Researchers have developed a technology that uses meta-optical devices to perform thermal imaging. The approach captures richer information about imaged objects, including spectral and polarization details, which could broaden the use of thermal imaging in fields such as autonomous navigation, security, thermography, medical imaging, and remote sensing. According to research team leader Zubin Jacob, the method is able to overcome limitations of traditional spectral imagers, which are often bulky and fragile due to their reliance on large filter wheels or interferometers. “We combined meta-optical devices and cutting-edge computational imaging algorithms to create a system that is both compact and robust while also having a large field of view,” he said. An artist’s interpretation of the spinning metasurface devices. The devices captured spectral and polarization details of thermal radiation as well as the intensity information that is acquired with traditional thermal imaging. Courtesy of Xueji Wang, Purdue University. The new spectro-polarimetric decomposition system uses a stack of spinning metasurfaces to break down thermal light into its spectral and polarimetric components. This allows the imaging system to capture the spectral and polarization details of thermal radiation, as well as the same intensity information acquired by traditional thermal imaging. The team showed that the optical system can be used with a commercial thermal camera to successfully classify various materials, a task that is typically challenging for conventional thermal cameras. “Traditional autonomous navigation approaches rely heavily on RGB cameras, which struggle in challenging conditions like low light or bad weather,” said first author Xueji Wang, a postdoctoral researcher at Purdue University. “When integrated with heat-assisted detection and ranging technology, our spectro-polarimetric thermal camera can provide vital information in these difficult scenarios, offering clearer images than RGB or conventional thermal cameras. Once we achieve real-time video capture, the technology could significantly enhance scene perception and overall safety.” Spectro-polarimetric imaging in the longwave-infrared is crucial for applications such as night vision, machine vision, trace gas sensing, and thermography. However, today’s spectro-polarimetric longwave-infrared imagers are bulky and limited in spectral resolution and field of view. To overcome these limitations, the researchers turned to large-area metasurfaces — ultrathin structured surfaces that can manipulate light in complex ways. After engineering spinning dispersive metasurfaces with tailored infrared responses, they developed a fabrication process that allowed these metasurfaces to be used to create large-area (2.5-cm diameter) spinning devices suitable for imaging applications. The resulting spinning stack measures “Integrating these large-area meta-optical devices with computational imaging algorithms facilitated the efficient reconstruction of the thermal radiation spectrum,” Wang said. “This enabled a more compact, robust, and effective spectro-polarimetric thermal imaging system than was previously achievable.” A commercial thermal camera being used in conjunction with the researcher’s spectro-polarimetric decomposition system, which uses a stack of spinning metasurfaces to break down thermal light into its spectral and polarimetric components. Courtesy of Xueji Wang, Purdue University. To evaluate their new system, the researchers spelled out “Purdue” using various materials and microstructures, each with unique spectro-polarimetric properties. Using the spectro-polarimetric information acquired with the system, they accurately distinguished the different materials and objects. They also demonstrated a three-fold increase in material classification accuracy compared to traditional thermal imaging methods, highlighting the system's effectiveness and versatility. According to the researchers, the method’s ability to distinguish temperature variations and identify materials based on spectro-polarimetric signatures could help boost safety and efficiency for a variety of applications, including autonomous navigation. “In security, for example, it could revolutionize airport systems by detecting concealed items or substances on people,” Wang said. “Moreover, its compact and robust design enhances its suitability for diverse environmental conditions, making it particularly beneficial for applications such as autonomous navigation.” In addition to working to achieve video capture with the system, the researchers seek to enhance the technique’s spectral resolution, transmission efficiency, and speed of image capture and processing. They also plan to improve the metasurface design to enable more complex light manipulation for higher spectral resolution. Additionally, they want to extend the method to room-temperature imaging since the use of metasurface stacks restricted the method to high-temperature objects. They plan to do this using improved materials, metasurface designs, and techniques such as antireflection coatings. The research was published in Optica (www.doi.org/10.1364/OPTICA.506813).
