Harnessing the materials used to make real-life invisibility cloaks could shrink cellphone antennas, leading to smaller gadgets.
Metamaterials 鈥� materials that possess properties which don鈥檛 exist in nature 鈥� can manipulate light or other electromagnetic waves with such dexterity that they can steer rays around objects as if they weren鈥檛 there at all. But most metamaterials can only pull such stunts on waves of a specific frequency 鈥� for example a particular colour of light.
An international team of physicists has now created metamaterials that can be tuned to a range of different frequencies as required. A cellphone antenna fashioned from the new material could be tuned to work very efficiently across a small frequency range, but retuned to a different band for .
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Tuned in
Design tricks for making 鈥渂roadband鈥� metamaterials that operate over a wide frequency range already exist, but they can鈥檛 be applied to many metamaterial designs, says at the University of California, San Diego, who helped create the new 鈥渇requency-agile鈥� design with , also at San Diego, and collaborators from in Durham, North Carolina, and in the Republic of Korea.
It was made by attaching a thin film of vanadium dioxide to a gold metamaterial structure. Applying a voltage to the film alters the frequency at which the gold metamaterial interferes with light waves, tuning it to a new 鈥渟etting鈥�.
It works because applying the voltage causes nanoscale 鈥減uddles鈥� of conducting vanadium metal to form within the insulating vanadium dioxide. They interact with the design鈥檚 electrical properties and alter the metamaterial鈥檚 tuning.
鈥淭he effect continues after the electrical current is gone because the metal puddles, once formed, will not readily disappear without some cause,鈥� says Driscoll, adding that there is evidence to suggest the effect should last for months or more.
That鈥檚 an improvement over other designs that can only remain tuned to a new frequency as long as the electric current is applied, he says.
At present the new design can be tuned to different frequencies in the terahertz frequency band, which could soon be used as an alternative to X-rays in medical imaging. But modifying it to work with the gigahertz frequencies used by cellphones should be straightforward, Driscoll says.
Adaptable material
鈥淢etamaterials are often narrowband, but at least with this scheme one could adapt the material to new frequencies,鈥� says , a metamaterial researcher at the University of St Andrews in the UK.
That removes an obstacle to the wider use of metamaterial antennas. Such antennas would be attractive because they could help to .
Because metamaterials can manipulate electromagnetic waves to a greater degree than normal materials 鈥� and even bend light 鈥渂ackwards鈥� 鈥� a small metamaterial antenna is as effective as a much larger standard antenna at transmitting or receiving waves.
But a narrowband metamaterial antenna has its drawbacks 鈥� it鈥檚 rare that engineers require an antenna tuned to just one frequency, says Driscoll. For instance, different countries have allocated to their cellphone networks.
He says a tunable metamaterial antenna would allow a wireless gadget to work 鈥渙utstandingly well鈥� at the frequencies used in one country, but also carry the option of retuning for use abroad.
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