Researchers have created a specially-formed material using genetic algorithms they say could shield objects from detection by infrared sensors, protect instruments and be modified to cover a variety of wavelengths.
“The metamaterial has a high absorption over broad bandwidth,” Jeremy A. Bossard, postdoctoral fellow in electrical engineering at Penn State, said. “Other screens have been developed for a narrow bandwidth, but this is the first that can cover a super-octave bandwidth in the infrared spectrum.” With a broader bandwidth, a single material could protect against electromagnetic radiation over a wider range of wavelengths.
Researchers examined silver, gold and palladium as potential choices for incorporation into the design but ultimately chose palladium because it provided better bandwidth coverage than the other options. The resulting metamaterial is comprised of a layer of palladium atop a silicon substrate or base. A polymide layer sits on top of the palladium, followed by a palladium screen layer with complicated cutouts known as sub-wavelength geometry designed to block various wavelengths. A polyimide layer caps the whole absorber.
“As long as the properly designed pattern in the screen is much smaller than the wavelength, the material can work effectively as an absorber,” Lan Lin, graduate student in electrical engineering, said. “It can also absorb 90 percent of the infrared radiation that comes in at up to a 55 degree angle to the screen.”
To design the elaborate screen pattern, researchers turned to a genetic algorithm. The team assigned a series of zeros and ones—a chromosome—to the pattern, and let algorithm randomly select patterns to create an initial population of potential designs. The algorithm then tested the patterns and eliminated all but the best ones. This process was repeated several times in order to select the best possible design.
Bossard says use of the genetic algorithm was key to both the success of the new metamaterial design and to its manufacturing potential.
“We wouldn’t be able to get an octave bandwidth coverage without the genetic algorithm,” he said. “In the past, researchers have tried to cover the bandwidth using multiple layers, but multiple layers were difficult to manufacture and register properly.”
The researchers expect manufacturing the new metamaterial to be relatively simple because it is made up of layers of metal or plastic that do not need complex alignment. The polyimide cap, in addition to protecting the screen, helps reduce any impedance mismatch that might occur when the wave moves from the air into the device.
“Genetic algorithms are used in electromagnetics, but we are at the forefront of using this method to design metamaterials,” said Bossard.
The research paper, “Near-Ideal Optical Metamaterial Absorbers with Super-Octave Bandwidth,” is published in the journal ACSNano. Other Penn State researchers who worked on this project include Seokho Yun, former postdoctoral fellow in electrical engineering, Liu Liu, graduate student in electrical engineering, Douglas H. Werner, McCain Chair Professor of Electrical Engineering, and Theresa Meyer, Distinguished Professor of Electrical Engineering and Materials Science and Engineering.
