New metamaterials are easily manufactured
UNIVERSITY PARK, Pa. – By combining theory and practice, two groups of Penn State scientists have designed low-loss optical metamaterials that are easily manufactured for custom applications with dispersions that can be tuned over large bandwidths.
Designing materials that allow a range of wavelengths to pass through while blocking others 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 electrical engineering postdoctoral fellow Jeremy A. Bossard.
The investigators 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. The most transformative innovation was the inclusion of nanonotches in the corners of the fishnet holes, which created a pattern that could be tuned to shape the dispersion over large bandwidths.
Top view shows a field emission scanning electron microscopy image of a portion of the fabricated metamaterial nanostructure. Scale bar: 3000 nm. The inset shows an enlarged unit cell. Scale bar: 200 nm.
“We introduced nanonotches in the corners of the airholes to give a lot more flexibility to independently control the properties of permittivity and permeability across a broad band,” said electrical engineering professor Douglas H. Werner. “The conventional fishnet doesn’t have much flexibility but is easy to fabricate.”
Theoretically, manipulating permittivity and permeability allows tuning of the metamaterial across a range of wavelengths and creates the desired index of refraction and impedance. But the researchers wanted to see whether that theoretical solution could become a reality.
Design constraints ensured that the material could be manufactured using electron-beam lithography and reactive ion etching. The initial material was a three-layer sandwich of gold, polyimide and gold on oxidized silicon. When the silicon dioxide mask and the electron beam resist are removed, what’s left is an optical metamaterial with the desired properties.
In this case, a bandpass filter was created, but the same principles can be applied to many optical devices used in optical communications systems, medicine, testing and characterization or even optical beam scanning, if the metamaterial is shaped to form a prism.
The metamaterial could also be used in conjunction with natural materials that do not have the desired properties for a specific optical application.
“All materials have a natural dispersion,” said Theresa S. Mayer, Distinguished Professor of Electrical Engineering and co-director of Penn State’s nanofabrication laboratory. “We might want to coat a natural material in some regions to compensate for the dispersion.”
Currently, the only way to compensate is to find another natural material that would do the job, Werner said. Only rarely does such a material exist.
The research was published in Scientific Reports (doi: 10.1038/srep01571).
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