Detecting light beyond the red range visible to our eyes is extremely challenging because infrared light carries very little energy compared to the ambient heat at room temperature. This obscures infrared light unless specialized detectors are cooled to extremely low temperatures, which is both expensive and energy-intensive. Now, researchers led by the University of Cambridge have demonstrated a new concept for detecting infrared light, showing how it can be converted into visible light that is easy to detect. Working with colleagues from the UK, Spain, and Belgium, the team used a single layer of molecules to absorb mid-infrared light within their vibrating chemical bonds. These shaking molecules can transfer their energy to visible light they encounter, "upconverting" it into emissions closer to the blue end of the spectrum, which can then be detected by modern visible-light cameras. Detecting light beyond the red range visible to our eyes is difficult because infrared light carries very little energy compared to the ambient heat at room temperature. This masks infrared light unless dedicated detectors are cooled to very low temperatures, a process that is both costly and energy-consuming. Now, researchers led by the University of Cambridge have presented a new concept for detecting infrared light, demonstrating how to convert it into visible light that is easy to detect. (Image: NanoPhotonics Cambridge/Ermanno Miele, Jeremy Baumberg) The results, published in the journal *Science* ("Mid-infrared light detection via molecular frequency upconversion with dual-wavelength hybrid nanoantennas"), open up new low-cost methods for sensing pollutants, tracking cancer, examining gas mixtures, and remotely sensing the outer universe. A key challenge for the researchers was ensuring that the vibrating molecules encounter visible light quickly enough. "This means we have to trap light tightly around the molecules, squeezing it into gaps surrounded by gold," said Angelos Xomalis of Cambridge's Cavendish Laboratory, the first author of the study. The researchers devised a way to sandwich a single layer of molecules between a mirror and small pieces of gold, made possible only by using "metamaterials"—substances that can twist and squeeze light into volumes a billion times smaller than a human hair. "Trapping these different colors of light together is tricky, but we wanted to find a way that wasn't expensive and could easily be scaled up to make practical devices," said co-author Dr. Rohit Chikkaraddy, also from the Cavendish Laboratory, who designed the experiments based on his simulations of light in these building blocks. Professor Jeremy Baumberg from the NanoPhotonics Centre at Cambridge's Cavendish Laboratory said, "It's like listening to slow-rippling earthquake waves by crashing them into violin strings to get an easy-to-hear high-pitched whistle—without breaking the violin." The researchers emphasize that while it is still early days, there are many ways to optimize the performance of these inexpensive molecular detectors, which could then capture a wealth of information in this spectral window. Many technologies, from astronomical observations of galactic structures to sensing human hormones or early signs of aggressive cancers, could benefit from this new detector advancement. The research was conducted by a team from the University of Cambridge, KU Leuven, University College London (UCL), the Faraday Institution, and the Polytechnic University of Valencia.