Ultrafast time-resolved X-ray diffraction
K. Sokolowski-Tinten1. C. Blome1, J. Blums1, A. Cavalleri2, C. Dietrich1,
A. Tarasevitch1, D. von der Linde1
1
Institute for Laser- and Plasmaphysics, University of Essen, 45117 Essen, Germany
Material Science Division, Lawrence Berekely Nat. Lab., Berkeley, CA 94720, USA
2
Abstract. Femtosecond laser-generated plasmas emit ultrashort X-ray pulses in the multi-keV
range, which allow the extension of X-ray spectroscopy into the litrafast time-domain. We
report here on the generation of such short X-ray pulses and their application for time-resolved
diffraction as a means to directly study ultrafast structural dynamics in laser-excited solids.
INTRODUCTION
Most of our knowledge on the atomic structure of matter is due to X-ray
spectroscopies, because the short wavelength of X-rays gives direct access to the
spatial scale of atomic arrangements. However, standard X-ray experiments (using for
example synchrotron radiation) provide essentially a static picture because the time
resolution of such experiments is rather limited. Most fundamental processes in nature,
such as chemical reactions and phase transitions involve dynamic changes of the
atomic arrangements, which occur typically on time-scales comparable with the
natural oscillation periods of atoms and molecules, that is femtoseconds to
picoseconds. Therefore, great efforts have been made during the past few years to
extend X-ray spectroscopy into the ultrafast time-domain.
One of the most successful approaches has been made possible by the recent
progress in ultrafast laser technology, namely chirped pulse amplification. MultiTerawatt, ultrafast table-top scale laser systems are now available in an increasing
number of laboratories all around the world. When focused onto solid targets, such
intense light pulses generate high-density plasmas emitting short bursts of hard X-rays
up to the MeV region [1]. This new kind of high brightness ultrashort pulsed X-ray
sources allow an extension of the established time-resolved techniques of ultrafast
optical spectroscopy into the hard X-ray range, thus providing simultaneously atomic
scale temporal and spatial resolution.
This contribution discusses the generation of femtosecond, multi-keV X-ray pulses
and their application for time-resolved diffraction as a means to directly study ultrafast
structural dynamics in laser-excited solids.
CP634, Science of Superstrong Field Inter actions, edited by K. Nakajima and M. Deguchi
© 2002 American Institute of Physics 0-7354-0089-X/02/$ 19.00
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SHORT X-RAY PULSES FROM LASER-PRODUCED PLASMAS
As has been first demonstrated by Ktihlke et al. [2], the high-density microplasmas, generated through the interaction of very intense light pulses with solid
targets, represent an efficient source of hard X-rays. Particularly interesting is the
strong characteristic line emission (for example Ka-emission) of these plasmas. This
line radiation is believed to result from the interaction of energetic electrons with the
non-excited target material underneath the thin plasma layer [3], a process quite
similar to an ordinary X-ray tube. Because those energetic electrons are produced by
direct acceleration in the strong laser field, a very short duration of the X-ray pulses,
of the order of the laser pulse duration, can be expected.
Femtosecond laser-plasma-driven X-ray sources are quite attractive, in particular
for small, university-scale laboratories, because they combine simplicity with low
cost, as compared to other, accelerator-based approaches [4]. A convenient
implementation of such a source uses a thin metallic wire, which is continuously
moved through the focus of the laser, as a target [5]. The left part of Fig. 1 shows a
photograph of the wire-target assembly used at our set-up for time-resolved
diffraction.
4
6
8
Photon Energy [keV]
2.746 2.748
2.75
2.752 2.754
FIGURE 1. Left: photograph of the Titanium wire-target assembly. Right: X-ray emission spectra of
the laser excited Titanium target (top: overview spectrum obtained by photon counting/pulse height
analysis with a X-ray CCD; bottom: higher resolution spectrum of the spin-orbit split K^-lines obtained
with a crystal spectrometer).
It should be kept in mind that due strong laser-induced ablation the target has to be
moved between two consecutive laser pulses in order to provide a fresh surface area
for each individual pulse. As a consequence the wire-target offers two main
advantages compared to other target schemes: (i) it allows a very compact design,
which simplifies shielding issues, and (ii) virtually infinite measurement times are
possible simply by providing a sufficiently long spool of wire. Our source, which runs
at 10 Hz repetition rate, can operate for nearly 70 h with a 500 m long spool of wire
and a pulling velocity of typically 200 urn per pulse!
