New miniature sensors developed at ANU could be the key to future safer medical imaging and security scans.

A team of physicists that developed a detector for terahertz radiation made from nanowires one hundredth the diameter of a human hair, a million times smaller than the current technology.

“The terahertz devices we have developed in this project have great potential for compact imaging and spectroscopy systems because of their nanoscale resolution and broad detection bandwidth,” said lead researcher, Associate Professor Lan Fu from the ANU Research School of Physics and Engineering.

“As well as being smaller, our nanowire detectors can also measure polarisation.”

Terahertz radiation can penetrate through materials such as plastics, wood and paper and is absorbed by water. Unlike X-rays, terahertz scanners use low-energy radiation which is safer because it cannot damage cells with ionisation – in the electromagnetic spectrum terahertz radiation falls between infrared and microwaves.

The ANU team and collaborators at Oxford University developed a system that used a single nanowire made from indium phosphide as a detector to measure the spectra of various materials. As well as frequency information, the nanowire yielded phase, amplitude and polarisation information.

“It’s the first time that nanowires have been used for terahertz time-domain spectroscopy,” said Dr Kun Peng, who recently received her PhD at the ANU Research School of Physics and Engineering.

The size of the nanowires – a few hundred nanometres in diameter and several micrometres in length – is also an advantage in overcoming a shortcoming of terahertz radiation: its millimetre-scale wavelength.

“I’m excited because I am making detectors and sources that are smaller than the wavelength of the radiation, which means we can overcome the diffraction limit by creating a near-field image,” Dr Peng said.

Nanowire detectors also promise cheaper and easier design and growth processes than for existing detectors, which are made of large crystals of indium phosphide, says Dr Peng.

“We have overcome quite a complex manufacturing process. The bulk indium phosphide detectors use additional processes to introduce defects in the crystals to reduce the carrier lifetime.”

“The nanowires have intrinsically short carrier lifetimes, because of their large surface to volume ratio, so they don’t need additional modification,” she said.

The nanowires were grown at ANU by Dr Qian Gao on indium phosphide wafers coated with a thin mask of silicon dioxide. Using acid, the mask was removed from specific areas, to precisely define the position and shape of the nanowire growth. Dr Gao then built up the indium phosphide nanowires on the exposed areas in a MOVPE (metal organic vapor-phase epitaxy) reactor.

The team is now exploring new nano-materials and structures, which promise greater flexibility and functionality for terahertz applications such as emitters, polarisers and on-chip integrated terahertz systems.

The research has been built on a long-term collaboration between the ANU group, led by Distinguished Professor Chennupati Jagadish, and the Oxford group, led by Professor Michael Johnston, with the support of Australian Research Council, Australian National Fabrication Facility, and the Australian Government International Postgraduate Research Scholarship.

The research is published in Nano Letters.