This image shows the design of Sun's lens with a gradient refractive index.
This image shows the design of Sun's lens with a gradient refractive index.

Engineers at Northwestern University and Oklahoma State University have used metamaterials and three-dimensional (3D) printing to develop a novel lens able to focus electromagnetic radiation at terahertz frequencies. Not only does this lens have better imaging capabilities than standard lenses, but it opens the door for more advances in the mysterious realm of the terahertz.

"Terahertz is somewhat of a gap between microwaves and infrared," said Cheng Sun, associate professor of mechanical engineering at Northwestern's McCormick School of Engineering. "People are trying to fill in this gap because this spectrum carries a lot of information."

The focal length of a lens is determined by its curvature and refractive index, which shapes the light as it enters the lens. Without components to counter imperfections, however, the resulting images can be fuzzy or blurred. "Typical lenses – even fancy ones – have many, many components to counter their intrinsic imperfections," explained Sun. "Sometimes modern imaging systems stack several lenses to deliver optimal imaging performance, but this is very expensive and complex."

In contrast, the lens developed by Sun and his team employs a gradient index, which is a refractive index that changes over space to create flawless images without requiring additional corrective components. Their work was supported by the US National Science Foundation and is reported in a paper in Advanced Optical Materials.

To produce the lens, the engineers employed a novel metamaterial that exhibits properties not readily available in nature. "Such properties originate from its tiny structures that are much smaller than the terahertz wavelength," said Fan Zhou, the paper's first author and a member of Sun's laboratory. "By assembling these tiny structures, we can create specific refractive index distribution."

In addition, the lens was manufactured using a 3D printing technique called projection micro-stereo-lithography. This technique offers a rapid, scalable and inexpensive way to produce the tiny features that are needed for the lens to operate in the terahertz frequency band. The printing technology allowed the researchers to fabricate the metamaterial so that it closely matched their detailed designs.

"For printing, we use a photo-polymer in liquid form," Sun said. "When we shine a light on the material, it 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 lens could enhance terahertz imaging, which is particularly useful for security applications, making it cheaper, more available and higher resolution. While X-rays can detect metal, they cannot detect plastic or chemicals. A terahertz scanner, on the other hand, can detect both these substances, allowing it to discover concealed weapons, biological weapons such as anthrax, and plastic explosives. And unlike X-rays, terahertz radiation is completely harmless to humans.

"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."

This story is adapted from material from Northwestern University's McCormick School of Engineering, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.