12
We have chosen titanium as target material. The Ti-Ka-emission (4.51 keV) of our
source allows Bragg diffraction on a wide variety of materials and overcomes the
inherent limitations with respect to lattice parameters and/or diffraction orders of
sources operating at longer wavelengths. The left part of Fig. 1 displays spectra of our
source in the keV-range. The top viewgraph represents an overview spectrum obtained
by operating an X-ray CCD-detector (thinned, back-illuminated) in the single photon
counting mode with pulse height analysis (to assure single-photon detection we had to
reduce the laser energy). The dominant features are the titanium Ka- and Kp-lines at
4.51 keV and 4.93 keV, respectively. The bottom viewgraph shows a high resolution
spectrum of the spin-orbit split Kai and Ka2-lines, which was obtained with a crystal
spectrometer.
As has been mentioned above, the X-ray-tube-like Ka-emission from laserproduced plasmas is caused by fast electrons accelerated in the intense laser field. The
efficiency of this process depends on the energy of the electrons and is therefore
strongly influenced by the details of the laser-plasma interaction. For example, it is
common experience [6,7] and also supported by theoretical calculations [8] that for
given laser- and material parameters the highest laser intensities not necessarily lead to
highest Ka-yield. This is demonstrated by the data shown in the left part of Fig. 2,
where we measured the relative yield of our titanium Ka-source (dots) as a function of
the position of the focusing optics (f = 150 mm) relative to the target.
-2.0
-1.5 -1.0 -0.5 0.0
Lens Position [mm]
0.5
-5
0
5
10
Delay Time [ps]
15
FIGURE 2. Left: normalized X-ray signal as a function of the lens position (relative to the focus; dots:
Ti- Ka-emission; squares: hard background). Right: Ti-Ka-yield in the two-pulse excitation scheme as a
function of the delay time between the plasma generating pre-pulse and the main pulse.
The Ka-signal has been normalized to the yield with the titanium target exactly in
the focal plane of the optics (zero position), corresponding to the highest laser
intensity. The highest Ka-flux is observed 0.5 mm away from the focal point, which
corresponds to approximately two times the Rayleigh length. At the same time the
background signal detected by the CCD (squares; arbitrarily normalized to fit into the
plot window of Fig. 2), which is due to hard X-rays and secondary radiation, is
significantly reduced. Therefore, optimizing the focusing conditions does not only
increase the Ka-flux, but allows at the same time a substantial improvement of the
signal-to-noise ratio.
A second way to tailor the plasma properties in order generate exactly those
electrons which have the highest efficiency in the production of K-shell holes is the
use of a double-pulse excitation scheme. This scheme separates the steps of plasma
13
production and electron acceleration. A first, medium intensity laser pulse is used for
plasma generation. After a certain delay, the main high intensity pulse interacts with
the pre-formed plasma to accelerate the electrons. The right part of Fig. 2 shows the
Ka-emission as a function of time-delay between the pre-pulse (I « 1015 W/cm2) and
the main pulse (I« 5xl0 16 W/cm2), which produces the fast electrons.
In agreement with results obtained at a silicon Ka-source at 1.8 keV [9] we observe
an enhancement of almost an order of magnitude at a delay time of just a few
picoseconds. In [9] this enhancement has been attributed to resonance absorption in
the slightly expanded plasma (scale length « K), which leads to a very efficient
production of electrons in the optimum energy range (note that the cross section for
ionization of K-shell electrons peaks for most materials at approximately 3 - 4 times
the Ka-energy).
TIME-RESOLVED X-RAY DIFFRACTION
It is an important advantage of laser-plasma driven X-ray sources that the laser
driving X-ray generation provides at the same time an absolutely synchronous source
for optical excitation. Therefore, the basic experimental concept of ultrafast timeresolved measurements, the so-called pump-probe scheme, which is well established
in the optical domain, can be directly extended to the hard X-ray regime: an optical
pump-pulse is used for excitation while an ultrashort X-ray pulse serves as a probe to
monitor the transient dynamics induced by the pump.
