Development of a nanophotonic fiber optic sensor to detect ultrafine dust

ENGINEERINGNET.BE – Inhaling ultrafine particles (UFPs) can pose serious health risks.

To prevent this, accurate ways to detect UFPs and thus monitor air quality are needed. Preferably through small, compact, accurate and inexpensive devices.

To this end, TU Eindhoven developed a nanophotonic sensor that is sensitive to minuscule changes in the environment around the sensor. So much so that it can detect a single nanoparticle. The sensor includes a semiconductor chip, which is placed on the tip of the fiber optic.

The sensor work is based on a photonic crystal, a periodic structure that can reflect light in all directions. “An irregularity has been added to this crystal: a ‘photonic crystal cavity’ (PhCC),” says PhD student Arthur Hendriks from TU Eindhoven.

This ensures that light can remain trapped in the crystal for a longer period of time. Hendriks: “Light is thus confined in a volume that is smaller than 1 µm3. This is known as the mode volume, and it must be very small to measure tiny nanoparticles.”

The researchers were able to place the PhCC on the tip of the fiber using a method previously developed by TU Eindhoven. When a small particle comes close to the PhCC in the crystal, it disrupts the cavity by changing the refractive index. “So the small particle changes the wavelength of the cavity, and we measure this change.”

“Our design offers unprecedented sensitivity compared to previous technologies,” says Hendriks. “Using the sensor, we were able to detect individual UFPs with diameters as small as 50 nanometers in real time.”

The next step for Hendriks and his colleagues is to hang the cavities, so that the quality factor and coupling efficiency become even higher. This could result in nanophotonic cavities with first-class properties, which are still readable via the fiber optic.

“Our approach could be used to detect even smaller particles. Or in other applications such as single-photon emitters and nano-optomechanical sensors,” says Hendriks. “Even the detection of single biological molecules may be possible.”

The next step for the UFP sensor is the European project LEARN, which focuses on monitoring and evaluating air quality in schools. TU Eindhoven plans to continue working on the sensor and testing it as part of this project.