|UNT System: Resource magazine >> Seeing the invisible|
Science fiction movies of the past depicted robots that could detect invisible objects with a beam of light. Today, people can detect some things in the dark by using electronic infrared technology. In the future, objects once hidden from view will be visible, thanks to research conducted by two University of North Texas scientists.
UNT physics professors Terry Golding, Ph.D., and Chris Littler, Ph.D., intend
to sharpen the vision of infrared sensors through research to improve
infrared detectors for military use.
Golding says events such as the Sept. 11 tragedy have shown the importance of infrared technology, which differentiates the temperatures of objects — their heat signatures — and displays them through color graphics.
"Firefighters employ infrared sensors to find people and pets through smoke and fire," he says.
Law enforcement and intelligence professionals also use infrared sensors to investigate and combat crimes. The military operates infrared missile guidance and satellite systems to protect the country.
"The next generation of infrared products will have higher accuracy to detect objects at greater distances in the dark and through fog and smoke," Golding says. "They will also increase the ability of medical diagnostic equipment to detect diseases."
Littler adds that the next generation of infrared technology will provide earlier detection of developing storm systems, allowing weather warnings to be issued further in advance. He says that improved sensors will also offer earlier warnings about volatile industrial"hot spots," such as excess friction in a motor or a restriction in a pipeline, that might cause equipment breakdown, injury or loss of life.
Seeing is detecting
compares infrared sensors to the human eye.
The difference is that infrared sensors can observe wavelengths much longer than those visible to humans, Littler says.
"The human eye is not able to recognize the difference between radiant heat of a person and a house, but infrared sensors can differentiate these various heat signatures," he says.
Golding says when light hits an infrared detector, infrared materials absorb it. These materials act like the retina of an eye, transforming light into electrical signals.
In the human eye, nerves relay these signals to the brain. Crystals in an infrared detector serve a similar purpose, sending information to a processor that decodes the signals into images.
In their efforts to increase the sensitivity of infrared detectors and improve their capacity to see at longer distances, Golding and Littler are working to develop the best possible infrared materials — the crystals that will imitate the function of the eye's network of light-sensitive nerves.
They begin their research with a Molecular Beam Epitaxy System — a vacuum-sealed incubator.
Using this chamber, the scientists deposit thin films of tellurium, cadmium and mercury — an alloy of infrared materials — onto a foundational base called a wafer. Scientists have found that the combination of these elements provides the most sensitive infrared detector.
"It's like spray-painting thin atomic layers on top of one another until they grow into a crystal," Golding says. "The objective is to find the right mixture of materials that will result in the creation of clear electrical signals when the material is exposed to infrared light."
The crystal challenge
for the crystal to conduct electricity, one atom in a million in the
alloy of tellurium, cadmium and mercury must be
Jack Dinan, Ph.D., a research scientist in the U.S. Army Night Vision and Electronic Sensors Directorate, says the Army chose an arsenic atom to give the tellurium, cadmium and mercury infrared mixture its conductive properties, but it proved to be a very difficult process to put arsenic atoms in just the proper place in an infrared crystal.
Military scientists have been searching for a solution to this problem based on the properties of the atoms themselves, says Dinan. But Golding proposed a solution that relies on the nuclei of atoms, not their properties.
"His theory is a classic example of thinking outside of the box," Dinan says. The Army found Golding's idea so innovative it is now funding UNT's infrared research project.
Golding's theory about the proper placement of an arsenic atom in an infrared crystal not only lies inside the nucleus of the atom, but also in the ability to change one substance into another.
"I looked at the Army's challenge to precisely place arsenic atoms within the infrared material," says Golding. "I realized I could combine the thinking of the ancient chemists, who tried to turn lead into gold, with modern chemists' understanding of the inner workings of atoms."
Since selenium, which has the same atomic structure as tellurium, can be converted into arsenic by irradiation, Golding proposed substituting a few selenium atoms for tellurium atoms and irradiating the crystal to provide the arsenic and thus the conductive properties needed.
With proper placement of the selenium-turned-arsenic atom, Golding and Littler's process has the potential to increase the detection range of an infrared sensor. They now intend to grow crystals with a higher sensitivity toward light and heat.
Golding says this technology will enable future generations of infrared detectors to see farther and better. With improved detection devices, objects once hidden from view will be revealed, and what is still science fiction today will become science fact.