Researchers Develop World’s First All-Metamaterial Optical Gas Sensor

The world’s first fully integrated, non-dispersive infrared (NDIR) gas sensor has been recently developed by researchers. The sensor is enabled by metamaterials, which are exclusively engineered synthetic materials.

The sensor requires only minimal energy to operate and does not contain any moving parts. It is one of the smallest NDIR sensors ever developed.

The NDIR sensor is suitable for smart home devices and new Internet of Things designed to identify and react to environmental changes. In addition, it can be used in upcoming medical monitoring and diagnostics instruments.

A paper describing the study findings will be presented at the Frontiers in Optics + Laser Science (FIO + LS) conference, which will be conducted in Washington, D.C., United States, from September 15^th to 19^th, 2019.

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

Alexander Lochbaum, Study Lead Author, Institute of Electromagnetic Fields, ETH Zurich, Switzerland

Part of the commercially most relevant kinds of optical gas sensors, NDIR sensors are used for detecting gas leaks, measuring air quality, assessing vehicle exhaust, and supporting a wide range of research, industrial, and medical applications. Reduced energy requirements, compact size, and potentially low cost of the new sensor pave the way for these as well as other kinds of applications.

Shrinking the Optical Pathway

Traditional NDIR sensors operate by first illuminating infrared light via air in a chamber until it arrives at a detector. In front of the detector, an optical filter is placed that removes all the light except the wavelength that gets absorbed by a specific gas molecule. This way, the concentration of that particular gas present in the air is indicated by the amount of light penetrating the detector.

Although a majority of the NDIR sensors can determine the concentration of carbon dioxide (CO₂), different kinds of optical filters can also be used for measuring a broad range of other gases.

In the recent past, the traditional infrared light source and detector have been replaced with microelectromechanical systems (MEMS) technology, which are tiny components bridging between electrical and mechanical signals. In the latest study, the scientists embedded metamaterials onto a MEMS platform to reduce the size of the NDIR sensor even further and significantly improve the optical path length.

Integral to the design is a form of metamaterial called a metamaterial perfect absorber (MPA) developed from an intricate layered arrangement of aluminum oxide and copper. MPA has a unique structure that allows it to absorb light coming from all angles. To manipulate this effect, the scientists developed a special multi-reflective cell.

This cell is able to “fold” the infrared light by reflecting it several times over. The new design made it possible to squeeze a light absorption path of around 50 mm long within a space measuring just 5.7 mm× 5.7 mm × 4.5 mm.

While traditional NDIR sensors need light to travel through a chamber measuring a few centimeters long to identify gas at relatively low concentrations, the latest design improves light reflection to achieve the same degree of sensitivity in a cavity measuring just more than half a centimeter long.

A Simple, Robust, and Low-Cost Sensor

By utilizing metamaterials for efficient absorption and filtering, the novel design is not only simpler but also stronger when compared to current sensor designs. Its core components include a metamaterial thermopile detector, an absorption cell, and a metamaterial thermal emitter.

The hotplate is periodically heated up by a microcontroller, causing the metamaterial thermal emitter to produce infrared light. The thermopile detects the light moving through the absorption cell, and the microcontroller subsequently gathers the electronic signal from the thermopile and transfers the information to a computer.

The main energy requirements come from the power required to heat the thermal emitter. The high efficiency of the metamaterial utilized in the thermal emitter allows the system to operate at relatively lower temperatures than the earlier designs. Therefore, each measurement requires less energy.

To test the device’s sensitivity, the scientists used it to determine different concentrations of CO₂ in a controlled atmosphere. They showed that the device can detect the concentrations of CO₂ with a noise-restricted resolution of 23.3 parts per million—a level equivalent to other systems available in the market.

But to do this, the sensor needed just 58.6 mJ of energy for each measurement, approximately five times reduction when compared to low-power thermal NDIR carbon dioxide sensors available in the market.

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.

Alexander Lochbaum, Study Lead Author, Institute of Electromagnetic Fields, ETH Zurich, Switzerland