Professors Mask Thermal Structures

With the development of a more stable form of a unique material, Harvard professors are now closer to finding a way to mask the thermal signatures of objects.

The research, conducted by applied physics professor and School of Engineering and Applied Sciences fellow Federico Capasso, is still in the experimental phases, but promises to have exciting real world applications down the road.

Every object emits a thermal signature—a certain amount of infrared light—that can be recognized by a thermal detector. That unique signature has practical applications—police, for example, use thermal imaging to find the location of concealed suspects before entering a building.

A previously undiscovered, stable form of Vanadium oxide, the unique material identified by scientists, is unusual in that as its temperature varies, its thermal signature changes in unusual ways. While normally thermal signatures, as displayed by thermal detectors, would appear red, orange, or yellow at high temperatures and blue and green at lower temperatures, as it approaches the range of 75-85 degrees Celsius, vanadium oxide begins to appear in the blue-green spectrum again, even though objects at that temperature normally appear strongly red.

According to Mikhail A. Kats, a lead author of the study and graduate student member of Capasso’s team, when the team started its research, advancing thermal camouflage was not the goal. Only when research began to uncover this property of vanadium oxide did Capasso and his team began to investigate the possibility.

“Slowly we moved to a slightly different direction; we started working on light absorbing structures in the infrared spectrum” said Kats.

The team can now reliably reproduce its results and is working to expand the range of temperatures vanadium oxide can be effectively controlled.

Capasso’s team’s work was made possible by the research of Shriram Ramanathan, a materials science professor at SEAS. According to Ramanathan, vanadium oxide is normally highly susceptible to internal stresses and cracking, making it difficult to work with. The research that Capasso and his team conducted was only feasible with reliably resilient samples. Ramanathan developed a new technique for producing vanadium oxide and as a result, he said, “stress generation can be carefully controlled...and we can get very reproducible results.”

Ramanathan said that with his technique, he thought there was “a good chance” that vanadium oxide coating might be offered for objects as usable products in the near future.

Kats said that such vanadium oxide coatings might be in use within five to ten years, but cautioned that there is still work to be done.

“We need to be able to control the [temperature] range over which this effect happens” he said, adding that thermal signature control can and will be achieved for much lower temperatures in the future.

This article has been revised to reflect the following correction:

CORRECTION: Oct. 31, 2013

An earlier version of this article misquoted study author Mikhail A. Kats as saying that his team’s findings counteract the law of thermodynamics. In fact, Katslater wrote in an email, while objects typically emit greater amounts of light as their temperature increases, his team demonstrated a structure that exhibits the opposite effect.