Nanofabrication

Theoretical and practical electrical engineers join forces to create broadband metamaterials

The union of theory and practice makes broadband, low-loss optical devices practical, which is why two groups of Penn State engineers collaborated to design optical metamaterials that have custom applications that are easily manufactured.

C60 just before impact with benzene deposited on AgMetamaterials are manufactured materials that derive their unusual properties from structure rather than only composition, and possess exotic properties not usually found in nature. Nanostructured metamaterials appear different for signals of different frequencies. They are dispersive, so that if researchers manipulate this material dispersion, they gain a comprehensive control of the device performance across a band of frequencies.

In the past, to control the optics of metamaterials, researchers used complicated structures including 3-dimensional rings and spirals that are difficult if not impossible to manufacture in large numbers and small sizes at optical wavelengths. From a practical perspective, simple and manufacturable nanostructures are necessary for creating high-performance devices.

"We must design nanostructures that can be fabricated," said Theresa S. Mayer, Distinguished Professor of Electrical Engineering and co-director of Penn State's nanofabrication laboratory.

Designing materials that can allow a range of wavelengths to pass through while blocking other wavelengths is far more difficult than simply creating something that will transmit a single frequency. Minimizing the time domain distortion of the signal over a range of wavelengths is necessary, and the material also must be low loss.

"We don't want the signal to change as it passes through the device," said Jeremy A. Bossard, postdoctoral fellow in electrical engineering.

The majority of what goes in must come out with little absorption or distortions to the signal waveform due to the metamaterial dispersion.

"What we do is use global optimization approaches to target, over wide bandwidths, the optical performance and nano fabrication constraints required by different design problems," said Douglas H. Werner, John L. and Genevieve H. McCain Chair Professor of Electrical Engineering. "The design methodology coupled with the fabrication approach is critically important."

The design team looked at existing fishnet structured metamaterials and applied nature-inspired optimization techniques based on genetic algorithms. They optimized the dimensions of features such as the size of the fishnet and the thicknesses of the materials. One of the transformative innovations made by the researchers was the inclusion of nanonotches in the corners of the fishnet holes, creating a pattern that could be tuned to shape the dispersion over large bandwidths. They reported their approach in the March 28, 2013 online issue of Scientific Reports.

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