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Researchers
at the Stanford Synchrotron Radiation Laboratory (SSRL)
and the German laboratory Berliner Elektronenspeicherring-Gesellschaft
für Synchrotronstrahlung (BESSY) have crafted
a technique to take X-ray images that reveal tiny
variations and lightning-quick changes in materials
a thousand times smaller than the thickness of a strand
of hair.
Their work
merited the cover of the Dec. 16 issue of Nature.
The technique--lensless X-ray holography--will be
valuable for researchers working with the world's
first X-ray free electron laser, the Linac Coherent
Light Source (LCLS), slated to begin experiments in
2009 at the Stanford Linear Accelerator Center (SLAC).
"We have demonstrated
the first direct imaging technique that will work
with LCLS, opening the door for taking pictures of
ultra-fast changes in the collective behavior of ensembles
of atoms and molecules," said SSRL physicist
Jan Luening. He and BESSY colleague Stefan Eisebitt
headed development of the technique.
"Our approach
is simple and it can be applied to a wide variety
of samples from thin films to small structures coming
from material science, biology or chemistry,"
Luening said.
State-of-the-art light
sources such as BESSY and SPEAR3 at SLAC achieve lensless
imaging by filtering light so that the only remaining
X-rays are "coherent"--that is, all the
X-ray light waves are in phase with each other (each
wave is peaking at the same time) and moving in the
same direction like a marching band in step. Because
it uses no lenses, the technique has the potential
to take direct images with 10 times better spatial
resolution than can be achieved with current X-ray
lenses and bring even finer details into view. Another
advantage to the technique is it entails much simpler
alignment and sample handling than do established
X-ray microscopy methods.
Lensless imaging will
be especially powerful at LCLS and other future X-ray
free electron lasers being planned in Germany and
other countries. X-ray free electron lasers will be
10 billion times brighter than today's brightest synchrotron
sources. And because laser light is inherently coherent,
X-ray filtering is unnecessary. In addition, LCLS
X-ray pulses will be extremely short--lasting only
femtoseconds, mere quadrillionths of a second.
This impressive combination
of properties not only makes LCLS a revolutionary
machine, it makes lensless imaging ideally suited
for obtaining "single shot" images of rapid,
intricate changes in nanometer-sized materials. Just
one pulse of X-ray light, rather than billions of
pulses, will be needed to capture a clear picture
of the action at that moment in time.
Scientists could take
a series of such images to create a "movie"
of the changes, analogous to time-lapse photography
for slow processes like a flower coming into bloom.
This confers a brand new capability to study the nonrepeatable
aspects of biological, physical and chemical processes
occurring on dizzyingly fast time scales. A few areas
of investigation include proteins attaching to each
other step by step and polymer chains assembling into
ordered clusters.
Holography is the key
The technique works
by shining a coherent beam of X-ray light through
two adjacent holes: one containing the sample to be
studied, the other a tiny "reference" hole.
The scattered light from both holes overlays to form
a single, holographic diffraction pattern. Holography
not only maps the intensities of the light, as do
normal diffraction patterns, it also encodes information
about the phases of the light that is otherwise intrinsically
lost.
"Without the
phases, it's like trying to predict what happens next
on a highway if you know where the cars are but not
their speed," explained Luening. "You simply
lack half of the important information. Holography
elegantly encodes this other half in the measured
intensities."
The information is
decoded by applying a standard mathematical procedure
known as Fourier transformation, yielding a complete
image of the sample.
The demonstration
experiment took place at BESSY in February 2004. The
obtained image revealed the randomly organized "north"
and "south" magnetic regions of a cobalt-platinum
film to a spatial resolution of 50 nanometers (50
billionths of a meter).
The work of the SSRL
authors is supported by the U. S. Department of Energy,
Office of Basic Energy Sciences.
CONTACT:
Neil Calder, Stanford Linear Accelerator Center: (650)
926-8707, neil.calder@slac.stanford.edu
COMMENT:
Jan Luening, Stanford Linear Accelerator Center: (650)
926-4539, jan.luening@slac.stanford.edu
RELEVANT WEB URLS:
STANFORD SYNCHROTRON RADIATION LABORATORY http://www-ssrl.slac.stanford.edu/
STANFORD LINEAR ACCELERATOR CENTER
http://www.slac.stanford.edu/
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