Shape-shifting

Oh, here’s one from BBC Future that I almost missed – the latest in ‘illusion optics’. I have a little video discussion of this too.

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In the tradition whereby science mines myth and legend for metaphors to describe its innovations, you might call this shape-shifting. Admittedly, the device reported in the journal Physical Review Letters by researchers in China is not going to equal Actaeon’s transformation into a stag, Metis into a fly, or Proteus into whatever he pleased. But it offers an experimental proof-of-principle that, using ideas and techniques related to invisibility cloaking, one object can be given the appearance of another. Oh, and the device does invisibility too.

This versatility is what marks out the ‘cloak’ made by Tie Jun Cui of the Southeast University in Nanjing, China, and his coworkers at Lanzhou University as distinct from the now considerable body of work on invisibility cloaks and other types of “transformation optics”. Surprisingly, perhaps, this versatility comes from a design that is actually easier to fabricate than many of the ‘invisibility cloaks’ made previously. The catch is that these shape-changes are not something you can actually see, but are apparent only when the transformed object is being detected from the effect it has on the electrical conductivity of the medium in which it is embedded.

The most sophisticated ‘invisibility cloaks’ made so far use structures called metamaterials to bend light around the hidden object, rather like water flowing around an obstacle in a stream. If the light rays from behind the object are brought back together again at the front, then to an observer they seem not to have deviated at all, but simply to have passed through empty space.

Researchers have also shown that, by rerouting light in other ways, a metamaterial cloak can enable so-called ‘illusion optics’ that gives one thing the appearance of another. However, with metamaterials this is a one-shot trick: the cloak would produce the same, single visual illusion regardless of what is hidden within it. What’s more, genuine invisibility and illusion optics are tremendously challenging to achieve with metamaterials, which no one really yet knows how to make in a way that will work with visible light for all the wavelengths we see. So at present, invisibility cloaks have been limited either to microwave frequencies or to simplified, partial cloaks in which an object may be hidden but the cloak itself is visible.

What’s more, each cloak only does one sort of transformation, for which it is designed at the outset. Cui and colleagues say that a multi-purpose shape-shifting cloak could be produced by making the components active rather than passive. That’s to say, rather than redirecting light along specified routes, they might be switchable so that the light can take different paths when the device is configured differently. You might compare it to a fixed rail track (passive), where there’s only one route, and a track with sets of points (active) for rerouting.

Active cloaks have not been much explored so far beyond the theory. Now Cui and his coworkers have made one. It hides or transforms objects that are sensed electrically, in a process that the researchers compare to the medical technology called electrical impedance tomography. Here, electrical currents or voltages measured on the surface of an object or region are used to infer the conductivity within it, and thereby to deduce the hidden structure. A similar technique is used in geophysics to look at buried rock structures using electrodes at the surface or down boreholes, and in industrial processes to look for buried pipes. It’s a little like using radar to reconstruct the shape of an object from the way it reflects and reshapes the echo.

Here, hiding an object would mean constructing a cloak to manipulate the electrical conductivity around it so that it seems as though the object isn’t there. And transforming its appearance involves rejigging the electric field so that the measurements made at a distance would infer an embedded object of a different shape. Cui and colleagues have built a two-dimensional version of such an illusionistic cloak, consisting of a network of resistors joined in a concentric ‘spider’s web’ pattern on an electrically conducting disk, with the cloaked region in a space at their centre.

To detect the object, an electrode at one position on the plate sets up an electric field, and this is measured around the periphery of the plate. Last year Cui and his colleagues made a passive version of an invisibility cloak, in which the resistor network guided electric currents around the central hole so as to give the impression, when the field was measured at the edges of the disk, that the cloak and its core were just part of the uniform background medium. Now they have wired up such a resistor network so that the voltage across each component, and thus the current passing through it, can be altered in a way that changes the apparent shape of the cloaked region, as inferred from measurements made at the disk’s edge.

In this way, the researchers could alter the ‘appearance’ of the central region to look invisible, or like a perfectly conducting material, or like a hole with zero conductivity. And all that’s needed is some nifty soldering to create the network from standard resistors, without any of the complications of metamaterials. That means it should be relatively easy to make the cloaks bigger, or indeed smaller.

In theory this device could sustain the illusion even if the probe signal changes in some way (such as its position), by using a rapid feedback mechanism to recalculate how the voltages across the resistors need to be altered to keep the same appearance. The researchers say that it might even work for oscillating electrical fields, as long as their frequency is not too high – in other words, perhaps to mask or transform objects being sensed by radio waves. Here the resistor network would be constantly tuned to cancel out distortions in the probe signal. And because resistors warm up, the device could also be used to manipulate appearances as sensed by changes in the heat flow through the cloaked region.

Reference: Q. Ma et al., Physical Review Letters 111, 173901 (2013).