EVANSTON, Ill. — 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.