The energy that the infrared detectors gather is all around us: With long wavelengths of light invisible to the naked eye, infrared radiation is emitted by everything from trees to Honda Civic engines to human bodies to dying stars.
But the detectors themselves are finicky gizmos whose readings are easily confused. They work when a photon of light strikes a semiconductor material, knocking an electron loose and creating a measurable electrical current. Because of their long wavelengths, though, infrared photons have very small energies; in an infrared photodetector, the semiconductor materials have very loosely bound electrons so a weak infrared photon can boot one out of orbit.
Therein lies the challenge. “When you get to energies that small, there are other things besides light that can also kick electrons out of the atom,” Saumil says. Like heat generated by the device itself. This phenomenon, known as thermal shaking, can muddle the readings of an instrument such as an infrared telescope. To counteract that effect, liquid nitrogen is often used to cool the entire device so that only infrared radiation emanating from the desired source is measured—a delicate, expensive undertaking.
Saumil’s approach has been to abandon the realm of classical physics entirely. His infrared photodetector is made of nanowires, each of which is about one ten-thousandth the width of a human hair. “Because we’re dealing with such small structures, this weird physics takes over so that it’s selective and only detects a small window of light frequencies. Plus, only photons of infrared light can knock out electrons, not thermal shaking,” he says. “That’s a quantum mechanical effect.”
To explain what they mean, Saumil and Supriyo take me to what’s called the “clean room” at VCU. We zip on bodysuits and don hairnets and booties to protect the delicate nanostructures being fabricated nearby. Saumil leads me to his workbench, where beakers of jade- and ruby-colored liquids await.
He screws a one-inch square of aluminum foil to the outside of a beaker, fills it with sulfuric acid, connects wires and then passes a current through the foil. This creates an oxide film on the aluminum that (though we’d need a high-powered microscope to see it) looks a lot like a honeycomb. Then into the same beaker Saumil pours solutions containing cadmium and sulfur, and zaps the thing again. The semiconductor material fills in the pores in the honeycomb, which works like a stencil.
The result is an array of nanowires with unique absorption properties. As the two materials bond, some atoms stretch and others compress, creating defect sites called electron traps. The only thing that can free an electron from its trap is a photon of infrared light.
The photodetector is small enough that Supriyo keeps a little dish of them on his office desk, like candy. He says they are cheap to manufacture, easily mass-produced and difficult to break, which makes them a potential tool for monitoring infrared radiation on a worldwide scale. Saumil imagines scattering them around the polar ice caps to study global warming. (Astronomical applications, though intriguing, seem unlikely in the near term; infrared telescopes are such high-stakes instruments that scientists will continue to cool them with liquid nitrogen.) Saumil’s infrared detector is also very sensitive. By switching semiconductor materials and using ultra-small structures, he’s able to tailor it to pick up on a specific frequency of infrared light, instead of all infrared light, a feature that caught the Army’s eye. For the last two summers, Saumil has worked with an Army lab, through the Student Temporary Employment Program, on potential remote sensing applications, and the Army Engineer Research and Development Center is considering pursuing an independent patent.
The tool is proving to be remarkably versatile. Saumil has discovered that he can use it to read other parts of the electromagnetic spectrum beyond the infrared, and even to spot beta particles emitted by some radioactive material, which could be useful in searching for nuclear weapons at shipping ports or for monitoring radiation levels in bone cancer patients. Still, before commercial production becomes an option, a few kinks need to be addressed, particularly the issue of the photodetector’s short shelf life: It only works for a few weeks. But Supriyo insists that with the proper packaging it could last for decades.