources are presented, aimed at understanding a recent experimental campaign to optimize emission from thin coating targets. Particular attention is given to control over the laser-plasma interaction conditions defined by the interplay between a controlled prepulse and the angle of incidence. It is found that the x-ray efficiency for poor-contrast laser systems in which a large preplasma is suspected can be enhanced by using a near-normal incidence geometry even at high laser intensities. With high laser contrast, similar efficiencies can be achieved by going to larger incidence angles, but only at the expense of larger x-ray spot size. New developments in three-dimensional modelling are also reported with the goal of handling interactions with geometrically complex targets and finite resistivity.
No preview · Article · Jul 2009 · Applied Physics A
[Show abstract][Hide abstract] ABSTRACT: Large amplitude coherent optical phonons have been investigated in laser-excited Bismuth by means of femtosecond time-resolved X-ray diffraction. For absorbed laser fluences above 2 mJ/cm2, the experimental data reveal an extreme softening of the excited A1g-mode down to frequencies of about 1 THz, only 1/3 of the unperturbed A1g-frequency. At even stronger excitation the measured diffraction signals no longer exhibit an oscillatory behavior presenting strong indication that upon intense laser-excitation the Peierls-distortion, which defines the equilibrium structure of Bismuth, can be transiently reversed.
[Show abstract][Hide abstract] ABSTRACT: Ultrafast time-resolved X-ray diffraction has been used to study the dynamics of coherent acoustic phonons in fs laser-excited Ge and Au, with the particular goal to clarify the interplay of the electronic and thermal pressure contributions. For semiconductors it is usually assumed that the electronic pressure is the dominant driving force. Our measurements reveal that in Ge the relative strength of the electronic pressure decreases with increasing laser fluence. Only for low fluences the electronic pressure dominates, while at high fluences the thermal pressure exceeds the electronic pressure. For the case of Au the data are well described within the established theoretical framework using the known values for those material parameters which determine the laser-induced pressure, namely the energy relaxation time and the electronic and lattice Grüneisen parameters.
No preview · Article · Dec 2008 · MRS Online Proceeding Library
[Show abstract][Hide abstract] ABSTRACT: We have characterized and compared the performance of different types of multi-layer optics for the focusing of femtosecond
X-ray pulses. Using X-ray pulses at 8keV, from a laser-driven plasma source we have measured the spatial distribution of
the diffracted X-rays directly after and in the focal plane of the various X-ray optical devices. For a Montel optic with
7.3× magnification we obtained the largest number of focused X-ray photons per unit angle. The performance of this optic in
the X-ray diffraction experiment on a thin germanium film is demonstrated.
No preview · Article · Sep 2008 · Applied Physics B
[Show abstract][Hide abstract] ABSTRACT: Following femtosecond laser excitation, large transient changes in the X-raydiffraction efficiency have been observed in thin
crystalline films of germanium. This behavior is explained in terms of the thermo-acoustic response of the impulsively heated
material, when an inhomogeneously strained surface layer is probed by divergent X-ray beams.
Full-text · Article · Mar 2007 · Applied Physics A
[Show abstract][Hide abstract] ABSTRACT: Ultrafast time-resolved X-ray diffraction has been used to study the electron-lattice energy exchange and the thermo-acoustic
relaxation in fs laser-excited Germanium.
[Show abstract][Hide abstract] ABSTRACT: We describe a high repetition rate laser driven X-ray source and the optimization of X-ray generation at low energy level. This source was used in time resolved X-ray diffraction to study the coherent acoustic phonon.
[Show abstract][Hide abstract] ABSTRACT: In the last few years, bent crystal X-ray mirrors have played an important role in time-resolved X-ray diffraction experiments when X-ray pulses from femtosecond laser-produced plasmas were used [1-4]. Improvements in manufacturing techniques have significantly increased the quality of this type of mirror.
Preview · Article · Jan 2005 · Journal of Physics Conference Series
[Show abstract][Hide abstract] ABSTRACT: In the last few years the generation of femtosecond pulses in the X-ray regime has become possible. These ultrashort X-ray
pulses have enabled femtosecond time- resolution to be extended to X-rays.