The tunable “metasurface” resembles an optical Swiss army knife

Engineers and colleagues at MIT report significant new advancements on a tunable metasurface, or flat optical device patterned with nanoscale structures, which they liken to a Swiss Army Knife while its passive predecessor can be seen as a single tool, like a flat blade screwdriver. The key to the work is a transparent material discovered by the team that rapidly and reversibly changes its atomic structure in response to heat.

“The applications opened up by the ability to quickly reconfigure metasurfaces are enormous,” said Yifei Zhang, lead author of an article reporting on the latest advances in a recent issue of Nature Nanotechnology. Zhang is a graduate student of the Department of Materials Science and Engineering (DMSE). “We are delighted that the current work overcomes several hurdles to implement these metasurfaces in real world applications. ”

According to Associate Professor Arka Majumdar of the University of Washington, Seattle, of these applications: “I am considering [that] this technology could revolutionize optical neural networks, depth detection and Lidar technology for self-driving cars. Majumdar was not involved in the search.

Electric switch

In the Nature Nanotechnology paper, MIT researchers describe the use of electrical currents to reversibly alter the material structure – and therefore optical properties – of the new metasurface. In the past, they used bulky lasers or an oven to provide the necessary heat. “This is important because we can now integrate the entire active optical device, as well as the electrical switch, on a silicon chip to form a miniaturized optical platform”, explains Juejun Hu, head of the work and associate professor of materials science and engineering. in DMSE.

The team also reports the demonstration “of a series of tunable optical functions using the platform,” Hu said. It is in particular a device of orientation of the beam where “by switching the material on different [internal] structures, we can send light in one direction relative to another, back and forth. Beam steering is key to self-driving cars, although Hu points out that the device he and his colleagues demonstrated is still fairly rudimentary. “It’s more a proof of principle. ”

Besides Zhang and Hu, the authors of the new article are Junhao Liang, Bilal Azhar, Mikhail Y. Shalaginov, Skylar Deckoff-Jones, Carlos Rios and Tian Gu, all from MIT DMSE; Clayton Fowler, Sensong An and Hualiang Zhang of the University of Massachusetts, Lowell; Jeffrey B. Chou, Christopher M. Roberts and Vladimir Liberman of MIT Lincoln Laboratory; Myungkoo Kang and Kathleen A. Richardson from the University of Central Florida, and Clara Rivero-Baleine from Lockheed Martin Corporation. Hu and Gu are also affiliated with MIT’s Materials Research Lab.

A new material

Phase change materials (PCM) change their structure in response to heat. They are used commercially in rewritable CDs and DVDs. Hu explains, “A laser beam changes the structure of the material locally, from amorphous to crystalline, and this change can be used to encode ones and zeros, digital information.”

However, conventional PCMs have limitations when it comes to optical applications. On the one hand, they are opaque. They won’t let the light pass. “This motivated us to research a new transparent phase change material for optical devices,” said Hu. Earlier this year, his team reported that adding another element, selenium, to a conventional PCM did the trick.

The new material, composed of germanium, selenium, antimony and tellurium (GSST), is the key to the new metasurface. The metasurface, in turn, is not just a thin film of GSST, it is a film of GSST of about half a square millimeter patterned with some 100,000 nanoscale structures. And these, in turn, “allow you to control the propagation of light. So you can turn a collection of these nanostructures into, say, a lens,” says Hu.

Harish Bhaskaran is a professor at the University of Oxford who was not involved in the research. He commented on the work as a whole and the advances reported in the new document:

“This is a very important area of ​​work because such tunable metasurfaces, that is, surfaces that can modulate the reflection of light even if they are nominally ‘flat’ or very thin, are extremely interesting. They can drastically reduce the volume of lenses, which of course are used in anything that manipulates light. [MIT’s] the use of low loss phase change materials (i.e. they absorb very little light) is a real way to make this a reality. The authors are also among the first to demonstrate dynamic tuning using electrically controlled radiators. “(In the same issue of Nature Nanotechnology a Stanford team also reports controlling metasurfaces with electric heating using a different approach.)

According to a News & Views article in the same issue of Nature Nanotechnology Referring to advances at MIT and Stanford, “this work makes a breakthrough in tunable PCM-based metasurfaces.” However, the authors of News & Views point out that both approaches have drawbacks.

The Hu team tackles some of these drawbacks. For example, the radiator used in their miniaturized optics platform is currently made of metal. But “metals are problematic for optics because they absorb light,” says Hu. “We are working on a new transparent silicon radiator.”

Hu describes the work as a whole as particularly exciting as it began with the discovery of a new material which the team then designed for a new application. “It ranges from material innovation to device integration, which in my opinion is quite unique. ”

The work was supported by the US Agency for Defense Advanced Research Projects and the US Air Force. The researchers also acknowledge the use of facilities provided by the MIT Materials Research Laboratory, MIT Microsystems Technology Laboratories, and the Harvard University Center for Nanoscale Systems.