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 the metamaterial this week in Applied Physics Letters, from AIP Publishing, and 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 U.K. “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 — an ability that’s essential to many existing and potential devices, 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 at Southampton’s Optoelectronics Research Centre & Centre for Photonic Metamaterials 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 metamaterials 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.
Distance Wireless Charging Without Being Close to the Charging Base Enhanced by Magnetic Metamaterials
Universitat Autònoma de Barcelona researchers have developed a system which efficiently transfers electrical energy between two separate circuits. The system, made with a shell of metamaterials which concentrates the magnetic field, could transmit energy efficiently enough to charge mobile devices without having to place them close to the charging base.
The research was published in the journal Advanced Materials.
Wireless charging of mobile devices is possibly one of the most desired technological milestones. Some devices can already be charged wirelessly by placing the mobile device on top of a charging base. The next step, charging devices without the need of taking them out of one’s pocket, might be just around the corner.
A group of researchers from the Department of Physics of Universitat Autònoma de Barcelona has developed a system which can efficiently transfer electrical energy between two separated circuits thanks to the use of metamaterials. This system is still in the experimental stage, but once it has been perfected and can be applied to mobile devices, it will be able to charge them wirelessly and at a longer distance than currently possible.
Today’s wireless devices make use of induction to charge through a special case adapted to the device and a charging base connected to an electrical socket. When the device is placed on top of the base, this generates a magnetic field which induces an electric current inside the case and, without the need of using any cables, the device is charged. If the device is separated from the base, the energy is not transferred efficiently enough and the battery cannot be charged.
The system created by UAB researchers overcomes these limitations. It is made up of metamaterials which combine layers of ferromagnetic materials, such as iron compounds, and conductor materials such as copper. The metamaterials envelop the emitter and receiver and enable transferring energy between the two, at a distance and with unprecedented efficiency.
With the use of metamaterial crowns researchers were able in the lab to increase the transmission efficiency 35-fold, “and there is much more room for improvement, since theoretically the efficiency can be increased even more if conditions and the design of the experiment are perfected” explains Àlvar Sánchez, director of the research.
“Enveloping the two circuits with metamaterial shells has the same effect as bringing them close together; it’s as if the space between them literally disappears”, states Jordi Prat, lead author of the paper.
Moreover, the materials needed to construct these crowns such as copper and ferrite are easily available. The first experiments conducted with the aim of concentrating static magnetic fields required the use of superconductor metamaterials, unfeasible for everyday uses with mobile devices. “In contrast, low frequency electromagnetic waves – the ones used to transfer energy from one circuit to the other – only need conventional conductors and ferromagnets”, Carles Navau explains.
A group of UK researchers has discovered a new type of optical activity by breaking the symmetry of metamaterials with reflected light
Optical activity–rotation of the polarization of light–is well known to occur within materials that differ from their mirror image. But what happens if this symmetry is broken by the direction of illumination rather than the material itself?
Curiosity about this question has led to the discovery of a new type of optical activity. As a group of University of Southampton researchers report in Applied Physics Letters, from AIP Publishing, breaking the symmetry of metamaterials with reflected light will enable novel applications because it causes optical activity of unprecedented magnitude–far exceeding previously known specular or “mirror-like” optical activity.
At the heart of the group’s work are metamaterials–materials constructed with unique shapes and symmetries that generate properties which don’t occur in their natural counterparts.