Imagine a cell phone charger that recharges your phone remotely without even knowing where it is; a device that targets and destroys tumors, wherever they are in the body; or a security field that can disable electronics, even a listening device hiding in a prosthetic toe, without knowing where it is.
While these applications remain only dreams, researchers at the University of Maryland have come up with a sci-fi seeming technology that one day could make them real. Using a “time-reversal” technique, the team has discovered how to transmit power, sound or images to a “nonlinear object” without knowing the object’s exact location or affecting objects around it. “That’s the magic of time reversal,” says Steven Anlage, a university physics professor involved in the project. “When you reverse the waveform’s direction in space and time, it follows the same path it took coming out and finds its way exactly back to the source.”
The time-reversal process is less like living the last five minutes over and more like playing a record backwards, explains Matthew Frazier, a postdoctoral research fellow in the university’s physics department. When a signal travels through the air, its waveforms scatter before an antenna picks it up. Recording the received signal and transmitting it backwards reverses the scatter and sends it back as a focused beam in space and time. “If you go toward a secure building, they won’t let you take cell phones,” Frazier says, so instead of checking everyone, they could detect the cell phone and send a lot of energy to it to jam it.”
What differentiates this research from other time-reversal projects, such as underwater communication, is that it focuses on nonlinear objects such as a cellphone, diode or even a rusty piece of metal – when a waveform bounces off them, the frequency changes. Most components electrical engineers work with are linear—capacitors, wire, antennas—because they do not change the frequency. With nonlinear objects, however, when the altered, nonlinear frequency is recorded, time-reversed and retransmitted, it creates a private communication channel because other objects cannot “understand” the signal. “Time reversal has been around for 10 to 20 years but it requires some pretty sophisticated technology to make it work,” Anlage says. “Technology is now catching up to where we are able to use it in some new and interesting ways.”
Not only could this nonlinear characteristic secure a wireless communication line, it could prevent transmitted energy from affecting any object but its target. For example, Frazier says, if scientists find a way to tag tumors with chemicals or nanoparticles that react to microwaves in a nonlinear way, doctors could use the technology to direct destructive heat to the errant cells—much like ultrasound is used to break down kidney stones. But unlike an ultrasound, that is directed to a specific location, doctors would not need to know where the tumors were and the heat treatment would not affect surrounding cells.
To study the phenomenon, the researchers sent a microwave pulse into an enclosed area where waveforms scattered and bounced around inside, as well as off a nonlinear and a linear port. A transceiver then recorded and time-reversed the frequencies the nonlinear port had altered and broadcast them back into the space. The nonlinear port picked up the time-reversed signal but the linear port did not. “Everything we have done has been in very controlled conditions in labs,” Frazier says. “It will take more research to figure out how to develop treatments,” Frazier says. “I’m sure there are other uses we haven’t thought of.”
ALICE & BOB GO NONLINEAR
by David Voss
Nonlinear devices added to linear systems often yield new, useful phenomena, an example being frequency-doubling crystals that combine two laser photons to create a photon of twice the energy. In a paper in Physical Review Letters, Matthew Frazier and colleagues at the University of Maryland, College Park, report experiments in which they put a nonlinear frequency-multiplying device into a chaotic bath of electromagnetic waves and find signal propagation effects that might launch a new kind of secure communication.
The equations that describe electromagnetic waves are linear and time invariant, which means that signals propagating forward in time can be recorded, played backwards (i.e., time reversed), and sent back along the incoming path, returning exactly to their source. Frazier et al. built a metal box with ports to couple microwave radiation in and out. Two of the ports are equipped with conventional linear antennas, but the third is an antenna incorporating a nonlinear element (in this case a diode). A scattering device in the box creates a chaotic electromagnetic environment to mask the signals and ensure complex signal paths.
