Engineers at the University of Wisconsin-Madison have developed a compact, single-shot, free-space-coupled spectrometer with hyperspectral imaging capabilities that can be integrated with a cellphone. The spectrometer is fabricated on top of and integrated with a CMOS chip. The miniaturized device measures 200 μm on each side and can lie directly on a sensor from a typical digital camera. The team’s spectrometer and hyperspectral imager are based on photonic-crystal (PC) slabs. The device uses arrays of photodetectors with different responsivities, which are created by optical interference in PC slabs positioned immediately on top of the photodetector pixels. The PC slabs enable a nonscanning, single-shot imaging method on a compact chip. Compact and low-cost devices like this miniaturized spectrometer could help turn ordinary cellphones into advanced analytical tools. Courtesy of Zongfu Yu. The researchers said that the PC slabs could be applied to any wavelength range by scaling the dimension of the PC. The PC slabs are compact, with sizes similar to light-sensing pixels in CMOS image sensors. The team demonstrated its spectrometer’s ability to perform hyperspectral imaging by using it to capture two distinct images of the numbers 5 and 9 from a snapshot of an overlaid projection that combined the pair of numbers into something indistinguishable to the naked eye. The researchers hope to boost the device’s spectral resolution as well as the clarity of the images it captures. For example, they could potentially improve spectral resolution by replacing silicon PC with silicon nitride or silicon carbide, which could create more diverse spectral features due to the lack of light absorption. Spatial resolution and angle tolerance could be improved by using even smaller PC slabs. These improvements could further enhance the sensing capabilities in this compact device. The research was published in Nature Communications (https://doi.org/10.1038/s41467-019-08994-5).