Fourier-transform Infrared (FTIR) spectroscopy is a basic tool for chemistry. Infrared radiation excites the bonds between atoms in organic molecules, making them vibrate at a frequency which is characteristic to the type of bond. The main advantage of FTIR is that it uses a burst of multifrequency infrared radiation, rather than slowly scanning across frequencies. Processors incorporated into the spectrometer analyse the waveform transmitted through the sample, using a mathematical technique called a Fourier transform to derive the frequencies which were absorbed.
Schematic representation of the FTIR chip. (left); with microscope view of the actual device (right)
Despite this being a very established technology, FTIR spectrometers are large instruments, usually confined to laboratories. Researchers from the University of Campinas’ device research laboratory in Brazil, collaborating with colleagues at the University of California San Diego, have now developed an FTIR spectrometer based on silicon photonics, a technique which uses the optical properties of silicon, whose components can be made using the same processes used to mass manufacture electronic components.
Resulting from the PhD project of Mario César Mendez Machado de Souza, the device uses silicon waveguides, a basic type of components in silicon photonics. The main problem that Souza had to overcome was that the refractive index of silicon, which determines how fast different wavelengths of light travel in it, is a thermo-optical effect; that is, it is temperature dependent. To achieve high resolution, the temperature needs to be high, and this induces non-linear changes in refractive index, making it hard to control.
"In practice, what happens when a thermo-optical effect is applied to a silicon-based infrared spectrometer with integrated photonics is that the Fourier transform mathematical operations used to convert the radiation spectrum data collected produce completely wrong results," Souza explained.
To overcome this, Sousa and his colleagues developed a laser calibration method to quantify and correct the distortions caused by nonlinearity. In a paper in Nature Communications, they explain how they developed an FTIR chip based on standard silicon photonic fabrication procedures measuring only 1 mm² in area.
In laboratory tests, the chip produced a broadband spectrum with a resolution of 0.38THz, comparable with the resolution of the few commercially available portable spectrometers. "The device we developed is far from optimised but still achieves resolutions comparable with those of the portable free-space optics-based spectrometers available in the market today," Souza said.
Sousa and his team are now attempting to integrate a light source and photodetectors into the same platform as the FTIR chip, which is compatible with standard, low-cost optical fibre technology. Such a device would be suitable for use in the field, where it could detect and analyse atmospheric pollutants as well as greenhouse gases, mounted, for example, on a drone.
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