3D printed Terahertz lens could revolutionise security imaging, useful for detection of concealed plastics and chemicals

Apr 29, 2016 | By Benedict

Researchers from Northwestern University have used 3D printing to develop a novel lens that picks up terahertz frequencies—those occupying a middle ground between microwaves and infrared light waves. It is hoped that the lens could help scientists to better understand the mysterious terahertz waves.

Design of the lens

Although we might not understand it fully, most of us learned something about the electromagnetic spectrum during school. But unless you happened to continue with physics beyond high school, you probably haven’t heard of terahertz waves, which occupy the frequency range between 300 GHz and 3 THz. That’s nothing to be ashamed of though: despite their importance in imaging and communications, terahertz waved are little understood and rarely studied.

Because of this lack of information, a group of researchers from Northwestern University have used metamaterials and a 3D printer to develop a lens which works with terahertz frequencies. “Typical lenses—even fancy ones—have many, many components to counter their intrinsic imperfections," said Cheng Sun, associate professor of mechanical engineering at Northwestern's McCormick School of Engineering. "Sometimes modern imaging systems stack several lenses to deliver optimal imaging performance, but this is very expensive and complex.”

Associate Professor of Mechanical Engineering Cheng Sun

Rather than pursue the expensive and complex route, Sun and his team created a gradient index which changes over space to create perfect images without additional corrective components. The process sounds simple, but is only possible thanks to a novel metamaterial which exhibits highly unconventional properties. “Such properties originate from its tiny structures that are much smaller than the terahertz wavelength,” said Fan Zhou, first author on the research paper and member of Sun's laboratory. “By assembling these tiny structures, we can create specific refractive index distribution.”

To manipulate the metamaterial, the researchers used a 3D printer and the projection micro-stereolithography 3D printing technique. This technology allowed the team to produce the tiny features needed for the lens to pick up terahertz frequencies. "For printing, we use a photo-polymer in liquid form," Sun explained. "When we shine a light on the material, [which] converts it into a solid. The material forms to the shape of the light, allowing us to create a 3D structure. You cannot accomplish a gradient index with traditional manufacturing processes.”

The development of the 3D printed lens could have important consequences for the future scientific study of terahertz waves, and could make terahertz imaging cheaper, of higher resolution, and more readily available. This could prove to be a boon for the security sector, since terahertz scanners can detect concealed plastics and chemicals. X-rays cannot detect such materials, and can be harmful to humans, while terahertz waves are completely harmless.

“This advance means we can unveil previously inaccessible information of some opaque materials in high resolution,” said Wei Cao, Sun's collaborator at Oklahoma State University. "This opens up an entirely new technique for a massive range of potential uses from biomedical research to security.”

The team’s research paper, titled “Additive Manufacturing of a 3D Terahertz Gradient-Refractive Index Lens,” has been published in Advanced Optical Materials. A similar study was also conducted last year at the Huazhong University of Science and Technology.

Posted in 3D Printing Application

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