Material looks cool while heating up

It’s a thermodynamic Houdini: the first material that appears to get colder as it heats up. Because of the unusual way the material interacts with infrared light, the finding, which appears October 21 in Physical Review X, could lead to camouflage against heat-sensing cameras and to efficient heating and cooling devices.

Physics textbooks explain that the hotter a body gets, the more light it radiates. This principle allows soldiers with infrared goggles to ferret out enemies even in total darkness.

But scientists are starting to learn how to design materials that do not always radiate more as they warm. To do this, researchers often try to find materials that naturally change the way they interact with light or electricity at certain temperatures. The compound vanadium dioxide makes such a transition around 70^o Celsius, switching abruptly from being an electrical insulator to a conductor.

Mikhail Kats, a graduate student at Harvard University, wondered how vanadium dioxide would interact with light above its transition temperature. So Kats and his colleagues deposited a 150-nanometer-thick layer of vanadium dioxide onto a wafer of sapphire.

Then the researchers heated the vanadium dioxide-sapphire sample and, with an infrared camera, measured how much infrared light the sample emitted as it warmed. The color gradually shifted from blue to red as the sample’s temperature increased from 60^o to 74^o, as is typical for a warming object. But then something strange happened: Even though the sample’s temperature continued to rise up to 100^o, the camera readout returned to an icy blue and stayed there.

“We saw this really dramatic effect,” Kats says. “You have an object that at 90^o looks the same as at 50^o.”

Because the material effectively conceals its temperature, it could allow soldiers and military aircraft to evade thermal sensors, Kats and his colleagues think. Such a material could also allow heaters to maintain a constant temperature by emitting less radiation in cold conditions and more in hot conditions. Kats says such technology could conserve energy on space satellites, which control their temperature solely through absorbing and radiating light.

The researchers also want to know how vanadium dioxide rearranges its internal structure as it warms to create its unusual interactions with light and electricity. Kats and his colleagues hope that understanding the fine-scale structure of vanadium dioxide will help them control how it radiates light.

Potential insights into how such “natural metamaterials” work excites Daniel Wasserman, an electrical engineer at the University of Illinois at Urbana-Champaign. “It’s a very clever paper,” he says. “It opens the door to some really interesting physics.”