In the group headed by Rachel Grange, Professor of Optical Nanomaterials in the Department of Physics, David Pohl and Marc Reig Escalé collaborated with other colleagues to develop a chip about two square centimetres in size. With it, they can analyse infrared light in the same way as they would with a much larger and bulkier spectrometer.
A conventional spectrometer splits the incident light into two paths before reflecting it off two mirrors. The reflected light beams are 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 create characteristic gaps in the infrared spectrum, scientists can use the resulting patterns to identify what substances occur in the test sample and in what concentration.
This same principle is behind the mini-spectrometer developed by the ETH researchers. However, in their device, the incident light is no longer analysed with the help of moveable mirrors; instead, it 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."
Depending on how the waveguide is configured, 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," Escalé says.
In contrast to other integrated spectrometers that can cover only a narrow range of the light spectrum, the device developed by Grange's group has a major advantage in that it can easily analyse a broad section of the spectrum.