Switching Between Transparent and Reflective

Modern optical devices require constant tuning of their light inter­action settings. For that purpose, there exist various mechanical appara­tuses that shift lenses, rotate reflectors, and move emitters. An inter­national research team that includes staff members of ITMO University and the University of Exeter have proposed a new meta­material capable of changing its optical pro­perties without any mechanical input. This development could result in a signi­ficant improvement in the reliability of complex optical devices while making them cheaper to manu­facture.

ITMO-researcher Ivan Sinev developed with colleagues a material capable of being invisible or reflective. (Source: ITMO)

Metamaterials open up incredible oppor­tunities. Thanks to their complex periodical structure, they are relatively inde­pendent from the properties of their components. Such structures can be volumetric or flat – in the latter case, they are referred to as metasurfaces. “Metas­urfaces allow us to achieve many interesting effects in the mani­pulation of light,” says Ivan Sinev, a senior researcher at ITMO University’s Depart­ment of Physics and Engineering. “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.”

In their search for materials for adaptive optical devices, the ITMO-researchers have joined forces with their colleagues from the University of Exeter in the UK, who have a lot of experience in working with phase-change materials. Among such materials is, for instance, the germanium antimony telluride (GeSbTe) compound, often used in DVDs. “We’ve made calcu­lations to see what this new composite material would look like,” says Pavel Trofimov, PhD student at the Depart­ment of Physics and Engineering. “We have an inclusion of GeSbTe embedded 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.”

Then, using the methods of e-beam litho­graphy, the scientists converted the layered structure into a metasurface: an array of microscopic disks that was then tested at the labora­tory on the subject of its ability to mani­pulate light. As the researchers expected, the combination of two materials into a complex periodic structure resulted in an important effect: the resulting surface’s transparency level could be changed throughout the experiment. The reason is that a silicon disk in the near-infrared region has two optical resonances, allowing it to strongly reflect IR beams directed onto its surface. The layer of GeSbTe has 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 two states: a crystal­line state in which its molecules are positioned in an ordered structure, and an amorphous state. If the layer of GeSbTe at the center of the meta­material is in the crystal­line state, the second resonance will disappear; if it is in the amorphous state, the disk will continue to reflect IR beams. “To switch between the two metasurface states, we’ve used a sufficiently powerful pulse laser,” explains Pavel Trofimov. “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 properties of this new metasurface can be used for various appli­cations. That includes, first and foremost, the creation of lidars. The principle of their creation can also serve as the basis in the production of special ultra-thin photo­graphic lenses, such as ones used in phone cameras. (Source: ITMO)

Reference: C. Ruiz de Gallareta et al.: Reconfigurable multilevel control of hybrid all-dielectric phase-change metasurfaces, Optica 7, 476 (2020); DOI: 10.1364/OPTICA.384138

Link: College of Engineering Mathematics and Physical Sciences, University of Exeter, Exeter, UK

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