Optical gas sensor comprised of metamaterials promises benefits for IoT

Metamaterial optical gas sensor
Researchers have developed a tiny non-dispersive infrared (NDIR) gas sensor, comprised of metamaterials, that has no moving parts and consumes little energy. (Alexander Lochbaum, ETH Zurich)

Researchers have developed the first fully-integrated, non-dispersive infrared (NDIR) gas sensor comprised of engineered synthetic materials known as metamaterials. The sensor reportedly has no moving parts, requires little energy to operate, and is among the smallest NDIR sensors ever created.

The sensor is being targeted for IoT applications and smart home devices to detect and respond to environmental changes. Other potential applications include future medical diagnostics and monitoring equipment.

“Our sensor design unites simplicity, robustness, and efficiency. Using metamaterials, we can omit one of the main cost drivers in NDIR gas sensors, the dielectric filter, and simultaneously reduce the size and energy consumption of the device,” said Alexander Lochbaum from the Institute of Electromagnetic Fields of ETH Zurich, Switzerland, and lead author on the paper, in a statement. “This makes the sensors viable for high-volume, low-cost markets such as automotive and consumer electronics.”

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NDIR sensors are typically used to assess vehicle exhaust, measure air quality, detect gas leaks and support various medical, industrial and research applications. Conventional NDIR sensors work by shining infrared light through air in a chamber until it reaches a detector. An optical filter positioned in front of the detector eliminates all light, except the wavelength that is absorbed by a particular gas molecule so that the amount of light entering the detector indicates the concentration of that gas in the air.

In recent years, engineers have replaced the conventional infrared light source and detector with microelectromechanical systems (MEMS) technology. In the new sensor, researchers integrate metamaterials onto a MEMS platform to further miniaturize the NDIR sensor and dramatically enhance the optical path length.

Key to the design is a type of metamaterial known as a metamaterial perfect absorber (MPA) made from a complex layered arrangement of copper and aluminum oxide. Because of its structure, MPA can absorb light coming from any angle. Researchers designed a multi-reflective cell that “folds” the infrared light by reflecting it many times over. This design allowed a light absorption path about 50 millimeters long to be squeezed into a space measuring only 5.7 × 5.7 × 4.5 mm.

Whereas conventional NDIR sensors require light to pass through a chamber a few centimeters long to detect gas at very low concentrations, the new design optimizes light reflection to achieve the same sensitivity level in a cavity that is just over half a centimeter long.

The use of metamaterials simplifies the sensor’s design. The main parts are a metamaterial thermal emitter, an absorption cell, and a metamaterial thermopile detector. A microcontroller periodically heats up the hotplate, causing the metamaterial thermal emitter to generate infrared light that travels through the absorption cell and is detected by the thermopile. The microcontroller then collects the electronic signal from the thermopile and streams the data to a computer.

The researchers tested the device’s sensitivity by using it to measure varying concentrations of carbon dioxide in a controlled atmosphere. They demonstrated it can detect carbon dioxide concentrations with a noise-limited resolution of 23.3 parts per million, a level on par with commercially available systems. The sensor required only 58.6 millijoules of energy per measurement, reportedly about a five-fold reduction compared to commercially available low-power thermal NDIR carbon dioxide sensors.

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