Artist's rendering of light interacting with BTS crystals. Image: Talia Spencer.
Artist's rendering of light interacting with BTS crystals. Image: Talia Spencer.

One of the leading challenges for autonomous vehicles is ensuring they can detect and sense objects – even through dense fog. Infrared cameras are much better at this than visible light cameras, able to offer improved visibility through the fog, smoke or tiny particles that can scatter visible light.

In air, infrared light – within a specific range called mid-wave infrared – scatters much less than visible and other infrared light waves. Infrared cameras can also see more effectively in the dark, when there is no visible light.

Currently, however, the deployment of infrared cameras is limited by their heavy cost and scarcity of effective materials. This is where materials that possess unique optical properties in the infrared and are scalable might make a difference in providing better object identification for several technologies, including autonomous vehicles.

Such a material has now been developed by scientists at the University of Southern California (USC)’s Viterbi School of Engineering and the University of Wisconsin, together with researchers from Air Force Research Laboratories, the University of Missouri and J.A. Woollam Co. Inc. The researchers describe the material in a paper in Nature Photonics.

The research group of Jayakanth Ravichandran, an assistant professor of materials sciences at the USC Viterbi School of Engineering, has been studying a new class of materials called chalcogenide perovskites. Among these materials is barium titanium sulfide (BTS; BaTiS3), a material rediscovered and prepared in large crystal form by Shanyuan Niu, a doctoral candidate in the Materials Science program at the USC Mork Family Department of Chemical Engineering and Materials Science.

Ravichandran's research group collaborated with the research groups of Mikhail Kats, an assistant professor of electrical and computer engineering at the University of Wisconsin-Madison, and Han Wang, an assistant professor of electrical engineering and electrophysics in USC's Ming Hsieh Department of Electrical Engineering, to study how infrared light interacts with this material. The researchers discovered that the material interacts in different ways with infrared light coming from two different directions.

"This is a significant breakthrough, which can affect many infrared applications," says Ravichandran.

This direction-dependent interaction with light is characterized by an optical property called birefringence. In simple terms, birefringence can be viewed as light moving at different speeds in two different directions through a material. In much the same way that sunglasses with polarized lenses block glare, BTS has the ability to block or slow down light depending on the direction in which it travels in the material. The researchers maintain that BTS has the highest birefringence among known crystals.

"The birefringence is larger than that of any known solid material, and it has low losses across the important long-wave infrared spectrum," says Kats.

The BTS material could be used to construct a sensor that filters out infrared light of certain polarizations to achieve better image contrast. It could also help filter light coming from different directions to allow sensing of a remote object's features. This could be particularly important for improving infrared vision in autonomous vehicles, which need to see the entire landscape around them, even in low visibility conditions.

"The hope is that in the future, a BTS-enhanced sensor in a car would function as retinas do to the human body," says Niu.

The researchers believe these infrared-responsive materials can also extend human perception. One possible application could be in the creation of imaging tools used by firefighters to generate an instant temperature map outside a burning building to assess where a fire is spreading and where emergency responders need to rescue trapped individuals.

At present, infrared equipment is too expensive for many fire stations. BTS, which is made of elements that are readily abundant in the earth’s crust, could make infrared equipment more affordable and effective. BTS is also safer for the user and the environment, as well as easier to dispose of, than the materials currently used in infrared equipment, which contain hazardous elements such as mercury and cadmium.

BTS could also be useful in devices that sense harmful molecules, gases and even biological systems. Other applications range from heat sensing to pollution monitoring to medicine.

"To date, the constraint of existing mid-infrared materials is a big bottleneck to translate many of these technologies," says USC's Wang. The researchers hope that intense research in this area will make several of these technologies a reality in the near future.

This story is adapted from material from the University of Southern California, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.