New chip could let smartphones conduct infrared spectroscopy

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  • Published: Oct 9, 2019
  • Source: ETH Zurich
  • Channels: Infrared Spectroscopy
thumbnail image: New chip could let smartphones conduct infrared spectroscopy

Infrared spectroscopy chip

The infrared spectroscopy chip developed by researchers at ETH Zurich.

Photo: ETH Zurich/Pascal A. Halder.

Nowadays, a smartphone can do almost anything: take photos or video, send messages, determine its present location and, of course, transmit telephone conversations. To these various abilities could soon be added conducting chemical analyses by infrared spectroscopy.

At first glance, the idea of using smartphones for chemical analyses seems a daring one. After all, infrared spectrometers generally weigh several kilograms and are difficult to integrate into a handheld device. But now, as they report in a paper in Nature Photonics, researchers at ETH Zurich in Switzerland have taken an important step towards turning this vision into reality. David Pohl and Marc Reig Escalé, part of a group headed by Rachel Grange, professor of optical nanomaterials, collaborated with other colleagues to develop a chip about 2cm2 in size that can analyze infrared light.

A conventional infrared spectrometer splits incident light into two paths before reflecting it off two mirrors; the reflected light beams are then recombined and measured with a photodetector. Moving one of the mirrors creates an interference pattern, which can be used to determine the proportion of different wavelengths in the incoming signal. Because chemical substances absorb characteristic wavelengths of infrared light, scientists can use the resulting patterns to identify what substances are present in a test sample and in what concentration.

This same principle is behind the mini-spectrometer developed by the ETH researchers. However, in their chip, the incident light is no longer analyzed with the help of moveable mirrors; instead, the chip 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," Pohl explains, "so this set-up lets us disperse the spectrum of the incident light in the same way."

Adjusting the the waveguide allows the researchers to 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," Escalé says.

Alongside its compact size, the chip offers two other advantages. It only has to be calibrated once, in contrast to conventional devices that need recalibration over and over again; and because it contains no moving parts, it requires less maintenance.

For the waveguide, the ETH researchers employed a material that is also used as a modulator in the telecommunications industry. Although this material has many benefits, it confines the light to the inside, which is less than ideal, as a measurement is possible only if some of the guided light can get out. For this reason, the scientists attached delicate metal structures to the waveguide that scatter the light to the outside of the device. "It required a lot of work in the clean room until we could structure the material the way we wanted," Grange explains.

Before the chip can actually be integrated into a smartphone or other electronic device, some technological progress still needs to be made. "At the moment we're measuring the signal with an external camera," Grange says, "so if we want to have a compact device, we have to integrate this as well."

Originally the physicists were aiming not at chemical analyses, but at a completely different application. In astronomy, infrared spectrometers provide valuable information about distant celestial objects. Because the Earth's atmosphere absorbs a high amount of infrared light, it would be ideal to station these instruments on satellites or telescopes in space. A compact, lightweight and stable measurement device that can be launched into space relatively inexpensively would naturally offer a substantial benefit.

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