394 research outputs found
Time-dependent energy absorption changes during ultrafast lattice deformation
The ultrafast time-dependence of the energy absorption of covalent solids
upon excitation with femtosecond laser pulses is theoretically analyzed. We use
a microscopic theory to describe laser induced structural changes and their
influence on the electronic properties. We show that from the time evolution of
the energy absorbed by the system important information on the electronic and
atomic structure during ultrafast phase transitions can be gained. Our results
reflect how structural changes affect the capability of the system to absorb
external energy.Comment: 7 pages RevTeX, 8 ps figures, submitted to Journal of Appl. Physic
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
Theory for the ultrafast ablation of graphite films
The physical mechanisms for damage formation in graphite films induced by
femtosecond laser pulses are analyzed using a microscopic electronic theory. We
describe the nonequilibrium dynamics of electrons and lattice by performing
molecular dynamics simulations on time-dependent potential energy surfaces. We
show that graphite has the unique property of exhibiting two distinct laser
induced structural instabilities. For high absorbed energies (> 3.3 eV/atom) we
find nonequilibrium melting followed by fast evaporation. For low intensities
above the damage threshold (> 2.0 eV/atom) ablation occurs via removal of
intact graphite sheets.Comment: 5 pages RevTeX, 3 PostScript figures, submitted to Phys. Re
Ablation of solids by femtosecond lasers: ablation mechanism and ablation thresholds for metals and dielectrics
The mechanism of ablation of solids by intense femtosecond laser pulses is
described in an explicit analytical form. It is shown that at high intensities
when the ionization of the target material is complete before the end of the
pulse, the ablation mechanism is the same for both metals and dielectrics. The
physics of this new ablation regime involves ion acceleration in the
electrostatic field caused by charge separation created by energetic electrons
escaping from the target. The formulae for ablation thresholds and ablation
rates for metals and dielectrics, combining the laser and target parameters,
are derived and compared to experimental data. The calculated dependence of the
ablation thresholds on the pulse duration is in agreement with the experimental
data in a femtosecond range, and it is linked to the dependence for nanosecond
pulses.Comment: 27 pages incl.3 figs; presented at CLEO-Europe'2000 11-15 Sept.2000;
papers QMD6 and CTuK11
Kikuchi ultrafast nanodiffraction in four-dimensional electron microscopy
Coherent atomic motions in materials can be revealed using time-resolved X-ray and electron Bragg diffraction. Because of the size
of the beam used, typically on the micron scale, the detection of
nanoscale propagating waves in extended structures hitherto has not
been reported. For elastic waves of complex motions, Bragg intensities
contain all polarizations and they are not straightforward to
disentangle. Here, we introduce Kikuchi diffraction dynamics, using
convergent-beam geometry in an ultrafast electron microscope, to
selectively probe propagating transverse elastic waves with nanoscale
resolution. It is shown that Kikuchi band shifts, which are sensitive
only to the tilting of atomic planes, reveal the resonance
oscillations, unit cell angular amplitudes, and the polarization
directions. For silicon, the observed wave packet temporal envelope (resonance frequency of 33 GHz), the out-of-phase temporal behavior of
Kikuchi's edges, and the magnitude of angular amplitude (0.3 mrad) and
polarization [011] elucidate the nature of the motion:
one that preserves the mass density (i.e., no compression or expansion)
but leads to sliding of planes in the antisymmetric shear eigenmode of
the elastic waveguide. As such, the method of Kikuchi diffraction
dynamics, which is unique to electron imaging, can be used to
characterize the atomic motions of propagating waves and their
interactions with interfaces, defects, and grain boundaries at the
nanoscale
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
Silicon clusters produced by femtosecond laser ablation: Non-thermal emission and gas-phase condensation
Neutral silicon clusters Si_n (up to n = 7) and their cations Si_n+ (up to n
= 10) have been produced by femtosecond laser ablation of bulk silicon in
vacuum and investigated using time-of-flight mass spectrometry. Two populations
of the Si_n+ clusters with different velocity and abundance distributions in
the ablation plume have been clearly distinguished. Possible mechanisms of
cluster formation (Coulomb explosion, gas-phase condensation, phase explosion)
are discussed
Short pulse laser-induced optical damage and fracto-emission of amorphous, diamond-like carbon
Short pulse laser damage and ablation of amorphous, diamond-like carbon films is investigated. Material removal is due to fracture of the film and ejection of large fragments, which exhibit a broadband emission of microsecond duration
Nonthermal fragmentation of C60
A theoretical study of the subpicosecond fragmentation of C60 clusters in
response to ultrafast laser pulses is presented. We simulate the laser
excitation and the consequent nonequilibrium relaxation dynamics of the
electronic and nuclear degrees of freedom. The first stages of the
nonequilibrium dynamics are dominated by a coherent breathing mode followed by
the cold ejection of single C atoms, in contrast to the dimer emission which
characterizes the thermal relaxation. We also determine the nonequilibrium
damage thresholds as a function of the pulse duration.Comment: 5 pages, 4 figures, submitted to Chem. Phys. Let
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