Media release

From: ARC Centre of Excellence for Transformative Meta-Optical Systems (TMOS)

Infrared innovation heats up, creating cheap robust spectrometers

A new filter for infrared light could see scanning and screening technology tumble in price and size.

Built on nanotechnology, the new heat-tunable filter promises hand-held, robust technology to replace current desktop infrared spectroscopy setups that are bulky, heavy and cost from $10,000 up to more than $100,000.

Because the new technology is built on silicon technology its manufacture can be scaled, with the potential to push costs down below $1 per filter, its developers said.

“The mid infrared is ripe for technological development,” said Professor Kenneth Crozier, Deputy Director of the ARC Centre of Excellence for Transformative Meta-Optics (TMOS), who leads the research group.

“There are high-performance systems available, but they are expensive. This smaller, lighter, low-power technology could open up a lot of applications, for example field testing of agricultural products, such as milk and olive oil, and screening and sorting of recycled materials.”

The research is published in Laser and Photonics Reviews. https://onlinelibrary.wiley.com/doi/10.1002/lpor.202400853

Many materials can be easily identified from the way they absorb different parts of the infrared spectrum, so infrared spectroscopy is useful for monitoring for contaminants, for example in industrial processes.

Spectroscopy relies on spreading the light source into a spectrum, which is conventionally done with a grating or prism to spread the infrared light: different parts of the spectrum are sent into the sample by tilting the grating back and forth.

For robustness, the TMOS team instead opted for a non-moving component to select specific parts of the infrared spectrum, in the form of a band-pass filter. Drawing on other research within TMOS that had used heating to vary component behaviour, they devised a filter with temperature-dependent band-pass wavelength.

This is made possible by making the filter from silicon, whose refractive index shifts smoothly with temperature.

“The great thing about it is that it is very stable and reversible,” said Ben Russell, PhD student in TMOS.

To generate bandpass behaviour from the silicon the team used a metasurface, a layer of silicon approximately 1.5 microns thick with an array of nanoscale features carved into it, sitting on a layer of sapphire (Al₂O₃) 470 microns thick.

Initially Russell modelled the dimensions of the metasurface, coming up with two possible solutions: parallel grooves 1002 nm deep, spaced 1683 nm apart, and crossed grooves, 1060 nm deep and 1684 nm apart(?with the same dimension?). While the parallel grooves gave better spectral properties, they were sensitive to the polarisation of the light, whereas the two-dimensional, crossed pattern was not.

Russell then created prototypes of both by carefully etching the pattern into an off-the-shelf silicon-on-sapphire wafer – which needed a few repeats, as he initially underestimated the etching rate.

Sure enough, the finished prototype behaved as modelled, displaying a linear wavelength shift of 80 nanometers, across the standard operating temperatures from 25 Celsius to 420 Celsius. More extreme heating and cooling to cryogenic temperatures extended this to 140 nanometres– although these extremes are unlikely to be of practical use in the future.

With stable temperature tuning achieved, the team tested the spectroscopic capabilities of the filter on a number of everyday items, for example successfully measuring polymide tape and a zinc selenide window.

They were also pleased to be able to easily distinguish between two clear, recyclable plastics of different composition, LDPE and PET.

“Folks in environmental monitoring, agriculture, and industrial process control and safety already understand the utility of portable infrared spectroscopy tools. The proliferation of compact, inexpensive, and reliable spectrometers will be a boon, and our results bring us another step closer.”