Materials best known for their use as invisibility cloaks and super-lenses can now have their properties fixed with external stimuli, thanks to research performed in the US and the South Korea.

The new "memory metamaterials", made by Tom Driscoll of the University of California at San Diego and colleagues, can have their electromagnetic properties temporarily modified depending on the level of applied voltage or light. According to the researchers, such tuning could allow for a "set-and-forget" approach to complex metamaterials for applications where it is impractical to maintain an external stimulus.

Harry Potter physics

Metamaterials are engineered structures that respond to electromagnetic waves in unusual ways. For example, they can be designed to have a refractive index that varies throughout, even taking on a negative value in some cases. This particular ability of metamaterials led to them being used in 2004 to make the first super-lens, which can beat the so-called diffraction limit, and the first invisibility cloak for microwaves in 2006.

One of the problems with most metamaterials is that they can only be designed to operate at a single "resonant" frequency. Although there are "frequency agile" metamaterials that allow their resonant frequency to be tuned with a certain stimulus, the tuning is lost as soon as the stimulus is taken away. Driscoll – whose group includes others from San Diego, Duke University in North Carolina, US, and ETRI in Daejeon, South Korea – solves this issue by creating memory metamaterials that can remember the new frequency that they should operate at.

Like many other metamaterials, memory metamaterials contain an array of conductive rings, called split-ring resonators (SRRs), which provide the basic electromagnetic properties. However, in memory metamaterials the SRRs are patterned onto vanadium dioxide, which has a metal-to-insulator phase transition that can be controlled with light or an applied voltage.

A new phase

It is the phase of vanadium dioxide, which can last for long periods after the light or voltage is withdrawn, that provides the "memory". The specific phase alters vanadium dioxide's capacitative properties, which in turn control the SRRs' resonant frequency. Until the phase changes back, the resonant frequency is set.

To test their memory metamaterials, Driscoll and colleagues examined them with terahertz spectroscopy before and after they applied an electrical pulse. They found that the resonant frequency shifted from 1.65 THz by as much as 20%, and persisted for at least 10 minutes. "Such persistent tuning is likely to be useful in reconfigurable metamaterial devices, enabling a kind of set-and-forget approach to the reconfiguration process," says Driscoll.

The researchers suggest that materials other than vanadium dioxide could push the effect to higher frequencies, and perhaps even the visible part of the spectrum.

This research was published in the latest edition of Science.