In an advance for smartphone spectroscopy, researchers at ETH Zurich have developed a chip about 2 cm2 in size that can be used with a smartphone to analyze infrared (IR) light in the same way a conventional spectrometer would. The researchers demonstrated an integrated single-waveguide Fourier transform spectrometer with an operational bandwidth of 500 nm in the near- and short-IR wavelengths. The prototype device, with a footprint of less than 10 mm2, exploits the electro-optic properties of thin-film lithium niobate in order to retrieve a complete spatial interferogram. Approximately 2 cm in length, this chip makes it possible to precisely analyze the infrared spectrum. Courtesy of ETH Zurich/Pascal A. Halder. The structure of the miniature device is based on the same principles as conventional spectrometers. However, instead of analyzing the incident light with the help of moveable mirrors, the mini-spectrometer makes use of special waveguides with an optical refractive index that can be adjusted externally via an electric field. “Varying the refractive index has an effect similar to what happens when we move the mirrors, so this setup lets us disperse the spectrum of the incident light in the same way,” researcher David Pohl said. The ETH researchers used thin-film lithium niobate, a material that is used as a modulator in the telecommunications industry, to build their spectrometer. As a waveguide, thin-film lithium niobate confines the light to the inside of the device. Since measurements can only be taken if some of the guided light can get out, the researchers compensated for this characteristic of the material by attaching metal structures to the waveguides that scatter the light to the outside of the device. Depending on how the waveguide is configured, the researchers can examine different parts of the light spectrum. “In theory, our spectrometer lets you measure not only infrared light, but also visible light, provided the waveguide is properly configured,” researcher Marc Reig Escalé said. The “spectrometer on a chip” has to be calibrated only once, compared to conventional devices that require calibration repeatedly. The miniature spectrometer does not rely on any moving parts, which minimizes its maintenance requirements. There is still some technological progress to be made before the mini-spectrometer can actually be integrated into a mobile or other electronic device. “At the moment we’re measuring the signal with an external camera, so if we want to have a compact device, we have to integrate this as well,” professor Rachel Grange said. Originally the research team was targeting not chemical analysis, but astronomy, for the application of its device. Because Earth’s atmosphere absorbs a high amount of IR light, a compact, lightweight, stable IR spectrometer that could be launched and stationed in space relatively inexpensively could provide valuable information about distant celestial objects. The research was published in Nature Photonics (https://doi.org/10.1038/s41566-019-0529-9).