An ultracompact, mid-infrared spectrometer that uses a 10-nm thick, single tunable black phosphorus photodetector shows promise as an on-chip device for sensing, surveillance, and spectral imaging. The device is the result of a collaboration among scientists from Yale University, Bar-Ilan University (Israel), and the National Institute of Materials Science (Japan). The spectrometer has an active area footprint of only 9 × 16 µm2. It measures light in the 2- to 9-µm spectral range and was constructed at the scale of the operational wavelength. The light-matter interactions in its black phosphorus photodetector can be tuned to capture the different spectral components. According to the researchers, most miniaturized spectrometers require large arrays of photodetection elements to capture the different spectral components of incident light and reconstruct the spectrum. “This spectrometer shows an advantage over conventional light-splitting spectrometers because the light doesn’t need to be split into different parts spatially,” Shaofan Yuan, a researcher at Yale University, said. Conventional spectrometers split the light into wavelengths. An optical micrograph of two typical on-chip spectrometers. Scale bar: 8 µm. Courtesy of Yale University. The device leverages an algorithm that provides it with a distinct spectral learning process. Using the wavelength scale of the spectrometer and a bias-dependent responsivity matrix learned from the spectra of a tunable blackbody source, the researchers reconstructed unknown spectra from the corresponding photoresponse vectors. The ultracompact, single-detector spectrometer has the potential to reconstruct the spectra of both monochromatic and broadband light, and it does not need to be paired with bulky interferometers, gratings, or infrared lasers. It is also electrically reconfigurable. These characteristics could make the ultracompact, single-detector spectrometer a highly suitable and affordable choice for use in on-chip mid-infrared spectroscopy. Its ability to continuously measure spectral information (though with moderate resolution) could make it potentially more effective at detecting potential hazards to automobiles, drones, and satellites than infrared cameras. The ultracompact spectrometer could also be used in remote sensing. “It is very exciting to realize such a high-performance spectrometer with the ultimate compactness,” said Doron Naveh, a professor at Bar-Ilan University. “We expect that the principle of leveraging advances in hardware and software simultaneously as shown in this work will lead to commercial applications in medicine, agriculture, and food quality control.” The research was published in Nature Photonics (www.doi.org/10.1038/s41566-021-00787-x).