01.01.2021

Diamond encrusted sensors

An Adelaide artist has inspired a path towards rugged diamond-based magnetic field sensors that could be used in underwater monitoring, brain scanning, and mineral exploration applications.

A consortium of scientists had been trying to embed diamond into optical fibres when they realised that artist Karen Cunningham had shown them the way through her creative use of nanoparticles in blown glass.

The diamonds the researchers were using feature deliberate spaces and impurities in their crystal structure called nitrogen vacancy (NV) centres. These gaps have incredibly useful light-emitting properties that pose significant opportunities to sensing applications.

An NV centre emits red light at a particular wavelength if shone with a green laser beam, but if the centres are within a magnetic field the brightness of the emitted photons changes ever so slightly. Monitoring this variation can therefore indicate the strength of a magnetic field, providing an incredibly precise sensor. However, monitoring these subtle signals usually requires complex optical microscopes that are difficult to take into the field.

Professor Heike Ebendorff-Heidepriem from the University of Adelaide was working with a consortium led by Professor Brant Gibson from RMIT University to find a way to take NV diamond sensors out into the real world. The team, which also included researchers from the University of Melbourne, University of South Australia, and Defence Science and Technology Group, started embedding the diamonds into optical fibres that would both create a protective casing for the diamond and carry light towards a detector, providing a means to monitor magnetic fields found out of the lab.

However, there were obstacles to overcome. The fibre fabrication process requires a stage where the material is molten – this liquid glass is highly corrosive and rapidly degrades the diamond particles rendering them all but useless as sensors.

Because diamond burns at high temperatures, the team had been restricted to using unconventional “soft glasses” such as tellurite – these don’t perform as well as conventional glass fibres and aren’t as widely used in industry, but they are easier to process at lower temperatures. If the sensors were ever to be usable, a more robust glass would be required.

Another problem was that the haphazard distribution of the diamond particles – caused by mixing them into the molten glass – would end up blocking a lot of the light signals before they could be detected.

For more than ten years, the team had been trying to find a way around these issues.

But then, upon visiting Karen Cunningham ahead of an exhibition in Adelaide in 2017, Heike saw that the artist had managed to incorporate diamond particles into her art by sprinkling diamond across the surface of widely used silicate glass before manipulating the glass further.

“For us, it was the lightbulb moment, and we knew we had found a way make diamond sensors in more conventional glass fibres,” Heike said.

It inspired Heike to invent an entirely new technique for introducing particles into optical fibres which she has named interface doping. To develop the technique, the team began working with ANFF Optofab’s optical fibre specialists using silicate glass as a proof-of-concept towards a more robust fibre. Initially, a rod of silicate glass is produced and dip- coated with diamond particles. The diamond-encrusted rod is then inserted into a tube made of the same silicate glass and they are drawn out together to form the fibre. Once drawn out, the team are left with a ring of diamond particles that runs the length of the glass fibre.

But the artistic influence took the researchers further – Karen also utilised larger-than-typical diamond particles, measuring microns across instead of nanometres. The team also began using similar sized particles and found that they actually provided a higher level of sensitivity to magnetic field changes due to a higher concentration of NV centres in the bigger particles which provided a larger signal.

With this successful first step demonstrated, the consortium is now furthering their work by integrating diamond into conventional glass fibre types for future use within telecom networks.

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