New promise for terahertz wave imaging

Much of the electromagnetic spectrum has provided science and medicine with ways of looking inside matter. Terahertz waves, lasting a millionth of a millionth of a second and sitting between microwaves and infrared on the EM spectrum, are one the most recent additions to the imaging armoury, and have very attractive properties particularly for medicine.

With much less energy than x-rays, they do not damage DNA or living tissue; moreover, they interact in specific ways with matter making them a useful tool for spectroscopy to characterise materials. They can also pass through nonconductive materials, which could make them useful for security screening.

However, terahertz waves have until now been difficult to use, because they require complex, bulky and expensive equipment to both generate and analyse. Researchers at Princeton University in New Jersey, US, have now developed a pair of compact microchips that can generate and receive terahertz waves that could, they claim, revolutionise their use, moving away from devices with arrays of mirrors, lenses and lasers and towards compact desktop or even handheld systems.

In articles published in the IEEE Journal of Solid-State Circuits, Princeton electrical engineer Kaushi Sengupta and colleagues describe how they have rethought the functioning of a solid-state antenna to create devices that send and receive terahertz waves. The paper that describes the generation of the waves was published last September, while the receiver paper appeared in February. The key to generating the waves is to create a wide range of wavelengths, which the team achieved by using very precise timing to send out sharp pulses of radiation. In the receiver, they noted that the incident terahertz waves interact with a metal structure inside the chip to set up a complex series of subtle electromagnetic fields that are normally ignored, but could be used as a signature to interpret the original wave. "Instead of directly reading the waves, we are interpreting the pattern created by the waves," Sengupta explained. "It is somewhat like looking for a pattern of raindrops by the ripples they make in a pond."

"The system is realised in the same silicon chip technology that powers all modern electronic devices and smartphones to tablets, and therefore costs only a few dollars to make on a large scale," Sengupta added. Both chips could be made small enough to fit on a fingertip.

The team is now working on extending the functionality of the chips to the lower part of the terahertz radiation band.