Device can be integrated with drones, smartphones and other equipment to detect chemical compounds and monitor greenhouse gases
One of the most used research instruments to identify and analyze chemical substances, Fourier transform infrared spectrometers (FTIR) are bulky, which makes them impossible to use in the field to detect compounds.
In order to reduce the size of the equipment, several attempts have been made to develop miniaturized FTIR spectrometers, which can be integrated into drones, cell phones and other devices, in order to enable the monitoring of greenhouse gases remotely, for example. The manufacturing cost of these miniaturized FTIR spectrometers, however, is still high, which makes their use on a large scale unfeasible.
To overcome these limitations, a group of researchers from Unicamp's Device Research Laboratory (LPD), in collaboration with colleagues from the University of California in San Diego, in the United States, developed an FTIR spectrometer based on silicon photonics – the same technology used today to manufacture computer chips, smartphones and other electronic devices.
Result of a search for doctorate and from one research internship abroad, carried out by student Mário César Mendes Machado de Souza, with scholarships from FAPESP and under the guidance of professor Newton Frateschi, the new spectrometer was described in an article published in the magazine Nature Communications..
“Silicon photonics technology offers a platform for manufacturing high-performance, low-cost miniaturized spectrometers,” Souza, author of the project and first author of the article, told FAPESP Agency.
According to Souza, FTIR spectrometers allow the identification of chemical compounds by irradiating a beam of infrared light on a sample and, subsequently, measuring the amount of light and wavelengths absorbed. The absorption pattern (spectrum) provides information about the chemical composition of the sample.
In recent years, several projects have begun to emerge aimed at developing this instrument based on integrated photonics technology, which uses light especially in the infrared spectrum. The attempts had not progressed until then due to several technical challenges, explained Souza.
One of them is that silicon waveguides are highly dispersive, that is, each wavelength travels at a different speed in this material and, therefore, they have different refractive indices (speed).
In order to tune the refractive index of silicon optical waveguides, the thermo-optical effect has been used, which consists of passing a current over the waveguide in order to heat it. As the device needs to be operated at high temperatures to achieve high resolution, this technique becomes non-linear, that is, a change in temperature also changes the refractive index in a non-proportional way.
“In practice, what happens when applying the thermo-optical effect to a silicon-based infrared spectrometer with integrated photonics is that when performing mathematical operations, called Fourier transforms, to convert the collected data into the radiation spectrum , the result is completely wrong”, summarized Souza.
The researchers were able to overcome these challenges by creating a laser calibration method that allows them to quantify and correct distortions caused by the dispersion and nonlinearity of silicon waveguides. As a proof of concept, they developed a 1 square millimeter (mm2) FTIR spectrometer chip based on standard silicon photonics manufacturing procedures.
The chip was tested on a laboratory bench and produced a broadband test spectrum with a spectral resolution of 0,38 terahertz (THz), which is comparable to the resolution of commercial portable spectrometers that operate in the same range today. wavelength, the researchers say.
“We developed a device that is not remotely optimized and, even so, already achieves resolutions comparable to those of commercial portable spectrometers that exist today, based on free space optics”, compared Souza.
The researchers' goal now is to implement a device that is fully functional and integrated with photodetectors, light sources and optical fibers.
“The idea is that both the light source and the spectrometer detector are integrated into the same platform,” said Souza.
THE ARTICLEThe article Fourier transform spectrometer on silicon with thermo-optic non-linearity and dispersion correction (doi: 10.1038/s41467-018-03004-6), by Mario CMM Souza, Andrew Grieco, Newton C. Frateschi and Yeshaiahu Fainman, can be read in the magazine Nature Communications. em www.nature.com/articles/s41467-018-03004-6.
