Metamaterials Embedded with Geometrical Optics Could Simplify Optical Devices

3D-printed metamaterials developed by a Tufts University engineering team display properties not found in conventional materials. The fabrication methods used by the team demonstrate how stereolithography-based 3D printers can be used to create 3D optical devices through a process that fuses metamaterials with geometrical optics, or MEGO. The MEGO devices can be fabricated at a lower cost than devices made using typical fabrication methods.

The researchers employed a hybrid fabrication approach that combined 3D printing, metal coating, and etching to create metamaterials with complex geometries and novel functionalities for wavelengths in the microwave range. For example, they created an array of tiny mushroom-shaped structures, each holding a small patterned metal resonator at the top of a stalk. This arrangement permitted microwaves of specific frequencies to be absorbed, depending on the geometry and spacing of the mushroom structures. Use of such metamaterials could be valuable in applications such as sensors in medical diagnosis or as antennas in telecommunications or detectors in imaging applications.

In one case, the researchers created a surface capable of selective high-frequency energy absorption by patterning hemispherical surfaces that resemble the compound eye of a moth. The hemispherical device can absorb electromagnetic signals from any direction.

3D-printed hemispherical metamaterial can absorb microwaves at select frequencies. Courtesy of Hojat Rezaei Nejad, Tufts University, Nano Lab.

Other devices developed by the team include parabolic reflectors that selectively absorb and transmit certain frequencies. Reflectors like these could simplify optical devices by combining the functions of reflection and filtering into one unit. “The ability to consolidate functions using metamaterials could be incredibly useful,” said professor Sameer Sonkusale. “It’s possible that we could use these materials to reduce the size of spectrometers and other optical measuring devices so they can be designed for portable field study.” The researchers validated the functionality and performance of the devices through simulation and measurement using a terahertz continuous-wave spectrometer.

Metamaterials extend the capabilities of conventional materials in devices by making use of geometric features arranged in repeating patterns at scales smaller than the wavelengths being detected or influenced. New developments in 3D-printing technology could make it possible to create many more shapes and patterns of metamaterials at ever-smaller scales.

The researchers believe that other MEGOs that absorb, enhance, reflect, or bend waves in new ways could be created using patterned 3D printing. The current Tufts study utilizes stereolithography. Other 3D-printing technologies, such as two-photon polymerization, could provide printing resolution down to 200 nm, which would enable the fabrication of even finer metamaterials that could detect and manipulate electromagnetic signals of even smaller wavelengths, potentially including visible light. As resolution in 3D printing improves, MEGO devices could reach terahertz frequencies.

“The full potential of 3D printing for MEGOs has not yet been realized,” said researcher Aydin Sadeqi. “There is much more we can do with the current technology, and a vast potential as 3D printing inevitably evolves.”

The research was published in Microsystems & Nanoengineering (https://doi.org/10.1038/s41378-019-0053-6).