To create a compact, flexible sensor for agriculture and other industries, an international engineering team combined the tunability of phase change materials with reconfigurable image-processing metasurfaces. The resulting device provides image processing functionality that can be dynamically reconfigured by a temperature change of just a few degrees.

According to the team, comprising researchers from City University of New York (CUNY), the University of Melbourne, the Royal Melbourne Institute of Technology (RMIT), and the ARC Centre of Excellence for Transformative Meta-Optical Systems (TMOS), the use of computational metasurfaces for reconfigurable image processing could replace traditional optical lens applications for sensing.

The reconfigurable sensor can rapidly switch between high-contrast edge detection and detailed IR imaging, without creating large volumes of data or using bulky external processors.

The researchers achieved reconfigurability by leveraging the insulator-to-metal phase transition of a thin layer of vanadium dioxide (VO₂), a material that undergoes an abrupt structural change when the temperature it is exposed to exceeds a certain threshold. The device design, which is based on adding a thin layer of VO₂ within a thicker metasurface, enables a large change in the optical properties of the metasurface, while simultaneously minimizing absorption losses when the VO₂ is in the insulating phase. The metasurface’s response is altered by the device’s filter, which is made with VO₂.

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The flat-optics sensor system can rapidly switch between edge detection for imaging the outline of an object, such as a fruit, and extracting detailed IR information. Courtesy of Lincoln Clark/TMOS.

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“Materials such as vanadium dioxide add a fantastic tuning capability to render devices ‘smart’,” said Madhu Bhaskaran, a professor at RMIT. “When the temperature of the filter is changed, the vanadium dioxide transforms from an insulating state to a metallic one, which is how the processed image shifts from a filtered outline to an unfiltered infrared image.”

The sensor prototype developed by Bhaskaran and her team at RMIT is one of the few metasurface-based devices that can be reconfigured.

“While a few recent demonstrations have achieved analog edge detection using metasurfaces, most of the devices demonstrated so far are static,” said CUNY researcher Michele Cotrufo. “Their functionality is fixed in time and cannot be dynamically altered or controlled. Yet, the ability to dynamically reconfigure processing operations is key for metasurfaces to be able to compete with digital image processing systems.”

The reconfigurability of the metasurface is reinforced by the nearly optimal performance metrics of the device. Besides strong, controllable tunability, the device provides throughput efficiency, full isotropy, a relatively large numerical aperture of about 0.26, and a response that is almost polarization-independent. These features are combined with a filter design that is compatible with large-scale manufacturing.

“It also operates at temperatures compatible with standard manufacturing techniques, making it well-placed to integrate with commercially available systems, and therefore move from research to real-world usage rapidly,” researcher Shaban Sulejman said.

According to Ann Roberts, professor at the University of Melbourne and chief investigator at TMOS, flat optics technologies are poised to have transformative impact on numerous industries. “Traditional optical elements have long been the bottleneck preventing the further miniaturization of devices,” she said. “The ability to replace or complement traditional optical elements with thin-film optics breaks through that bottleneck.”

The compact, lightweight sensor could find use in augmented reality, remote sensing, and bio-medical imaging applications. For example, the sensor could be fitted to a drone for remote crop monitoring, enabling farmers to pinpoint which crops require additional resources like irrigation, fertilization, or pest control, thereby improving yields.

The use of reconfigurable image processing metasurfaces could be extended to devices capable of performing other temperature-controlled operations, including bandpass filtering, convolution, direction sensing, polarization imaging, and quantitative phase microscopy.

RMIT holds a granted U.S. patent and has a pending Australian patent application for its method of producing VO₂ films, which may be suitable for a broad range of applications.

The research was published in Nature Communications (www.doi.org/10.1038/s41467-024-48783-3).

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