Traditional infrared images may look colorful, with warm objects appearing redder and whiter than their surroundings, but these images do not display the object’s real color. Instead, they are based on the amount of thermal radiation the camera picks up.
The ability to identify different colors of the infrared spectrum would capture more information about the objects being imaged, such as their chemical compositions.
A team of researchers from DukeUniversity led by MaikenH. Mikkelsen, the Nortel Networks assistant professor of electrical & computer engineering and physics, demonstrated a technique for absorbing small bands of the electromagnetic spectrum from visible light through the near infrared. The method is easily scalable, can be applied to any surface and is less expensive than current light absorption technologies.
A curved object covered with the coating that absorbs all red light, which leaves the object with a green tint. A closer look reveals that the surface is covered with tiny 100 nanometer silver cubes. Image Credit: Maiken Mikkelsen and Gleb Akselrod, Duke University
The technique would allow advanced thermal imaging systems be produced faster and with higher sensitivity than products on the market currently. It could also be used in a wide variety of other applications, such as masking the heat signatures of objects.
"By borrowing well-known techniques from chemistry and employing them in new ways, we were able to obtain significantly better resolution than with a million-dollar state-of-the-art electron beam lithography system," says Mikkelsen. "This allowed us to create a coating that can fine-tune the absorption spectra with a level of control that hasn't been possible previously, with potential applications from light harvesting and photodetectors to military applications."
From left to right Thang Hoang, postdoctoral researcher; Jiani Huang, graduate student; Maiken Mikkelsen, professor of electrical and computer engineering and physics and Gleb Akselrod, postdoctoral researcher. Image Credit: Duke University
"This doesn't require top-down fabrication such as expensive lithography techniques and we don't make this in a clean room," says Gleb Akselrod, a postdoctoral researcher in Mikkelsen's laboratory. "We build it from the bottom up, so the whole thing is inherently cheap and very scalable to large areas."
The technology employs the phenomenon known as plasmonics. The researchers first coat a surface with a thin film of gold. Then, they put down a few-nanometer-thin layer of polymer, followed by a coating of silver cubes, each one about 100 nanometers (billionths of a meter).
When light strikes the new surface, a specific color gets trapped on the surface of the nanocubes in packets of energy called plasmons, and eventually dissipates into heat. The team was able to control the thickness of the polymer film, the size and number of silver nanocubes, which enable the coating to absorb different wavelengths of light from the visible spectrum to the near infrared.
"What is so attractive about the film/nanocube system is its remarkable simplicity and flexibility," says David R. Smith, the JamesB. Duke professor of electrical and computer engineering at Duke. "The unique absorbing properties of the nanocubes can be predicted with straightforward formulas, making it easy to quickly determine recipes for surface coatings that provide desired spectral properties. The nanocube system eliminates, or at least vastly reduces cost and manufacturing issues, so that we can focus on impacting exciting application areas such as photovoltaics or thermal coatings."
By coating photodetectors to absorb only specific wavelengths of infrared light, inexpensive cameras could be made to see different infrared colors.
"We haven't made the device that's actually going to take that energy and convert it to an electrical signal yet," says Akselrod. "That's going to be the next step."
