Invisibility becomes reality as scientists bend light waves

With almost magical potential, researchers pursue 'Harry Potter' cloak, bending light waves to make objects appear to vanish

PUBLISHED : Friday, 27 December, 2013, 3:18am
UPDATED : Friday, 27 December, 2013, 8:34am

A new way of assembling things, called metamaterials, may in the not too distant future help to protect a building from earthquakes by bending seismic waves around it. Similarly, tsunami waves could be bent around towns, and soundwaves bent around a room to make it soundproof.

While the holy grail of metamaterials is still to make objects and people invisible to the eye, they are set to have a more tangible commercial impact playing more mundane roles, from satellite antennas to wirelessly charging cellphones.

Metamaterials are simply materials that exhibit properties not found in nature, such as the way they absorb or reflect light. The key is in how they're made. By assembling the material, from photonic crystals to wire and foam, at a scale smaller than the length of the wave you're seeking to manipulate, the wave can, in theory, be bent to will.

This makes metamaterials the tool of choice for scientists racing to build all sorts of wave-cloaking devices, including the so-called invisibility cloak. A team of scientists at Zhejiang University in Hangzhou last month demonstrated a device that made fish "invisible". The same technology was also used to make a cat "disappear". The device was made of a hexagonal array of glass-like panels, which obscured the object from view by bending light around it.

"The invisibility cloak was just one more thing we were discovering, that we have all this flexibility in this material and here's another thing we can do," David Smith of Duke University, regarded as one of the founding fathers of metamaterials, said in a telephone interview. "But we're equally interested in seeing this transition in making a difference in people's lives."

Indeed, Smith's own journey from laboratory to factory illustrates that while metamaterials have for some become synonymous with "Harry Potter" cloaks, their promise is more likely to be felt in a range of industries and uses, from smaller communication devices to quake-proof buildings.

At the heart of both metamaterials and invisibility are waves. If electromagnetic waves - whether visible light, microwave or infrared - can be bent around an object it would not be visible on those wavelengths. It was long thought you couldn't control light in this way with natural materials as their optical properties depended on the chemistry of the atoms from which they were made.

Watch: How does China's invisibility cloak work?

It was only when Smith and his colleagues experimented with altering the geometry of material in the late 1990s that they found they could change the way it interacted with light. With that, says Andrea Alu, an associate professor at the University of Texas at Austin, scientists found "it may be possible to challenge rules and limitations that were for centuries considered written in stone".

The past decade has seen an explosion in research that has built on Smith's findings to make objects invisible to at least some forms of light.

"There have now been several demonstrations of cloaking at visible wavelengths, so cloaking is truly possible and has been realised," says Jason Valentine of Vanderbilt University, who made one of the first such cloaks. These, however, have limitations, such as only working for certain wavelengths or from certain angles. But the barriers are falling fast, says Valentine.

In the past year, for example, Duke University's Yaroslav Urzhumov has made a plastic cloak that deflects microwave beams using a normal 3D printer, while Alu has built an ultra-thin cloak powered by electric current.

However, scientists on the mainland believe they have an edge. "The competition is no longer about the theory, but the materials. Chinese scientists have a natural advantage there," said Professor Wang Guoping, who is working on an invisibility cloak in the physics department at Wuhan University, Hubei province. In the US, funding for this kind of research is coming from the military. Urzhumov said in an e-mail interview that the US Department of Defence is "one of the major sponsors of metamaterials and invisibility research in the US".

The Defence Advanced Research Projects Agency, which commissions advanced research for the Department of Defence, has funded research into metamaterials since 2000, according to the department's website.

Military interest in metamaterials was primarily in making a cloak, said Miguel Navarro-Cia of Imperial College London, who has researched the topic with funding from the European Defence Agency and US military.

But an invisibility cloak needn't be a sinister tool of war.

Vanderbilt's Valentine suggests architectural usage. "You could use this technology to hide supporting columns from sight, making a space feel completely open," he said.

Other potential uses include rendering parts of an aircraft invisible for pilots to see below the cockpit, or to rid drivers of the blind spot in a car.

Military or not, this is all some way off.

"Most invisibility cloaks, essentially, are still in the research stage," says Ong Chong Kim, director at the National University of Singapore's Centre for Superconducting and Magnetic Materials.

Ong and others say that while metamaterials may not yet be making objects invisible to the eye, they could be used to redirect other kinds of waves, including mechanical waves such as sound and ocean waves. French researchers earlier this year, for example, diverted seismic waves around specially placed holes in the ground, reflecting the waves backward.

Ong points to the possibility of using what has been learned in reconfiguring the geometry of materials to divert tsunamis from strategic buildings.

Elena Semouchkina, a pioneer on cloaking devices at Michigan Technological University, points to screening antennas so they don't interfere with each other, protecting people from harmful radiation or acoustic pressure, and even preventing buildings from destruction from seismic waves.

Metamaterials could also absorb and emit light with extremely high efficiency - for example, in a high-resolution ultrasound - or redirect light over a very small distance. This, says Anthony Vicari of Lux Research, "could be used to improve fibre optical communications networks, or even for optical communications within microchips for faster computing".

Indeed, there is clearly a growing appetite for commercialising the unique properties of metamaterials.

One of the first to do so was the now-defunct Rayspan, a California-based company whose antennas found their way into Wi-fi routers from networking manufacturer Netgear and also a smartphone from LG Electronics.

The antennas were smaller, flatter and performed better than other options, but integrating them into the rest of the phone proved difficult, said former Rayspan executives. A spokesman for LG said the project was no longer active and that LG had no plans to apply metamaterials in other products.

"One thing from my experience as an entrepreneur is that technology gets very excited about what it's doing in the lab," said Maha Achour, who co-founded Rayspan, "but the reality when you commercialise things is completely different."

The company's patents have since been sold to an undisclosed buyer.

The lessons have been learned. Now, the focus has shifted to using metamaterials in products in markets where they can more easily gain a commercial foothold.

Smith, who built the first metamaterials in 1999, has led the charge, teaming up with Intellectual Ventures, a patent portfolio firm, to spin off two companies: Kymeta, making flat-panel antennas for satellite communications, and Evolv Technologies, which hopes to make a lighter, faster and portable airport scanner. Kymeta, in partnership with satellite operators Inmarsat and O3b Networks, hopes to ship in early 2015.

The two fields were chosen from a shortlist of 20 potential markets, Smith said. "They're the same metamaterials behind the cloak, but we were looking for more near-term applications."

The next likely consumer use of metamaterials could be in the wireless charging of devices, an area which is attracting industry attention.

Mark Gostock of ISIS Innovation, an Oxford University research commercialisation firm, said he was in talks with several manufacturers to licence the technology of ISIS. Samsung Electronics has filed several patents related to metamaterials and wireless charging, but declined to comment.

Other companies that cite metamaterials in their patent filings include Harris, NEC, Hewlett-Packard and Panasonic.

Eventually, says Wil McCarthy, chief technology officer of Denver-based smart window maker RavenBrick and holder of a patent he hopes will bring metamaterials to polarising windows, metamaterials will be incorporated without much fanfare.

"The people buying these products will have no idea how they work, and won't know or care that they're doing things that were previously considered impossible," he says.