When an object is heated, it gets hotter and emits more thermal radiation, right? Well, not always. Researchers at Harvard University have developed a thin-film/substrate structure that emits decreasing amounts of thermal radiation when heated over the temperature range of 75–85°C. Viewing this process through an infrared camera, the object appears to be getting colder even though it is really heating up.
Mikhail Kats, a graduate student in the group of Federico Capasso at Harvard University, calls this phenomenon “a very unusual situation—almost pathological.” He explains that the amount of radiation emitted at all frequencies from an object at temperature T (Kelvin scale) is proportional to its emissivity times T 4. “So the only way you can see the effect that we saw is if the emissivity goes down faster than T 4 goes up.” This is a huge change in emissivity. The structure that shows such dramatic properties is a thin (150 nm) film of VO2 on a sapphire substrate.
“Vanadium oxide is extremely special because the insulating and metallic phases have very different dielectric properties that give you these spectacular changes in the interaction with radiation over very small temperature windows,” said Shriram Ramanathan, a Harvard faculty member and co-author of the research published in the October–December 2013 issue of Physical Review X (DOI: 10.1103/PhysRevX.3.041004; 041004).
Scientists have long known that VO2 undergoes an insulator-to-metal transition (IMT) at 67°C in the bulk material, and they are exploring this phase transition as an on/off switch for optical and electronic applications in thin-film form. But by observing the transition from insulator to metal slowly, in steps of 0.5°C, and in the particular configuration of a thin film of VO2 on sapphire, Kats, Ramanathan, Capasso, and their colleagues were able to observe this unusual negative differential thermal emittance phenomenon in great detail.
As the VO2 thin-film/sapphire substrate heats up in the phase transition region, small islands of metal begin to form in the insulator matrix, forming what the researchers call a “naturally disordered metamaterial.” At one point in the phase transition, a lot of IR absorption is exhibited and hence a lot of emissivity, Kats said, while at a point further along in the phase change less IR absorption occurs and therefore less emissivity. Thin-film interference causes incoming infrared radiation to bounce back and forth between the VO2 thin film and the sapphire substrate, making both components essential in causing the negative differential thermal emittance.
So instead of artificially combining a metal with an insulator in the manual construction of a metamaterial, the researchers have found a way to do this more naturally. “You don’t have to pattern very complicated features, and you don’t need to worry about mixing and matching composite materials,” Ramanathan said. “It’s a powerful way to think about designing metamaterials by exploiting electronic disorder at the appropriate length scale.”
Like other metamaterials that have been touted for their ability to hide or camouflage an object, it is possible that this VO2/sapphire structure could be used as a coating on a tank, for example, to make it blend in with the landscape surrounding it. A tank that would normally show up easily in an IR camera because it is hotter than its surroundings might be made to blend in by using a little bit of heat to drastically change its IR emissivity and make the vehicle look colder in an IR camera. Kats has also proposed the possibility of making a rewritable IR “blackboard” held at a temperature within the IMT range. A laser beam or soldering iron could be used to write a message on the blackboard by changing the local emissivity; the message could only be seen by thermal imaging, and would be invisible to the naked eye. Temperature control of satellites in space, where the only way an object can heat up or cool down is by absorbing or emitting radiation, is another possible application down the road.
Kats talks about future plans to modify the VO2 or change the substrate to produce a whole family of structures that could be effective in different circumstances. “We need to be able to do this either over a larger temperature range or at lower temperatures,” he said. “If you want to put this on a person for temperature regulation or for camouflage, you need to do it not at 75°C but 35°C. A lot of this is going to depend on how we can control the system to change the transition temperature.”
According to Richard Haglund, Professor of Physics at Vanderbilt University who was not involved in this research, “This paper suggests a broad applications potential for tunable thermo-optical systems based on the complementary properties of a phase-changing film and an appropriately selected or designed substrate. Given the potential of the VO2-sapphire system for infrared tagging, camouflage and identification schemes, and the range of possibilities for designer film-substrate systems with specific thermo-optical properties, this paper may well turn out to be Reference Number 1 in many papers yet to come.”
“I think that one of the most clever things about this work is that this team saw that the transition region, instead of being a necessary bridge from insulator to metal, could be a region with a wealth of really interesting physics,” said Dan Wasserman, an assistant professor in Electrical and Computer Engineering at the University of Illinois at Urbana-Champaign. “The idea of a naturally occurring, and dynamic, disordered metamaterial is fascinating, and they’ve utilized this transition region to show some really interesting macroscopic features of the material. I look forward to seeing what happens as they continue to explore this material system, in particular at the nanoscale.”
