By Heather Rock Woods
Researchers at 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.
Coherent x-rays shine through a cobalt-platinum sample and a
reference hole (right, foreground). The detector records the
resulting hologram pattern (center and left). A computer
(background) applies a mathematical Fourier transformation to
the hologram to obtain a complete image.
Their work merited the cover of the December 16 issue of
Nature magazine. 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 at
SLAC in 2009.
"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 Jan Luening (SSRL). He and Stefan Eisebitt (BESSY) 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
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
that it entails much simpler alignment and sample handling than
established X-ray microscopy methods do.
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. Rather than billions of pulses, just one pulse
of X-ray light 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 such as a flower coming into bloom. A brand new capability will
enable researchers 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 normal diffraction patterns
do—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
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).