The key to a safe high-quality medical scans: a golden bow tie

Wednesday 6 May 2020

Physicists have unveiled a design for a high-precision detector in the shape of a golden bow tie, that could enable a new generation of safe, compact scanners.

Two tiny triangles of solid gold connected by two nanowires form the bow tie detector, which operates in the terahertz region of the electromagnetic spectrum, between microwaves and infrared. 

Terahertz scanning offers a safer low-energy alternative to X-rays: it is not powerful enough to ionise materials. However, it still penetrates through materials such as plastics, wood and paper, is absorbed by water, and is reflected by metals, giving the technology the capability to analyse a wide range of samples.

As well as being stylish, the golden bow ties are able to detect the polarisation of the terahertz radiation, which adds another dimension to the detectors’ versatility, said Professor Chennupati Jagadish, who led the ANU team in this international collaboration.

“The polarisation gives you much more useful information, especially about biological molecules, for example their chirality,” he said.

“Complex molecules have their own terahertz fingerprints, so this technology can be used for finding cancer biomarkers, locating explosives or measuring moisture levels in crops.”

The team, from ANU, Oxford University, England, and Strathclyde University in Scotland, have unveiled their new detector design in the journal Science

The device overcomes a limitation in the resolution, or detail, of conventional terahertz imaging, which is linked to its millimetre-scale wavelength – a million times larger than X-rays, with nanometre-scale wavelengths.

The international team’s new design gets around this limitation with the microscopic scale of the bow ties. The pair of nanowires at their heart are indium phosphide wires one hundredth the size of a human hair, around 280 nanometres in diameter and ten micrometres long. Although each detector is much smaller than the terahertz waves (around 300 microns), an array of bow ties can be used to create a near-field image that bypasses the diffraction limit of the terahertz radiation’s wavelength. 

To detect the polarisation of the radiation the team combine two bow ties, set at right angles to each other, with their central nanowires crossing but not in contact – one bow tie is set slightly above the other. 

Although a simplistic-sounding design, the vertically-offset configuration took three years of collaboration to devise and manufacture. Lead author Dr Kun Peng created the nanowires at ANU, and showed how they could be used with terahertz radiation, although the signal levels were low, due to the tiny size of the wires.

Dr Peng then moved to Professor Michael Johnson’s group at Oxford University, where the metallic triangles were added to the design, as antennae to boost the signal level – gold being the obvious choice due to its high conductivity. Finally the devices were assembled using the precision manufacturing capabilities at Strathclyde University, led by Professor Martin Dawson.

The team are now creating nano-scale electronics to connect to the detector, so that the whole device can be built onto a single chip, in contrast with existing bulky terahertz scanners.

Image: Dimitar Jevtics

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