Light Changes Properties of Metamaterials

Metasurface: Scanning electron microscopy images of a 750 nm period grating fabricated by focused ion beam milling in a 300 nm thick amorphous germanium antimony telluride film on silica (Source: A. Karvounis et al., U. Southampton)

Metasurface: Scanning electron microscopy images of a 750 nm period grating fabricated by focused ion beam milling in a 300 nm thick amorphous germanium antimony telluride film on silica (Source: A. Karvounis et al., U. Southampton)

Invisi­bility cloaks have less to do with magic than with meta­materials. These human-engineered mate­rials have properties that don’t occur in nature, allowing them to bend and mani­pulate light in weird ways. For example, some of these materials can channel light around an object so that it appears invisible at a certain wavelength. These materials are also useful in applic­ations such as smaller, faster, and more energy efficient optics, sensors, light sources, light detectors and tele­communi­cations devices.

Now researchers have designed a new kind of meta­material whose properties can be changed with a flick of a switch. In their proof-of-principle experiment, the researchers used germanium antimony telluride (GST), the kind of phase-change material found in CDs and DVDs, to make an improved switchable meta­surface that can block or transmit particular wave­lengths of light at the command of light pulses. The researchers describe how its ability to switch properties can be used in a range of sophis­ticated optical devices.

“Techno­logies based upon the control and mani­pulation of light are all around us and of fundamental importance to modern society,” said Kevin MacDonald, a researcher at the Univer­sity of Southampton. “Meta­materials are part of the process of finding new ways to use light and do new things with it. They are an enabling technol­ogy platform for 21st century optics.” By dynamically controlling the optical properties of materials, you can modulate, select, or switch charac­teristics of light beams, such as intensity, phase, color and direction, he said.

Switchable meta­materials in general aren’t new. MacDonald and many others have made such materials before by combining metallic meta­materials with so-called active media such as GST, which can respond to external stimuli like heat, light or an electric field. In these hybrid materials, the metal component is structurally engineered at the nano­meter scale to provide the desired optical properties. Incor­porating the active medium provides a way to tune or switch those properties.

The problem is that metals tend to absorb light at visible and infrared wave­lengths, making them unsuitable for many optical device appli­cations. Melting points are also suppressed in nano­structured metals, making the metam­aterials susceptible to damage from laser beams. In addition, a typical metal is gold, which isn’t compatible with the CMOS technology that’s ubiquitous in making today’s integrated devices. In the new work, MacDonald and his colleagues have made a switchable meta­material that doesn’t use metal at all. “What we’ve done now is structure the phase-change material itself,” MacDonald said. “We have created what is known as an all-dielectric meta­material, with the added benefit of GST’s non­volatile phase-switching behavior.”

Pulses of laser light can change the structure of GST between a random, amorphous one and a crystal­line one. For GST, this behavior is non­volatile, which means it will stay in a particular state until another pulse switches it back. In rewritable CDs and DVDs, this binary laser-driven switching is the basis for data storage. The researchers created meta­material grating patterns in an amorphous GST film only 300 nm thick, with lines 750 to 950 nanometers apart. This line spacing allows the surfaces to selectively block the trans­mission of light at near-infrared wave­lengths between 1300 and 1600 nm. But when a green laser converts the surfaces into a crystal­line state, they become transparent at these wave­lengths.

The research team is now working to make meta­aterials that can switch back and forth over many cycles. They’re also planning increa­singly complex structures to deliver more sophis­ticated optical functions. For example, this approach could be used to make switchable ultra-thin meta­surface lenses and other flat, optical components. (Source: AIP)

Reference: A Karvounis et al.: All-dielectric phase-change reconfigurable metasurface, Appl. Phys. Lett. , online 2 August 2016, DOI: 10.1063/1.4959272

Link: Centre for Photonic Metamaterials, University of Southampton, United Kingdom

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