Because laser-plasma driven X-ray sources are essentially monochromatic, they are
particularly suited for time-resolved Bragg-diffraction experiments. A schematic of
such an experiment is shown in Fig. 3.
laser pulse to generate a plasma
sample
Bragg diffracted X-rays —^s^
variable delay
rocking curve X-ray CCD camera
FIGURE 3. Schematic of an optical pump X-ray probe experiment for time-resolved diffraction.
In our set-up near-infrared laser pulses from a 10 Hz amplified Ti:sapphire laser
system with pulse energies of about 150 mJ and a pulse duration of 120 fs are focused
onto the surface of the moving titanium wire. A small fraction is split off the main
laser pulse and (after passing through an optical delay line) is used for sample
excitation.
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The Ka-radiation from the plasma is emitted incoherently into the full solid angle.
Efficient use of the produced X-rays requires, therefore, re-collection and focusing of
the X-rays onto the surface of the sample under investigation with a spot size
comparable or smaller than the area excited by the optical pump. Focusing of the Karadiation is accomplished with the help of a toroidally bent crystal. As has been
discussed in detail by Misalla et al. [10], monochromatic point-to-point imaging of the
plasma source can be achieved in this way. The experimental geometry is shown in the
left part of Fig. 4.
toroidally bent crystal:
Si (311), 5x15mm2
topography
Rowland circle
FIGURE 4. Focusing of the keV-plasma emission. Left: experimental geometry. Right top:
topography of the bent crystal mirror; Right bottom: 1:1 image of the plasma source (FWHM: 85 |nn).
For a given crystallographic orientation of the mirror the horizontal and vertical
bending radii are determined by the imaging geometry on the Rowland-circle and the
requirement that the Bragg-condition has to be fulfilled for the chosen wavelength.
The results of an imaging experiment at our Ti-Ka-source using a bent Silicon
crystal with (311) surface orientation are shown in Fig. 4. In the lower right the spatial
distribution of the focused X-rays, as detected with our X-ray CCD (pixel size 27 um),
is displayed. The spot is nearly circular and exhibits a FWHM of approximately 85
jLim. This focus typically contains 30.000 detected Ka-photom per pulse.
The upper right part of Fig. 4 shows the local reflection characteristic of the mirror,
which is obtained by placing the CCD-detector away from the image plane close to the
mirror. This reflection characteristic provides direct information on the surface
topography of the mirror because of its very narrow rocking curve (angular
dependence of the diffraction efficiency for a fixed X-ray wavelength). Any spatial
deformation of the mirror surface leads to a deviation of the local incidence angle
from the Bragg-angle and thus to a reduction of the diffraction efficiency. The Siliconcrystal used in our set-up shows an excellent topography with a peak-to-peak nonuniformity of the reflectivity of only 10 %.
In the diffraction experiment the crystalline sample is placed in the image plane
under the appropriate Bragg-angle. The optical pump spot and the X-ray focus must
overlap spatially on the surface of the sample, and the diffracted X-rays are recorded
by an X-ray sensitive area detector. Please note, that the X-rays, incident from the Xray mirror onto the sample, cover an angular range much larger than the width of the
15
rocking curve^ which can be, therefore, recorded in a single exposure without any
angle-scanning of the sample.
A major problem in optical pump, X-ray probe diffraction experiments results from
the different penetration depths of optical radiation and multi-keV X-rays. In
semiconductors and metals the optical absorption depth is usually below one
micrometer, becoming even shorter at high levels of excitation due to strong nonlinear
contributions to the overall absorption. Therefore, only a very thin layer of the order of
100 nm near the surface can be optically excited. X-rays, on the other hand, have
typically a penetration depth of a few microns or more.
To overcome this mismatch between pump- and probe depths we used thin
crystalline films grown on silicon substrates. Using surfactant mediated growth
techniques [11] highly perfect single-crystalline films can be obtained on large size
substrate (4" wafers). Most important, these films are stress free and grow with their
natural lattice constant. Therefore, it is easily possible in a diffraction experiment to
separate the contributions of the thin film from the bulk substrate if the difference in
the lattice constants and the corresponding difference in the Bragg-angles is
sufficiently large.
This is the case for the thin Germanium films grown on Silicon, which we used in
the diffraction experiment discussed in the next chapter. An example of the diffraction
pattern of such a Ge/Si-heterostructure is shown in Fig. 5.
