Tiny spectrometer could help smartphones do a number of new functions

Published in journal Nature Communications is a study wherein scientists have presented details of their invention that could pave way for smartphones to perform a lot more functions than they are capable of performing today.

Researchers at TU Eindhoven have managed to reduce the size of spectrometer substantially so much so that it can be fit inside a smartphone. The spectrometer could enable smartphones to perform functions such as check cleanliness of air, freshness of food even malignancy of a tumor.

The underlying technology is spectrometry that has a number of applications. Spectrometry is analysis of visible and invisible light. Scientists explain that every material on Earth including human tissues have their own ‘footprint’ in terms of light absorption and reflection and that’s what can be analysed through spectrometry.

While smaller spectrometers have been built in the past, all precise spectrometers are large and hence they can’t be used everywhere. Further, they split up the light into different colors (frequencies), which are then measured separately. Just after the light is split, the beams, which have different frequencies, still overlap each other; highly precise measurements can therefore only be made some tens of centimeters after the splitting.

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The Eindhoven researchers developed an ingenious sensor that is able to make such precise measurements in an entirely different way using a special ‘photonic crystal cavity’, a ‘trap’ of just a few micrometers into which the light falls and cannot escape. This trap is contained in a membrane, into which the captured light generates a tiny electrical current, and that is measured.

To be able to measure a larger frequency range, the researchers placed two of their membranes very closely one above the other. The two membranes influence each other: if the distance between them changes slightly, then the light frequency that the sensor is able to detect shifts too. Researchers incorporated a MEMS (a micro-electromechanical system). This electromechanical mechanism allows the distance between the membranes to be varied, and thereby the measured frequency. Ultimately, then, the sensor covers a wavelength range of around thirty nanometers, within which the spectrometer can discern some hundred thousand frequencies, which is exceptionally precise. This is made possible by the fact that the researchers are able to precisely determine the distance between the membranes to just a few tens femtometers (10^-15 meters).

To demonstrate the usefulness, the research team demonstrated several applications, including a gas sensor. They also made an extremely precise motion sensor by making clever use of the fact that the detected frequency changes whenever the two membranes move in relation to each other.