Tiny tunable IR filter could shrink bulky thermal sensors onto chips

Australian researchers have developed a tiny, electrically tunable infrared filter that could help shrink bulky thermal sensing systems onto portable chips – potentially opening the door to handheld pollution detectors, compact multispectral cameras and next-generation chemical sensing devices.

The technology, developed by the Australian Research Council Centre of Excellence for Transformative Meta-Optical Systems researchers at the TMOS nodes at The University of Western Australia (UWA) and The Australian National University (ANU), works in the long-wave infrared region – the part of the spectrum associated with thermal radiation emitted by objects that are near room temperature.

Their results were published in Advanced Materials Technologies.

While conventional thermal cameras mainly measure heat intensity, the new device was designed to help infrared systems distinguish between different materials and gases based on their spectral fingerprints.

Lead author, TMOS PhD Student Oleg Bannik at the UWA node, said one useful way to think of the technology “is as colour vision for thermal imaging.”

“Instead of seeing only hot and cold, a camera could compare several carefully selected infrared bands, similar to how the human eye combines red, green and blue wavelengths to perceive colour,” said Bannik.

“That could allow systems to tell the difference between gases, chemicals or materials that look identical in ordinary thermal images.”

For decades, infrared spectroscopy was restricted to labs, military systems and expensive industrial equipment. These were machines with mirrors, lenses and moving parts – but were often too bulky and power-hungry to leave controlled environments.

Gold and silicon ‘sandwich’

The device itself is a microscopic ‘sandwich’ of suspended gold and silicon membranes perforated with nanoscale holes.

By electrically changing the tiny gap between the layers – across distances smaller than a micron – the researchers could continuously tune which infrared wavelengths passed through the structure.

The work relies on a phenomenon known as “extraordinary optical transmission”, in which light passes through tiny holes in metallic films far more efficiently than expected.

In the team’s device, nanoscale movements strongly alter the infrared response of the system.

“The most counterintuitive part is that changing a gap by only a few hundred nanometres can strongly tune infrared light with wavelengths around ten microns,” said Bannik.

“Tiny nanoscale motions end up controlling much larger infrared waves through near-field plasmonic interactions.”

In laboratory testing, the researchers tuned the transmission peak from around 8 µm to 9.8 µm in wavelength using voltages below 10 V. Simulations suggest the tuning range could eventually extend beyond the long-wave infrared region.

Unlike many existing infrared filtering systems, which rely on comparatively large moving optical components, the new approach operates using extremely small membrane motions and low power consumption.

“It is important to understand that, by itself, the device is only a tunable spectral filter, not a complete sensing system,” said Bannik. “However, when combined with a thermal detector, it can add entirely new capabilities to infrared cameras and sensing platforms.”

He says environmental monitoring is one of the strongest potential applications, particularly for detecting methane leaks and industrial emissions.

“The technology could also benefit industrial safety, thermal imaging, medical diagnostics and defence systems where identifying materials matters more than simply measuring temperature,” he added.

“The most realistic applications are non-contact diagnostics and advanced thermal imaging,” said Bannik.

These lightweight, low-power infrared sensors could also be especially useful in drones and portable field systems.

Further development

Still, significant engineering challenges remain before the technology leaves the lab, particularly around manufacturing reliability and contamination control at extremely small scales. “One of the hardest engineering challenges was maintaining extremely flat suspended membranes separated by gaps smaller than a micron,” said Bannik.

He added that the work points to a future where bulky infrared spectrometers could eventually be replaced by compact, chip-scale systems. “The idea that nanometer-scale motion can give machines a richer understanding of the thermal and chemical world is both scientifically fascinating and potentially very useful in real-world sensing systems.”