170 nm Ge on Si
•2
0.10
m
0.05
(O
0.00
24
25
26
Diffraction Angle [°]
FIGURE 5. Bragg-diffraction profile (rocking curve, Ill-reflection) from a 170 nm thick, singlecrystalline Germanium film grown on Silicon using the laser-driven Ti-Ka X-ray source. Insert: image
directly recorded on the X-ray CCD-detector.
The data shown in Fig. 5 represent the angular diffraction profile (rocking curve) of
the 111-Bragg reflection of a 170 nm thick Germanium film on Silicon obtained from
a two-minutes integration on the CCD (the insert shows the CCD-image). The Braggpeaks of the Germanium film (QB = 24.88°) and the Silicon substrate (0B = 26°) are
well separated. Note, that the significantly larger width of the Germanium-peak is
related to the small thickness of the overlayer and not to structural imperfections.
X-RAY PULSE DURATION
As in an ordinary, all-optical pump/probe experiment the temporal resolution of an
optical pump/X-ray probe experiment is determined by the duration of the probe pulse.
16
While there are reliable and very sophisticated methods to accurately measure the
pulse shape of optical pulses (for example FROG), comparable methods for the multikeV X-ray range are not yet available.
Nevertheless some information on the X-ray pulse duration can be obtained from a
time-resolved diffraction experiment. In the most general case the observed transient
changes of the diffraction signal represent the convolution of the actual material
response with the X-ray probe pulse. If the material response is sufficiently fast the
measured transients directly provide the X-ray pulse duration.
A process, which should give such a very fast material response, is femtosecond
laser-induced melting of semiconductors. It has been investigated for nearly 20 years
with ultrafast optical techniques [12-14] and there is strong evidence that a transition
from the ordered solid phase to the disordered liquid state is possible within just a few
hundred femtoseconds. Therefore, this process has become some kind of a test-case
for ultrafast X-ray techniques [15-19], in particular diffraction, and a number of recent
studies [18-20] have clearly demonstrated sub-picosecond X-ray response.
Results from our own work [19] are depicted in the left part of Fig. 6. It shows the
angular integrated X-ray reflectivity of the Ill-reflection of a 170 nm Germanium
film as function of pump-probe time delay for two different pump fluences.
1.0
0.2J/cnT
o 0.4 J/cm2
>> 0.9
0.8
oi
(T
I
0.6
0
-0.4
1
Delay Time [ps]
0.0
0.4
Delay Time [ps]
FIGURE 6. Left: angular integrated X-ray reflectivity of a 170 nm Germanium film (111-reflection) as
a function of the time delay between the optical pump pulse and the X-ray probe for two different pump
fluences. Right: fits of the measured diffraction data (0.2 J/cm2) assuming a step-like material response
and Gaussian-shaped X-ray pulses.
The most prominent feature is the rapid initial drop of the integrated diffraction
efficiency within a few hundred femtoseconds, a clear indication for a very fast loss of
order over a depth of about 40 nm. Here we do not discuss further the details of
ultrafast melting and the interesting material behavior observed on longer time-scales
(for this the reader is referred to [19]), but want to focus on the X-ray pulse duration.
To get an estimate on the pulse duration we assume the limiting case of a
completely instantaneous, step-like material response (which obviously over-estimates
the X-ray pulse duration!). In the right part of Fig. 6 the initial decrease of the
diffraction signal measured for a pump fluence of 0.2 J/cm2 has been fitted by a
convolution of a step-like material response with Gaussian-shaped X-ray pulses of
different duration. Satisfactory fits are obtained with X-ray pulse widths between 250
fs and 350 fs. As a result we can give an upper limit for the X-ray pulse duration of
about (300 ± 50) fs. To our knowledge, these are the shortest X-ray pulses in the
multi-keV range reported so far!
17
ACKNOWLEDGMENTS
The authors are indebted to I. Uschmann and E. Forster for providing the X-raymirror, and to M. Kammler and M. Horn-von-Hoegen for preparation of the
heterostructure samples. Financial support by the Deutsche Forschungsgemeinschaft,
the European Community, the German Academic Exchange Service, and the National
Science Foundation is gratefully acknowledged.
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