A silicon device with
thousands of slats of tiny mirrors lined up to resemble
a jalousie window could speed the
efficiency of NASA's next generation space telescope, the Constellation-X
Observatory, according to a team of scientists at the Massachusetts Institute
of Technology (MIT), in Cambridge.
The scientists created what they
call the Critical Angle Transmission grating, a device that uses nano mirrors to diffract X-ray and
extreme ultraviolet range beams without the energy absorption that typically
occurs with X-ray diffraction gratings, said Ralf Heilmann, associate director
of MIT's Space Nanotechnology Laboratory at the Kavli Institute of Astrophysics
and Space Research.
Scientists use
diffraction gratings in space telescopes such as NASA's Chandra
X-ray Observatory as a tool to aid
their search
for clues to the formation of the universe. It is work that must be
conducted in space because Earth's atmosphere absorbs X-ray energy. Diffraction
gratings, which are a common physical science tool, disperse X-ray wavelengths
in much the same way a prism refracts
visible light.
The MIT grating was
invented during research and development for NASA's Constellation-X program, a
proposed mission to fly several X-ray
telescopes working in unison. These telescopes will investigate black
holes, galaxy formation, the recycling of matter and energy, and dark matter
and dark energy.
Based on more than
two decades of work that included optics for Chandra, MIT scientists set out to
create a device that would disperse X-ray wavelengths without absorbing the
wavelengths' energy. The scientists found that tightly spaced silicon mirrors
reflect wavelengths rather than absorbing them, allowing the wavelengths to be redirected at angles
that allow a broader view of their spectra, said Mark Schattenburg, director of
the Space Nanotechnology
Laboratory.
"We're getting five
times more throughput than with
Chandra," Schattenburg said. "For every 10 X-rays through Chandra's telescope,
only one to two gets detected. At least five times more get through the nano
mirrors so we're boosting the percentage of efficiency."
The silicon slats are
as thin as 35 nanometers —
the size of some of the smallest computer chip transistors and wires under
commercial development. The thousands of long, thin slats are spaced about 150
nanometers apart at a shallow angle that allows the X-rays to skip from the
smooth sidewalls of the slats through the spaces between the slats.
Heilmann said the new
technology could be used in a wide array of instruments, from those involved in
plasma physics to the life and environmental sciences. The principles of
diffraction and efficient reflection below a critical angle apply to neutrons
and whole atoms as well, he said.
The
findings were published in the June 9 edition of Optics Express, and Heilmann
spoke June 23 at the SPIE (International Society for Optical Engineering)
Conference on Astronomical Telescopes and Instrumentation in Marseille, France.
The
MIT team already had notched a
major advancement between the Optics Express publication and the conference in
France, where Heilmann announced that the team had reduced the grating pitch — the distance between
the slats — from 574 nanometers
to 200 nanometers with the same results. The goal is to reduce the grating
pitch to 100 nanometers, he said.
The
team, which also includes MIT mechanical engineering graduate student Minseung
Ahn, now must fine-tune the fabrication of the device, expanding it from its
current size of a few square millimeters to accommodate plans for fitting it
with Constellation-X's 3.7 meter diameter telescope, Schattenburg said.
"We
have to make a square meter or two with very tight control of the
nanogeometry," he said. "We might have to change the spacing or make some
subtle change to squeeze the maximum resolution out of the telescope."
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