Metamaterial Can Change Optical Properties, Become Invisible or Reflective in IR Spectrum

Researchers at ITMO University and the University of Exeter have developed a metamaterial capable of changing its optical properties without any mechanical input. The new metamaterial could improve the reliability of complex optical devices while also making them cheaper to manufacture.

Thanks to their complex periodical structure, metamaterials are relatively independent from the properties of their components. Such structures can be volumetric or flat, as is the case with metasurfaces.

“Metasurfaces allow us to achieve many interesting effects in the manipulation of light,” ITMO researcher Ivan Sinev said. “But these metasurfaces have one issue: How they interact with light is decided right in the moment when we design their structure. When creating devices for practical use, we would like to be able to control these properties not only at the outset, but during use, as well.”

Ivan Sinev, a senior researcher at ITMO University’s Department of Physics and Engineering. Courtesy of ITMO.News (news.itmo.ru).

In their search for materials for adaptive optical devices, the researchers embedded the compound germanium antimony telluride (GeSbTe), often used in DVDs, as a thin layer between two layers of silicon. “It’s a sort of sandwich: First we coat a blank substrate with silicon, then put on a layer of phase-change material, and then some more silicon,” ITMO researcher Pavel Trofimov said.

Then, using e-beam lithography, the scientists converted the layered structure into a metasurface consisting of an array of microscopic disks. When they tested the ability of the metasurface to manipulate light, they found that the combination of the two materials into a complex periodic structure resulted in a surface transparency level that could be changed throughout the experiment. The reason for this fluidity, the researchers said, is that a silicon disk in the near-infrared region has two optical resonances, allowing it to strongly reflect infrared beams directed onto its surface. The layer of GeSbTe made it possible to “switch off” one of the two resonances, making the disk nearly transparent to light in the near-infrared region.

Phase-change materials have a crystalline state in which the material’s molecules are positioned in an ordered structure, and an amorphous state. If the layer of GeSbTe at the center of the metamaterial were in the crystalline state, the second resonance would disappear. If it were in the amorphous state, the disk would continue to reflect infrared beams.

By strategic placement of the ultrathin GSbTe layer and reversible switching of its phase-state, the researchers were able to show individual, multilevel, and dynamic control of metasurface resonances.

“To switch between the two metasurface states, we’ve used a sufficiently powerful pulse laser,” Trofimov said. “By focusing the laser on our disk, we’re able to perform the switch relatively quickly. A short laser pulse heats up the GeSbTe layer nearly to the melting point, after which it quickly cools down and becomes amorphous. If we subject it to a series of short pulses, it cools down more slowly, settling into a crystalline state.”

They demonstrated their concept through the design, fabrication, and characterization of metadevices capable of dynamically filtering and modulating light in the near infrared, with modulation depths as high as 70% and multilevel tunability. They showed numerically how their approach could be re-scaled to shorter wavelengths through appropriate material selection.

The properties of the new metasurface could be used in lidar devices that scan spaces by emitting infrared pulses and receiving the reflected beams, and in the production of ultrathin photographic lenses, such as those used in phone cameras. The concept of hybrid all-dielectric/phase-change metasurfaces could pave the way for a range of design possibilities in terms of multilevel, reconfigurable, high-efficiency light manipulation.

The research was published in Optica, a publication of The Optical Society (OSA) (www.doi.org/10.1364/OPTICA.384138).