Mobile phones can do almost anything. With these devices, it might even be possible to ascertain a beer’s alcohol content or how ripe a piece of fruit is. The infrared spectrometers used today for chemical analyses generally weigh several kilograms and are difficult to integrate into a handheld device.
A chip about 2 square centimeters in size was developed that can be used to analyze infrared light in the same way as a conventional spectrometer, which 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 that 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 new mini-spectrometer; however, in the new device, the incident light is no longer analyzed with the help of movable 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 the mirrors are moved, enabling dispersion of 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 contrast to other integrated spectrometers that can cover only a narrow range of the light spectrum, the new device can easily analyze a broad section of the spectrum. The spectrometer has to be calibrated only once, compared to conventional devices that need repeated recalibration and because it contains no moving parts, it requires less maintenance.
Researchers employed a material that is also used as a modulator in the telecommunications industry. This material has many positive properties but as a waveguide, it confines the light to the inside. This is less than ideal, as a measurement is possible only if some of the guided light can get out. For this reason, the researchers attached delicate metal structures to the waveguides that scatter the light to the outside of the device.