Wildfires and threats in the sights - Microfocussing system set to transform infrared detectors

Researchers have developed a highly sensitive method for detecting hotspots in the environment, such as bushfires or military threats, by harnessing the focussing power of meta-optical systems.

The key to the approach is innovative lens technology thinner than a human hair, which can collect and process infrared radiation from fires and other heat sources with much improved efficiency. Crucially it does not need cryogenic cooling, unlike current sensors.

The result is sensor technology that promises to enhance devices in both the civilian and military spheres, said Dr Tuomas Haggren, lead researcher on the project.

“It’s elegant engineering with real-world payoff: a single layer that behaves like millions of tiny lenses, manufactured at scale,” said Dr Haggren, a research fellow at the Australian National University and the ARC Centre of Excellence for Transformative Meta‑Optical Systems (TMOS).

“It directly improves the cameras that communities rely on.”

The team propose that their technology could be deployed to provide constant surveillance for bushfires, by mounting the sensors onto telecom network towers. 

“Fire detection technologies are of national importance, and our solution addresses a critical gap in scalable, cost-effective bushfire detection,” said Dr Wenwu Pan, a research fellow at the University of Western Australia (UWA) and TMOS.

“The same advances enable compact, low-power sensors that provide 360-degree situational awareness on defence platforms.”

The sensors operate at wavelengths between 3 and 5 µm, known as the mid-wavelengths infrared (MWIR), which offer good visibility in both night and day, combined with good thermal contrast for identifying heat sources.

However, efforts to make MWIR cameras sharper have run into manufacturing and performance limits. Firstly, as pixels get smaller it’s harder to stop light spilling over between pixels and blurring the image. 

Secondly, there is a noise issue in trying to collect more light with larger detectors: the detectors function on the same principle as solar cells, with a PN junction converting the light to electrical signals. But PN junctions constantly generate low levels of signal, known as dark current: increasing the collection area generates proportionately more of this noise.

This dark current can be reduced with cryogenic cooling, but that is not a satisfactory solution for field or remote use, as it increases costs and reduces practicality significantly.

Instead the team came up with the idea that they could focus the light, so it could be collected by a smaller detector – reducing dark current. Better yet if an array of lenses were used – one for every pixel – then each pixel could be smaller, and separated from its neighbours, which would cut out spillover.

“The system brings together three key advances — mid-wave infrared sensing for round-the-clock, long-range detection; operation without cryogenic cooling for low power and high reliability; and real-time data for faster response,” said Associate Professor Gilberto Umana-Membreno, from UWA and TMOS. 

But how to create the thousands of tiny lenses needed for this idea? The answer is by using a metasurface, a surface covered in an array of nanoscopic shapes smaller than the wavelength of light, that can produce remarkable effects not possible with natural materials.

“These flat metalenses allow us to bring photolithographic, wafer-scale optics directly into the detector stack – a practical way to boost performance,” Associate Professor Gilberto Umana-Membreno said.

The team used electromagnetic modelling to design a flat metasurface that concentrates mid-infrared light onto each detector pixel, improving sensitivity and reducing noise. The design is detailed in the Journal of Electronic Materials. 

Various nano-pillar designs were tested through simulation to optimise light focusing efficiency and showed great promise for increasing accuracy and reducing losses, Dr Wenwu said.

“By patterning a flat single-layer film we concentrate more light where it’s needed.”

The new design promises widespread impact – beyond heat detection, infrared sensors are used for remote sensing, night vision, environmental monitoring, national security, defence, meteorology, astronomy, spectroscopy and medical imaging.

As well as simple focussing, metalenses can be designed to carry out advanced optical processing, by separating and manipulating different components of light based on polarisation, phase or wavelength.

“The project is highly eligible for a range of grants, and would support scalable roll out.” Associate Professor Gilberto Umana-Membreno said.

“There are significant commercial opportunities.”