A new, nanoscale, 3D structure to control light

Date:
February 2, 2022
Source:
Penn State
Summary:
Metamaterials, made up of small, repeated structures, engineered
to produce desired interactions with light or sound waves, can
improve optical devices used in telecommunications, imaging and
more. But the functionality of the devices can be limited by the
corresponding design space.



FULL STORY ========================================================================== Metamaterials, made up of small, repeated structures, engineered to
produce desired interactions with light or sound waves, can improve
optical devices used in telecommunications, imaging and more. 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. Their
findings published online in Advanced Functional Materials.

"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 an electric field oscillating in a specific direction
-- called 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 at Penn State 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." Eric B. Whiting,
an electrical engineering doctoral candidate, was first author on 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
of the Materials Research Institute.

The U.S. Department of Energy, the Defense Advanced Research Projects
Agency and the Laboratory Directed Research and Development program at
Sandia National Laboratories supported this work.

========================================================================== Story Source: Materials provided by Penn_State. Original written by
Gabrielle Stewart. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Eric B. Whiting, Michael D. Goldflam, Lei Kang, Michael B. Sinclair,
Katherine M. Musick, Sawyer D. Campbell, D. Bruce Burckel,
Douglas H.

Werner. Broadband Asymmetric Transmission of Linearly
Polarized Mid‐Infrared Light Based on Quasi‐3D
Metamaterials. Advanced Functional Materials, 2022; 2109659 DOI:
10.1002/adfm.202109659 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220202162747.htm

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