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Terahertz (THz) waves are a promising starting point for noninvasive imaging technologies for two main reasons. Not only can THz waves penetrate most uncharged materials, this portion of the electromagnetic spectrum coincides with the characteristic absorption bands of many materials and molecules. However, conventional THz imaging technologies work best with flat samples. Imaging real objects with three-dimensional curved surfaces requires THz detectors to be rotated around samples, making imaging systems bulky and costly.
Now Yukio Kawano and his team have come up with a straightforward alternative: an array of flexible THz imaging devices based on CNT films that can be wrapped around a three-dimensional object. The researchers simply cut a CNT film down to the size of the object to be imaged and added detector elements to the film via electrode metal evaporation. Arrays of devices were then mounted on a flexible plastic film, which was wrapped around objects for imaging.
“Our THz scanner successfully performed multi-view imaging of cylindrical samples such as a PET bottle and a syringe – and was able to identify a breakage and an impurity,” Kawano explains. “We also fabricated a wearable THz scanner for human monitoring applications.”
The CNT layer absorbs THz waves over a wide frequency range and generates an electrical signal, which is collected and displayed as an intensity map. The metal electrodes play an important dual role – as well as connecting the CNT layer to external electronics, they also act as a sink for any heat induced by the THz radiation.
“The unique advantage of our device is that it works as a flexible, portable, and wearable terahertz camera,” says Kawano. “It also operates at room temperature and is based on large-area CNT films, which do not require high-cost nanofabrication.”
Michael S. Fuhrer of ARC Centre of Excellence in Future LowEnergy Electronics Technologies, Monash University, Australia, believes that the results are exciting.
“The work... brings THz sensors based on the photothermoelectric effect in carbon nanomaterials out of the lab, showing they can be manufactured easily and inexpensively in large-area flexible arrays and are sensitive enough to use low power THz sources such as the radiation from warm objects,” he says.
Fuhrer believes the findings could open the door to a wide range of applications such as medical diagnostic imaging and product quality testing.
Indeed, the researchers now plan to develop a large-scale array of CNT THz scanners with higher integration. Kawano hopes that this will enable the demonstration of more advanced applications of the approach – such as high-speed, multi-view inspections of industrial products such as PET bottles, syringes, pharmaceutical products, and even food. The team will also investigate the potential for real-time monitoring of health conditions.