A new nanoscale 3D structure to control light

Metamaterials, made up of small, repeating structures designed to produce desired interactions with light or sound waves, can improve optical devices used in telecommunications, imaging and more. But the functionality of devices can be limited by the corresponding design space, according to Lei Kang, assistant research professor of electrical engineering at Penn State.

Kang and interdisciplinary collaborators from Penn State and Sandia National Laboratories exploited three dimensions of design space to create and test a metamaterial with robust optical properties. Their findings published online in Advanced functional materials.

“It’s not easy to effectively explore the design space for 3D metamaterial components or unit cells,” Kang said. “But we have developed a variety of complex optimization techniques in our lab, and our collaboration with Sandia National Laboratories has enabled the fabrication of very complex 3D structures at the nanoscale. This unique combination of advanced capabilities provides a good strategy for exploring 3D unit cells that can lead to sophisticated metamaterial functionality.

One such feature enables asymmetric light transmission, in which light waves exhibit different power levels depending on their direction of travel through a material. Achieving this phenomenon for light with an electric field oscillating in a specific direction — called linear polarization — often required large components due to design issues, according to Kang. He said a nanoscale device that allows asymmetric transmission of linearly polarized light could lead to much more efficient optical devices, advancing technology applications in communications and more.

To identify an ideal unit cell design, the team developed a genetic algorithm-based computational optimizer, which identifies new configurations by mimicking natural selection, along with self-designed and commercial software to target robust performance in settings defined.

However, applying this approach to a 3D space presented unique obstacles and advantages when designing the optimizer. Generating designs in an extra dimension, while providing an extra degree of freedom for the development of functional materials, required a higher computational load. The Penn State researchers also had to consider manufacturing limitations: a simpler design would be easier to make but potentially deficient in function, while a complex design that works ideally might be impractical or impossible to construct at the time. nanometer scale.

In a recommendation designed to address these challenges, the optimizer simulated numerous arrangements of connected gold particles inside the walls of a cube-shaped unit cell, targeting those that best supported the robust asymmetric transmission of linearly polarized light over a wide range of frequencies.

Researchers at Sandia National Laboratories fabricated the optimized design by constructing many nanoscopic unit cells with cube-shaped cavities on a silicon nitride base. A gold pattern was then stenciled and deposited on two interior walls of each unit cell.

Sandia’s team then tested the sample material by illuminating it with linearly polarized light. They found that the design performed just as well as its computationally-simulated, optimized counterpart, resulting in asymmetric transmission of light over a wide range of frequencies.

This behavior made the design promising for use in optical isolators, according to Sawyer Campbell, assistant research professor of electrical engineering.

“As components of optical devices, optical isolators control and transmit light in a single direction, like a diode in an electrical circuit,” Campbell said. “These components are extremely important in telecommunications, control systems and other areas.”

The researchers said they aim to continue developing metamaterials using their optimization techniques and a variety of fabrication methods.

“Creating more complicated 3D structures would allow us to take these findings further,” Kang said. “New combinations of our advanced optimization methods and state-of-the-art 3D fabrication techniques could further propel the optical capabilities of metamaterials.”

Eric B. Whiting, a doctoral candidate in electrical engineering, was the first author of the paper. Other contributors to this work included Michael D. Goldflam, Michael B. Sinclair, Katherine M. Musick, and D. Bruce Burckel with Sandia National Laboratories; and Douglas H. Werner, John L. and Genevieve H. McCain Chair Professor of Electrical Engineering at Penn State and Principal Investigator of the project. Werner is also a faculty member at the Materials Research Institute.

The U.S. Department of Energy, the Defense Advanced Research Projects Agency, and the Sandia National Laboratories laboratory-led research and development program supported this work.

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