New Terahertz Imaging Technique Reveals Tiny, Hidden Objects

In an article published today in Science Advances, a team of UK researchers revealed a new way to see small or hidden objects using a technique known as terahertz imaging. This could lay the foundation for a new kind of imaging device with a wide range of applications from studying bacteria to performing quality control in electronics manufacturing.

A terahertz image taken with the newly-developed, noninvasive, sub-wavelength terahertz imaging setup. The image size is 2x2mm and the structure was hidden behind a silicon wafer.
Image Credit: Stantchev, et al.

The electromagnetic spectrum contains a rich collection of tools that we can use to explore the world around us. From x-ray to visible to infrared and radio, each type of light reveals different kinds of information and has applications in its own particular niche.

In recent years, the often-ignored terahertz part of the spectrum has been gaining attention. Terahertz radiation falls between the infrared and microwave portions of the spectrum. Most of its fame comes from the fact that it can “see through” clothing, paper, plastic, ceramics—materials that don’t conduct electricity. Terahertz imaging devices have revealed invisible details about famous paintings and defects in space shuttle panels. Lab experiments have shown a range of possible applications like detecting explosive chemicals, identifying hidden weapons (the “milimeter wave” scanners found at many modern airports operate in the terahertz range), and even screening for skin cancer. Terahertz radiation is non-ionizing and doesn’t damage tissue or DNA, so scientists are exploring various medical and dental imaging applications as well.

The electromagnetic spectrum, with the frequency of the radiation given along the bottom.
Image Credit: Vardanyandv (CC BY-SA 4.0)

Along with its promising applications come some drawbacks. The key components of terahertz imaging designs are complicated and expensive to make. Traditional devices don’t work very well for objects smaller than the wavelength of the radiation, which ranges from 0.15mm to 1.5mm. Techniques that do work for small objects are pretty slow and aren’t ideal for biological materials, especially if you want to keep them intact.

In proof-of-concept experiments, a team of researchers from the University of Exeter, University of Glasgow, and British defense firm QinetiQ Limited recently demonstrated a new technique for imaging objects with terahertz radiation. The fast, high-resolution technique is capable of imaging tiny objects and hidden structures like some biological features.

For their experiments, the UK team hid an object behind a silicon wafer. Then they illuminated the wafer with two different sources—a terahertz beam, and a special kind of pulsed light that projected patterns of light and dark areas onto the wafer. As a part of the wafer was illuminated in a pattern, it became conductive. Since terahertz radiation only sees through non-conductive materials, the illuminated parts of the wafer don’t let any of the beam though.

The terahertz radiation travels through the non-conducting part of the wafer, and then passes through or is scattered by the object hidden underneath. A single-pixel detector records the terahertz signals that make it through. By combining information from the patterns they used and the signals that were detected, the researchers were able to reconstruct the hidden object.

Using this method, the UK team was able to resolve features significantly smaller than the wavelength of their beam, about 25% of the size. This resolution was limited by the thickness of the wafer, so with a thinner wafer the resolution would be even better.

As an example application, the team repeated the process with a printed circuit board hidden behind the wafer. They were able to see tiny (micrometer-sized) cracks in the circuit board. They also had a little fun producing the image at the top of this story!

Although the particular design of this experiment would be expensive to reproduce and manufacture as imaging devices, the research provides a framework that will hopefully inspire cheaper, more commercially viable designs. Such devices could help us see tiny and hidden parts of our world in even better detail.

Kendra Redmond

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