First All-Metamaterial Optical Gas Sensor

Researchers have developed the first fully-inte­grated, non-dispersive infrared (NDIR) gas sensor enabled by specially engineered synthetic materials known as meta­materials. The sensor has no moving parts, requires little energy to operate and is among the smallest NDIR sensors ever created. The sensor is ideal for new Internet of Things and smart home devices designed to detect and respond to changes in the environ­ment. It also could find use in future medical diag­nostics and monitoring equipment.

The miniature all-metamaterial optical gas sensor (golden capsule) next to a one-cent coin. (Source: A. Lochbaum, ETHZ)

“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. “This makes the sensors viable for high-volume, low-cost markets such as automotive and consumer elec­tronics.”

NDIR sensors are among the commer­cially most relevant types of optical gas sensors, used to assess vehicle exhaust, measure air quality, detect gas leaks and support a variety of medical, industrial and research appli­cations. The new sensor’s small size, poten­tially low cost, and reduced energy requirements open new oppor­tunities for these and other types of 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. Though most NDIR sensors measure carbon dioxide, different optical filters can be used to measure a wide range of other gases.

In recent years, engineers have replaced the conven­tional infrared light source and detector with microelectro­mechanical systems (MEMS) tech­nology, miniscule components that bridge between mechanical and electrical signals. In the new work, 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. To take advantage of this, the 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 millimeters.

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 accomplish the same level of sensi­tivity in a cavity that is just over half a centimeter long. By using metamaterials for efficient filtering and absorption, the new design is both simpler and more robust than existing sensor designs. Its 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. The light 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 primary energy requirement comes from the power needed to heat the thermal emitter. Thanks to the high efficiency of the metamaterial used in the thermal emitter, the system works at much lower temperatures than previous designs, so less energy is needed for each measurement. 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. However, to do this the sensor required only 58.6 millijoules of energy per measurement, about a five-fold reduction compared to commer­cially available low-power thermal NDIR carbon dioxide sensors.

“For the first time, we realize an inte­grated NDIR sensor that relies exclu­sively on meta­materials for spectral filtering. Applying meta­material tech­nology for NDIR gas sensing allows us to rethink the optical design of our sensor radically, leading to a more compact and robust device,” said Lochbaum. (Source: OSA)

Reference: A. Lochbaum et al.: Ultra-Compact All-Metamaterial NDIR CO2 Sensor, Frontiers in Optics/Laser Science 2019, Washington, USA

Link: Institute of Electromagnetic Fields (IEF), ETH Zurich, Zurich, Switzerland

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