This is a cross-sectional scanning electron microscopy image of the surface of the metamaterial, showing a 750nm-period grating fabricated by focused ion beam milling in a 300nm-thick amorphous GST film. Image: Karvounis/Gholipour/MacDonald/Zheludev, Optoelectronics Research Centre, University of 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. Metamaterials are also useful for creating 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) to make an improved switchable metasurface that can block or transmit particular wavelengths of light, all under the command of light pulses. The researchers describe the metamaterial in a paper in Applied Physics Letters, and also explain 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 in the UK. "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, scientists can modulate, select or switch various characteristics of light beams, including intensity, phase, color and direction -- an ability that's essential to many existing and potential devices, MacDonald said.
Switchable metamaterials in general aren't new. MacDonald and many others have made such materials 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 metal component in 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. What is more, gold is often used as the metal component, but gold isn't compatible with the CMOS (complementary metal–oxide–semiconductor) technology that's ubiquitous in today's integrated devices.
In this new work, MacDonald and his colleagues at Southampton University's Optoelectronics Research Centre & Centre for Photonic Metamaterials have managed to create a switchable metamaterial that doesn't contain any metal. "What we've done now is structure the phase-change material itself," MacDonald explained. "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 from random and amorphous to regular and crystalline. 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 forms the basis for data storage.
The researchers created metamaterial grating patterns in an amorphous GST film only 300nm thick, with lines 750–950nm apart. This line spacing allows the metamaterial surface to selectively block the transmission of light at near-infrared (NIR) wavelengths (between 1300nm and 1600nm). But shining a green laser on the surface of the GST film converts it into a crystalline state, which is transparent to these NIR wavelengths.
The research team is now working to make metamaterials that can switch back and forth over many cycles. They're also planning to fabricate increasingly complex structures that can deliver more sophisticated optical functions. For example, this approach could be used to make switchable ultra-thin metasurface lenses and other flat, optical components.
This story is adapted from material from the American Institute of Physics, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.