Terahertz radiation is a growing field of technology that enables faster materials analysis than X-ray examination, and provides non-destructive, internal analysis of a raft of different types of materials. Now researchers have developed a way of manufacturing lenses operating at this frequency that are simple and inexpensive, but are claimed to produce near-flawless images which could vastly improve biomedical imaging as well as biological and explosive security scanning.

Terahertz frequencies are located between the microwave and infrared frequencies in the in the electromagnetic spectrum, at a wavelength range between 1 mm to 0.1 mm, and have some particularly remarkable properties. Many ordinary materials and living tissue, for example, are semi-transparent to this radiation and produce their own unique "fingerprints," that allows them to be individually identified as well as imaged and analyzed.

"Terahertz is somewhat of a gap between microwaves and infrared," says Northwestern University's Associate Professor of Mechanical Engineering, Cheng Sun. "People are trying to fill in this gap because this spectrum carries a lot of information."

A number of new devices have been developed recently for terahertz imaging, such as one detector using graphene, and another that applies the technique to RFID tags. The most commercially attractive devices, however, are those that use this sub-millimeter radiation to supplement security and protection services, particularly in regard to explosive or biomedical hazard detection.

To further develop new technologies for these applications, the team at Northwestern created a lens using metamaterials and a 3D printing technique known as projection micro-stereo-lithography. This a microscopic process where a dynamic photo-mask is applied over a UV-sensitive polymer and a high-powered flash of UV laser light is applied to solidify the pattern traced on the polymer and join it to the layer below.

Able to rapidly produce the minuscule features needed for the lens to function at terahertz frequencies, this 3D printing technology gave the researchers the absolute precision needed to mold the metamaterial to their designs.

"Typical lenses – even fancy ones – have many, many components to counter their intrinsic imperfections," says associate professor Sun. "Sometimes modern imaging systems stack several lenses to deliver optimal imaging performance, but this is very expensive and complex. (In our lens) we use a photo-polymer in liquid form. 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 3-D structure."

The focal lengths of ordinary lenses are a product of the curvature ground or molded into them. This, in turn, determines the refractive index of the lens (in simple terms, the amount the light bends as it enters and leaves the lens). Often, however, additional components are required in a lens system to counter the imperfections in most lenses to avoid a fuzzy or blurry image reproduction.

In the Northwestern University's terahertz lens, though, a gradient index is employed to produce ultra-sharp images without additional corrective components. This is where a continuous change of the refractive index is produced within the lens material itself; the human eye is an example of a natural lens with a gradient index capability, as it can focus on objects at varying distances.

"Such properties originate from its tiny structures that are much smaller than the terahertz wavelength," says Fan Zhou, a member of Sun's team. "By assembling these tiny structures, we can create specific refractive index distribution. You cannot accomplish a gradient index with traditional manufacturing processes."

According to the researchers, the new lens could make terahertz imaging cheaper, with higher resolution, and more widely available than it is currently. Particularly useful for security because, unlike X-rays, terahertz scanners can detect plastic, biological materials, and chemicals to help uncover biological weapons such as anthrax, or plastic explosives such as Semtex. Terahertz radiation is also completely harmless to humans and can be used over and again without irradiation danger, unlike X-ray imaging.

The result of the research were recently published in the journal Advanced Optical Materials.

The video below shows the Northwestern 3D printer in action.

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