Collaboration Makes Easily Manufactured Optical Metamaterials
UNIVERSITY PARK, Pa., March 29, 2013 — Controlling the optics of metamaterials involves using complicated structures that are difficult to manufacture in large numbers and at small sizes at optical wavelengths. However, engineers are collaborating to change that with a nanonotch, fishnet-structured metamaterial that can be tuned to shape the dispersion over large bandwidths.
From a practical perspective, simple and manufacturable nanostructures are necessary for creating high-performance devices. By combining theory and practice, two groups of Penn State scientists have collaborated to design low-loss optical metamaterials that have custom applications that are easily manufactured.
The fabricated free-standing metamaterial filter thin-film mounted on an optical frame. Images courtesy of Penn State.
Designing materials that 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 electrical engineering postdoctoral fellow Jeremy A. Bossard.
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 nanofabrication 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.”
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.
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.
“We introduced nanonotches in the corners of the air holes to give a lot more flexibility to independently control the properties of permittivity and permeability across a broad band,” Werner said. “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.
Theory may provide a solution, but the researchers set out to find if that solution could become a reality. They placed constraints on the design to ensure 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, the researchers were left with 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.
This is a tilted view, field emission scanning electron microscopy image of the fabricated metamaterial nanostructure. Scale bar: 1000 nm.
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, supported by the National Science Foundation’s Materials Research Science and Engineering Center and National Nanotechnology Infrastructure Network, appears in Scientific Reports (doi: 10.1038/srep01571).
For more information, visit: www.psu.edu
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