23 August 2012
Researchers have made breakthroughs in designing invisibility cloaks and perfect lenses by making them out of artificial structures called metamaterials. These wonder devices have often lacked any sort of on-off button, but a new metamaterial design allows certain properties to be tuned with an external light source. As described in Physical Review Letters, the device can switch from a normal flat mirror to a focusing/defocusing mirror without changing its shape.
A typical metamaterial is a periodic assembly of small components, or “meta-atoms.” A common example of a meta-atom is a split-ring resonator (SRR), which is a metal ring with a small gap. Previous work has shown that an array of SRRs can be used to create a negative index of refraction, which scatters light in ways not possible with natural materials.
It has recently been shown that the overall properties of SRR-based metamaterials can be tuned. However, Ilya Shadrivov of the Australian National University in Canberra and his colleagues wanted to take this a step further by remotely controlling individual meta-atoms. In their case, the meta-atoms are SRRs integrated with varactors, which are electronic components with variable capacitance. Changing the capacitance alters the SRR resonance and the effect it has on light. The team connected each varactor to a photodiode so that each meta-atom could be controlled with a nearby LED. In experiments, the team showed that incoming microwaves reflect off their SRR array like a flat mirror. But by increasing the brightness of selected LEDs, the angle of reflection could be altered. The array could even focus or defocus the microwaves, as if it were a parabolic mirror.