Military camouflage has taken a step forward with the discovery of a coating that, when heated past a certain temperature, looks colder to thermal cameras. A team of applied physicists at Harvard School of Engineering and Applied Sciences (SEAS) tested the coating by placing a material on a hot plate and watching it through an IR camera as the temperature rose. At first, the sample behaved as expected, giving off more IR light as it was heated: At 60 °C, it appeared blue-green to the camera; by 70 °C, it was red and yellow; and by 74 °C, it was a deep red. Then something strange happened: The thermal radiation plummeted. At 80 °C, the sample looked blue, as if it could be 60 °C, and at 85 °C, it looked even colder. The researchers also found that the effect was reversible and repeatable, many times over. A new coating intrinsically conceals its own temperature from thermal cameras. Courtesy of Mikhail Kats, Harvard SEAS. "The effect is so large that our sample emits half of the thermal radiation at 100 °C that it does at 75 °C — a remarkable contrast to the behavior of conventional thermal emitters," the team said in a paper on the work published this week in the American Physical Society's open-access Physical Review X. "We now call the effect 'negative differential thermal emittance.' " Principal investigator Federico Capasso predicts that, with only small adjustments, the coating could be used as a new type of thermal camouflage or as a kind of encrypted beacon to allow soldiers to covertly communicate their locations in the field. The secret to the technology lies within a very thin film of vanadium oxide, a phase-change material that undergoes a structural and electronic phase transition at approximately 70 °C. At room temperature, pure vanadium oxide is electrically insulating, but at slightly higher temperatures, it transitions to a metallic, electrically conductive state. During that transition, the optical properties change, too, meaning special temperature-dependent effects — such as IR camouflage — also can be achieved. Although the insulator-metal transition has been recognized in vanadium oxide since 1959, it is difficult to work with: In bulk crystals, the stress of the transition often causes cracks and can shatter the sample. Recent advances in materials synthesis and characterization — such as those by Shriram Ramanathan, Harvard SEAS associate professor of materials science — have allowed the creation of extremely pure samples of thin-film vanadium oxide, enabling a burst of new science and engineering to take off in just the past few years. “Thanks to these very stable samples that we’re getting from professor Ramanathan’s lab, we now know that if we introduce small changes to the material, we can dramatically change the optical phenomena we observe,” said Mikhail Kats, a graduate student in Capasso's group and lead author of the paper. “By introducing impurities or defects in a controlled way via processes known as doping, modifying or straining the material, it is possible to create a wide range of interesting, important and predictable behaviors.” The researchers say a vehicle coated in vanadium-oxide tiles could mimic its environment like a chameleon, appearing invisible to an IR camera with only very slight adjustments to the tiles’ actual temperature — a far more efficient system than the approaches in use today. Tuned differently, the material could become a component of a secret beacon, displaying a particular thermal signature on cue to an IR surveillance camera. Capasso’s team suggests that the material could be engineered to operate at specific wavelengths, enabling simultaneous use by many individually identifiable soldiers. And, because thermal radiation carries heat, the team believes a similar effect could be used to deliberately speed up or slow down the cooling of structures ranging from houses to satellites. The team sees its most significant contribution as the discovery that nanoscale structures that appear naturally in the transition region of vanadium oxide can provide a special level of tunability, which can be used to suppress thermal radiation as the temperature rises. “To artificially create such a useful three-dimensional structure within a material is extremely difficult,” Capasso said. “Here, nature is giving us what we want for free. By taking these natural metamaterials and manipulating them to have all the properties we want, we are opening up a new area of research, a completely new direction of work. We can engineer new devices from the bottom up.” Besides Capasso, Kats and Ramanathan, co-authors included research associate Patrice Genevet and graduate students Romain Blanchard, Shuyan Zhang and Changhyun Ko, all of Harvard SEAS. For more information, visit: www.seas.harvard.edu