Metamaterials, made up of small, repeated structures, engineered to produce desired inter­actions with light or sound waves, can improve optical devices used in telecommu­nications and imaging. But the func­tionality of the 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 leveraged three dimensions of design space to create and test a meta­material with robust optical properties. 

“It’s not easy to efficiently explore 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 allowed for fabrication of very complex 3D structures at the nanometer scale. This unique combination of advanced capa­bilities provides a good strategy to explore 3D unit cells that can lead to sophisticated metamaterial func­tionalities.” One such functionality is enabling asymmetric transmission of light, in which light waves exhibit different power levels dependent on their direction of travel through a material. Realization of this phenomenon for light with linear polarization has often required bulky components due to design challenges, according to Kang. He said a nanoscale device permitting asymmetric transmission of linearly polarized light could lead to signi­ficantly more efficient optical devices, advancing techno­logical applications in communi­cations and more.

To identify an ideal unit cell design, the team developed a compu­tational optimizer based on a genetic algorithm, which identi­fies new configurations by mimicking natural selection, with both self-designed and commercial software to target robust performance within set para­meters. Applying this approach to a 3D space, however, presented unique obstacles and benefits when designing the optimizer. Generating designs in an additional dimension, while providing an additional degree of freedom for developing func­tional materials, required a higher computational load. The researchers also had to account for fabrication limitations: A simpler design would be easier to make but potentially deficient in function, while a complex design that performs ideally could be impractical or impossible to construct at the nanoscale.

In a recommen­dation engineered to meet these challenges, the optimizer simulated many arrangements of connected gold particles on the inside of a cube-shaped unit cell’s walls, targeting those that best supported robust asymmetric trans­mission of linearly polarized light across a wide frequency range. Researchers at Sandia National Laboratories fabricated the optimized design constructing many nanoscopic unit cells with cube-shaped cavities atop a silicon nitride base. A gold pattern was then stenciled and deposited onto two inside walls of each unit cell. 

The Sandia team then tested the sample material by illuminating it with linearly polarized light. They found that the design performed as well as its computationally optimized and simulated counter­part, resulting in asymmetric trans­mission of the light across a wide range of frequencies. This behavior made the design promising for use in optic isolators, according to Sawyer Campbell, assistant research professor of electrical engi­neering. “As components in optical devices, optic isolators control and transmit light in only one 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 expand on these findings,” 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.” (Source: PSU)

Reference: E. B. Whiting et al.: Broadband Asymmetric Transmission of Linearly Polarized Mid-Infrared Light Based on Quasi-3D Metamaterials, Adv. Func. Mat., online 7 January 2022; DOI: 10.1002/adfm.202109659

Link: Materials Research Institute (D. Werner), Pennsylvania State University, University Park, USA