Researchers develop tiny infrared spectrometer

Scientists in Switzerland have developed an infrared spectrometer small enough to fit on a computer chip. The technology could allow smart phones to perform chemical analysis.

Today's infrared spectrometers weigh several pounds. They are often bulky and difficult to integrate with mobile devices.

The compact infrared spectrometer developed by researchers at ETH Zurich -- described this week in the journal Nature Photonics -- promises to change that.

The new chip, measuring two centimeters by two centimeters, analyzes infrared light the same way a conventional spectrometer does. Spectrometers break infrared light into different wavelengths, creating a digitized frequency spectrum -- a signature that can be interpreted by a computer. Because different chemicals produce different spectral signatures, a spectrometer reading can be used to identify the substance from which the light passed through or bounced off of.

Conventional spectrometers split light waves into two separate beams. The beams recombine after reflecting off of two mirrors. The recombined light is then measured by a photodetector.

One of the two mirrors can be moved to create an interference pattern, which allows the photodetector to measure the proportion of different wavelengths in light and produce a digitized frequency spectrum.

The new technology forgoes the mirrors. Instead, the spectrometer uses "an optical refractive index," which can be externally augmented using an electric field. No moving parts are necessary.

"Varying the refractive index has an effect similar to what happens when we move the mirrors, so this set-up lets us disperse the spectrum of the incident light in the same way," ETH researcher David Pohl said in a news release.

Scientists suggest the technology could be tweaked to measure other portions 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," said researcher Marc Reig EscaleÌ.

Because it contains no moving parts, the device doesn't need to be constantly re-calibrated, making it easier and cheaper to use.

For the spectrometer chip, scientists used a material, thin-film lithium niobate, that is often used as a modulator in telecommunications. But because the material's waveguide properties trap light on the inside, researchers had to augment the film with tiny metal structures designed to scatter the light and let some of it escape. The light's spectral qualities can only be measured if some of the light can get out.

While the new technology is small enough to fit in a mobile phone, scientists have yet to integrate the chip with an 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," said lead researcher Rachel Grange, a professor of optical nanomaterials at ETH Zurich.

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