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Inqui r y I ssue 1
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2015
Inqui r y I s sue
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t’s often said that necessity is the mother
of invention. Such was the case for Ames Laboratory
physicist Adam Kaminski who took the research
challenge he was facing and turned it into a new
solution that will help advance his research.
Two years ago the National Science Foundation closed
the synchrotron in Stoughton, Wisc. More recently,
Brookhaven National Lab closed its synchrotron light
source to make way for a more advanced and powerful
facility. Concerned that this would leave him without the
low-energy light source he needed to study the electronic
properties of new materials, he improvised, and the result
was the development of a new technique that provides a
homegrown, laboratory-based solution.
Kaminski uses a technique called angle-resolved
photoemission spectroscopy (ARPES) in which light energy
(photons) is directed at a sample being studied. The photons
for Synchrotron Light Source
cause electrons in the sample to be emitted into a vacuum.
An electron analyzer measures the energy and momentum
of these electrons, providing details about the electron
properties within the material.
Besides using synchrotron beam lines, lasers could
provide the input energy needed, but there were problems
with the existing technology. High-energy, tunable lasers
offered variable photon energy, but lacked the resolution
necessary for good results. Low-energy lasers provided
excellent resolution, but the fixed photon energy limited
their usefulness.
So Kaminski, who admittedly knew little about lasers,
set about finding a way to make a low-energy laser that was
tunable. In searching the literature, he found that such a
tunable laser had been suggested, but had never been used
in ARPES systems. The laser used a potassium beryllium
fluoroborate (KBBF) crystal to quadruple the frequency of
infrared laser converting photons to the required “vacuum
ultra-violet (UV)” range.
Obtaining such a crystal wasn’t easy. Kaminski found
that the main source for the KBBF crystals, China, had
embargoed their export. However, he found a research
group at Clemson University that was able to grow him
the crystal he needed. He was also able to obtain funding
through the DOE Office of Science to build the new system.
As an added bonus, the crystal growth and preparation was
commercialized by Advanced Photonic Crystals, LLC. This
will make them available in U.S. for applications such as UV
photo lithography, spectral analysis and defense.
In simple terms, Kaminski’s system uses a pair of
lasers, with the first acting as a pump for the second one.
The resulting beam consists of very short pulses (one
quadrillionth of a second) and very high (400 kW) peak
power and is directed into a vacuum chamber that contains
lenses, mirrors and the abovementioned “magic” crystal. This
process quadruples the energy of the photons. By tuning the
wavelength of the second laser and rotating the crystal, one
can then tune the energy of the produced UV photons. The
beam is then focused at the sample in an ultra-high vacuum
chamber and a connected electron analyzer measures the
electrons emitted from the sample.
“Development of a laboratory-based solution was really
important,” Kaminski said. “Our beam is smaller, photon
flux is higher by one or two orders of magnitude, and energy
resolution is better by a factor of five.”
For certain experiments, such as Kaminski’s, that can
translate into significantly better data. As illustrated by the
graphs (above), synchrotron results of magnesium diboride
show a surface band that curves relatively smoothly.
Results from the tunable laser ARPES shows a dramatically
enhanced plot with a sharp peak and a slight dip before
leveling off.
“Our system has significant advantages,” Kaminski said.
“It offers much higher resolution. When a researcher has a
sample they want tested, we can usually do it the next day. ”
Kaminski has performed ARPES measurements for a
number of research groups at Ames Laboratory as well as
researchers at Sandia National Laboratory and Princeton.
“It’s great to have the capability to perform measurements
right here in the Ames Laboratory,” he said, “and it’s busy 24/7!”
B Y K E R R Y G I B S O N
I
Solution
Homegrown
At left is a graph of synchrotron results of magnesium diboride that show a surface band that curves relatively smoothly.
At right are results of the same material from the tunable laser ARPES that shows a dramatically enhanced plot with a
sharp peak and a slight dip before leveling off.
Adam Kaminski stands next to the equipment
he assembled to conduct angle-resolved
photoemission spectroscopy in his lab using
laser light and a potassium beryllium
fluoroborate crystal to excite electrons in
the material being studied.