Press Release

Seeing the Universe in a Brand New Light

By SpaceRef Editor
March 12, 2002
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Scientists at Northwestern University have developed a
novel device that could lead to an ultraviolet (UV) light detector
approximately 10 times more sensitive than the UV detectors now on the
Hubble Space Telescope, allowing astronomers to observe important objects
throughout the universe for the first time.

Ultraviolet rays are a high-energy form of light familiar to most people
because they can give us a sunburn. They also can provide important clues
to unlocking some of the secrets of our universe. But UV rays can only be
observed in space using special detectors, and the light is difficult to
detect, because of interference from the more common visible and infrared
light rays. Existing UV detectors use filters to prevent interference from
these longer wavelength rays and, as a result, are not very sensitive or
efficient.

To address this problem, the National Aeronautics and Space Administration
(NASA) recently awarded a grant to Northwestern so that Mel Ulmer,
professor of physics and astronomy, and Bruce Wessels, professor of
materials science and engineering, could develop further a device that is
sensitive to UV rays and also naturally insensitive to visible and infrared
light. To achieve these properties, the device uses an advanced
semiconductor material called gallium nitride. (Detectors that are
insensitive to the visible and infrared – the bulk of the light emitted by
the sun – are called “solar blind.”)

“Our semiconductor material can detect light across the entire UV range and
is currently six times more efficient than detectors used in the Hubble
Space Telescope,” said Ulmer. “The Hubble’s UV detectors are not solar
blind, so they use filters to block the visible and infrared in order to
see the UV, resulting in an efficiency of only 5 percent. Another drawback
of the Hubble detectors is their much smaller field of view.”

Once optimized, a large UV detector based on gallium nitride could be used
by astronomers to see deeper into the universe and learn more about planets
and young stars, as well as an elusive material known as the “cosmic web.”
The cosmic web is presumably a structure of gas and dark matter that
theoreticians say exists throughout the universe but has not been imaged
directly yet.

“The cosmic web is one of the missing building blocks of the universe,” said
Ulmer. “It is possible that with a detector properly sensitive to the
ultraviolet – a light between visible light and very high energy x-rays –
we could see it. This fundamental information would be key to understanding
how galaxies and the universe formed and evolved.”

Samples of the gallium nitride material already have been converted
successfully into a camera-like device called a phototube that is 30
percent efficient or six times better than the Hubble’s detectors, but now
Ulmer and Wessels are working to improve the conductivity of this material
to increase its efficiency, as well as improve the solar blindness of the
resulting detectors.

“By adding electrically active impurities to gallium nitride we have
improved the optical properties of the material, making it very sensitive
to ultraviolet light, but we know we can do even better,” said Wessels, an
expert in semiconductor thin films. “Our goal is to make the device 50
percent efficient or 10 times better than the detectors used in the
Hubble.” Efficiencies as high as 90 percent are theoretically possible.

Ulmer and Wessels will work with Oswald Siegmund, associate director of the
Space Sciences Laboratory at the University of California, Berkeley, to
convert samples of their improved material into phototubes that can then be
tested. The resulting detectors, which require a vacuum to test, work by
converting UV light shining on the material into emitted electrons that are
collected within the phototubes. The electrical signal, which is
proportional to the amount of UV light hitting the detector, is processed
and results in an image. In addition to its increased UV sensitivity and
insensitivity to visible and infrared light, another major advantage of the
gallium nitride material is that it works at room temperature.

“This thin film material, while only a few microns thick, is very robust and
also could be used in UV light-emitting diodes, lasers, advanced electronic
devices, flame detection and biomedical applications,” said Wessels.

The research is being supported by a three-year $300,000 grant from NASA.

(Source contacts: Mel Ulmer at 847-491-5633 or m-ulmer2@northwestern.edu and
Bruce Wessels at 847-491-3219 or wessels@elmo.tech.nwu.edu)

SpaceRef staff editor.