A signal sent into one linear port of the chamber will bounce around and eventually hit the nonlinear antenna, which responds by producing signals at new frequencies. These new frequencies are then recorded at the other linear port, and then played backwards into the box, whereupon they reverse their propagation and return precisely focused onto the nonlinear element, regardless of how complex the path is. Among the applications envisaged by the authors is a communications network in which messages broadcast by Alice in a wide area are picked up by Bob with a nonlinear receiver at a secret location (not even known to Alice). Only that location will receive the reversed playback; Eve the eavesdropper will only detect garbled signals from the chaotic wave environment.
Rather than hiding objects from view, it hides events
by Ian Sample / 13 July 2011
The latest device, which has been shown to work for the first time by Moti Fridman and Alexander Gaeta at Cornell University, goes beyond the more familiar invisibility cloak, which aims to hide objects from view, by making entire events invisible.
Fridman declined to discuss the cloak, details of which were posted on the arxiv database on Tuesday, because the paper has been submitted to Nature, which has strict rules about what can and cannot be said before an article is published. There is enough in the paper to draw out the basic principle though. The first generation of invisibility cloaks remain a work in progress. They fool the eye by bending light around an object, much as water flows around a pebble in a stream. So far as an observer is concerned, the object simply isn’t there. That, at least, is the idea. So far, few invisibility cloaks work with visible light, and those that do hide only small objects, such as paperclips, in polarised light.
The next generation of cloaks demonstrated by Fridman’s group work in a different way. Instead of bending light around an object, they create a blindspot in time, during which an event can happen without being noticed. The theoretical prospect of a “space-time” cloak – or “history editor” – was raised by Martin McCall and Paul Kinsler at Imperial College in a paper published earlier this year. The physicists explained that when light passes through a material, such as a lens, the light waves slow down. But it is possible to make a lens that splits the light in two, so that half – say the shorter wavelengths – speed up, while the other half, the longer wavelengths, slow down. This opens a gap in the light in which an event can be hidden, because half the light arrives before it has happened, and the other half arrives after the event.
Writing in July in a special issue of Physics World devoted to invisibility, McCall and Kinsler describe the ultimate bank heist, where a robbery takes place under the watchful gaze of CCTV cameras that completely miss the crime because it is hidden by a space-time cloak. Switch the cloak on, and half the light scattering off the bank vault into the CCTV camera arrives before the break-in begins, while the second half arrives after the robber has tidied up and fled. The camera sees nothing but an unchanging scene. Fridman’s demonstration is not quite so dramatic. He used one set of lenses to prise open a gap in a beam of light, by slowing down long wavelengths, such as red, and speeding up short wavelengths, such as blue. With a second set of lenses, he then closed the gap, so at the end of the experiment, the light beam looked exactly as it did at the start.
Fridman’s cloak is not about to aid the perfect crime. The longest event it could hide would last only around 1.25 microseconds. A test described in the paper hid an event – some interference caused by another light beam – that was even faster. It is worth remembering these are early days for invisibility cloaks. The first rudimentary device came out of Duke University only five years ago.
A team led by scientists at Duke University’s Pratt School of Engineering has demonstrated the first working “invisibility cloak.” The cloak deflects microwave beams so they flow around a “hidden” object inside with little distortion, making it appear almost as if nothing were there at all. Cloaks that render objects essentially invisible to microwaves could have a variety of wireless communications or radar applications, according to the researchers. The team reported its findings on Thursday, Oct. 19, in Science Express, the advance online publication of the journal Science. The research was funded by the Intelligence Community Postdoctoral Fellowship Program.
The researchers manufactured the cloak using “metamaterials” precisely arranged in a series of concentric circles that confer specific electromagnetic properties. Metamaterials are artificial composites that can be made to interact with electromagnetic waves in ways that natural materials cannot reproduce. The cloak represents “one of the most elaborate metamaterial structures yet designed and produced,” the scientists said. It also represents the most comprehensive approach to invisibility yet realized, with the potential to hide objects of any size or material property, they added.
