Genetic algorithm used to design broadband metamaterial
May 7, 2014
The engineers say this allows the metamaterial to shield objects from view by infrared sensors and protect instruments, for example.
“The metamaterial has a high absorption over broad bandwidth,” said Jeremy A. Bossard, postdoctoral fellow in electrical engineering. “Other screens have been developed for a narrow bandwidth, but this is the first that can cover a super-octave [more than doubling] bandwidth in the infrared spectrum.”
Having a broader bandwidth means that one material can protect against electromagnetic radiation over a wide range of wavelengths, making the material more useful. The researchers found that palladium provided better bandwidth coverage than silver or gold.
The new metamaterial is actually made of layers on a silicon substrate or base. The first layer is palladium, followed by a polyimide (plastic) layer and a palladium screen layer on top. The screen has elaborate, complicated cutouts — sub-wavelength geometry — that serve to block the 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,” said Lan Lin, graduate student in electrical engineering. “It can also absorb 90 percent of the incoming infrared radiation at up to a 55 degree angle to the screen.”
To design the necessary screen for this metamaterial, the researchers used a genetic algorithm (emulates natural selection). They described the screen pattern by a series of zeros and ones — a chromosome — and let the algorithm randomly select patterns to create an initial population of candidate designs. The algorithm then tested the patterns and eliminated all but the best. The best patterns were then randomly tweaked for the second generation. Again the algorithm discarded the worst and kept the best. After a number of generations, the good patterns met and even exceeded the design goals. Along the way the best pattern from each generation was retained.This evolved metamaterial can be easily manufactured because it is simply layers of metal or plastic that do not need complex alignment. The clear cap of polyimide serves to protect the screen, but also helps reduce any impedance mismatch that might occur when the wave moves from the air into the device.
“This kind of device could be used as a coating to hide reflective objects, as a component in solar or thermophotovoltaic energy conversion systems, or as a custom infrared emitter,” Bossard explained to KurzweilAI in an email interview. “The absorption range in the infrared can also be customized depending on the device application.
“The prototype we made to demonstrate the technology was fabricated using electron-beam lithography, which is expensive on a large scale. However, other methods for fabrication are under development that could be employed to fabricate the metamaterial economically on a production scale.”
The researchers report their results in ACS Nano.
Abstract of ACS Nano paper
Nanostructured optical coatings with tailored spectral absorption properties are of interest for a wide range of applications such as spectroscopy, emissivity control, and solar energy harvesting. Optical metamaterial absorbers have been demonstrated with a variety of customized single band, multiple band, polarization, and angular configurations. However, metamaterials that provide near unity absorptivity with super-octave bandwidth over a specified optical wavelength range have not yet been demonstrated experimentally. Here, we show a broadband, polarization-insensitive metamaterial with greater than 98% measured average absorptivity that is maintained over a wide ±45° field-of-view for mid-infrared wavelengths between 1.77 and 4.81 μm. The nearly ideal absorption is realized by using a genetic algorithm to identify the geometry of a single-layer metal nanostructure array that excites multiple overlapping electric resonances with high optical loss across greater than an octave bandwidth. The response is optimized by substituting palladium for gold to increase the infrared metallic loss and by introducing a dielectric superstrate to suppress reflection over the entire band. This demonstration advances the state-of-the-art in high-performance broadband metamaterial absorbers that can be reliably fabricated using a single patterned layer of metal nanostructures.