Scientists Invent 'Invisible' Metamaterial With Bonus Reflect Mode
An international research team has proposed the creation of a new metamaterial with adjustable optical properties — without the need for mechanical (or manual) input — which may improve optical device reliability while also lowering manufacturing costs, according to a cover article published in the journal Optica.
'Invisible' metamaterial on the horizon
Accelerated development in physics and materials science throughout recent decades has brought society a broad spectrum of available materials. But now the ones designing complex devices are less constrained by the limits of traditional materials like metals, glass, wood, or minerals. This is where metamaterials — studied at ITMO University and elsewhere — open up novel opportunities for new uses.
Consisting of complex periodical structures, metamaterials are relatively independent from the properties of their constitutive components, and can have volumetric or flat structures. The latter are called metasurfaces.
"Metasurfaces allow us to achieve many interesting effects in the manipulation of light," said Senior Researcher at ITMO University's Department of Physics and Engineering Ivan Sinev, to Optics. "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."
Forging new 'metasurfaces'
While seeking materials appropriate for adaptive optical devices, ITMO University researchers use their sizable experience working with silicon metasurfaces to join forces with their colleagues at the University of Exeter in the United Kingdom — who are also very experienced in working with phase-change materials. One such material is the germanium antimony telluride (GeSbTe) compound found in DVDs.
"We've made calculations to see what this new composite material would look like," said Pavel Trofimov, a doctoral student in the physics and engineering departments, to Optica. "We have an inclusion of GeSbTe embedded as a thing 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."
After this scientists use a method of e-beam lithography to convert the newly-layered structure into a metasurface. Finally, it became an array of microscopic disks to undergo testing in the laboratory — to see how well it manipulates light.
Phase-change: invisible order, chaotic reflection
Just as the researchers expected, the combination of two materials into a complex periodic structure created an important function: the newly-fashioned surface's transparency level can be altered throughout the experiment. This is because of a silicon disk in the near-infrared region with two optical resonances — which lets it strongly reflect infrared beams pointed at its surface. The layer of GeSbTe gives scientists the ability to "turn off" one of the two resonances, which makes the disk almost transparent to light in the near-infrared part of the spectrum.
Materials capable of phase-change have two states: one is crystalline, and involves molecules arranged in an ordered structure, while the other is an amorphous state. When the GeSbTe layer in the center of the metamaterial is arranged in a crystalline state, the second resonance disappears — when it's amorphous, the disk will reflect IR beams.
"To switch between the two metasurface states, we've used a sufficiently powerful pulse laser," said Pavel Trofimov to Optics. "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."
The new metasurface's properties open the door to a variety of applications. Most interesting is the creation of lidars — devices that scan a space via the emitting of infrared pulses and then receiving reflected beams. The principle of this metamaterial's creation might also advance the production of special ultra-thin photographic lenses, like the ones we all use in smartphone cameras.
As materials science continues to pile up breakthroughs, the expanding world of cutting-edge electronics — from consumer-grade smartphones to advanced military hardware — might be taking its first steps into a major industrial revolution.