Earlier scientific approaches to achieving “invisibility” often relied on limiting the reflection of electromagnetic waves. In other schemes, scientists attempted to create cloaks with electromagnetic properties that, in effect, cancel those of the object meant to be hidden. In the latter case, a given cloak would be suitable for hiding only objects with very specific properties. “By incorporating complex material properties, our cloak allows a concealed volume, plus the cloak, to appear to have properties similar to free space when viewed externally,” said David R. Smith, Augustine Scholar and professor of electrical and computer engineering at Duke. “The cloak reduces both an object’s reflection and its shadow, either of which would enable its detection.”
The team produced the cloak according to electromagnetic specifications determined by a new design theory proposed by Sir John Pendry of Imperial College London, in collaboration with the Duke scientists. The scientists reported that theoretical work in Science earlier this year. The principles behind the cloaking design, though mathematically rigorous, can be applied in a relatively straightforward way using metamaterials, said cloak designer David Schurig, a research associate in Duke’s electrical and computer engineering department. “One first imagines a distortion in space similar to what would occur when pushing a pointed object through a piece of cloth, distorting, but not breaking, any threads,” Schurig said. “In such a space, light or other electromagnetic waves would be confined to the warped ‘threads’ and therefore could not interact with, or ‘see,’ objects placed inside the resulting hole.”
The researchers used a mathematical description of that concept to develop a blueprint for a cloak that mimics the properties of the imagined, warped space, he said. “You cannot easily warp space, but you can achieve the same effect on electromagnetic fields using materials with the right response,” Schurig continued. “The required materials are quite complex, but can be implemented using metamaterial technology.” While the properties of natural materials are determined by their chemistry, the properties of metamaterials depend instead on their physical structure. In the case of the new cloak, that structure consists of copper rings and wires patterned onto sheets of fiberglass composite that are traditionally used in computer circuit boards. To simplify design and fabrication in the current study, the team set out to develop a small cloak, less than five inches across, that would provide invisibility in two dimensions, rather than three. In essence, the cloak includes strips of metamaterial fashioned into concentric two-dimensional rings, a design that allows its use with a narrow beam of microwave radiation. The precise variations in the shape of copper elements patterned onto their surfaces determine their electromagnetic properties.
The cloak design is unique among metamaterials in its circular geometry and internal structural variation, the researchers said. All other metamaterials have been based on a cubic, or gridlike, design, and most of them have electromagnetic properties that are uniform throughout. “Unlike other metamaterials, the cloak requires a gradual change in its properties as a function of position,” Smith said. “Rather than its material properties being the same everywhere, the cloak’s material properties vary from point to point and vary in a very specific way. Achieving that gradient in material properties was a fairly significant design effort.” To assess the cloak’s performance, the researchers aimed a microwave beam at a cloak situated between two metal plates inside a test chamber, and used a specialized detecting apparatus to measure the electromagnetic fields that developed both inside and outside the cloak. By examining an animated representation of the data, they found that the wave fronts of the beam separate and flow around the center of the cloak. “The waves’ movement is similar to river water flowing around a smooth rock,” Schurig said. Moreover, the observed physical behavior of the cloak proved to be in “remarkable agreement” with that expected based on a simulated cloak, the researchers said.
Although the new cloak demonstrates the feasibility of the researchers’ design, the findings nevertheless represent a “baby step” on the road to actual applications for invisibility, said team member Steven Cummer, a professor of electrical and computer engineering at Duke. The researchers said they plan to work toward developing a three-dimensional cloak and further perfecting the cloaking effect. Although the same principles applied to the new microwave cloak might ultimately lead to the production of cloaks that confer invisibility within the visible frequency range, that eventuality remains uncertain, the researchers said. To make an object literally vanish before a person’s eyes, a cloak would have to simultaneously interact with all of the wavelengths, or colors, that make up light, he said. That technology would require much more intricate and tiny metamaterial structures, which scientists have yet to devise.