Hiding behind water (Image: Dimitar Dilkoff/AFP/Getty)

When J. K. Rowling described Harry Potter’s invisibility cloak as “fluid and silvery”, she probably wasn’t thinking specifically about silver-plated nanoparticles suspended in water. But a team of theorists believe that using such a set-up would make the first soft, tunable metamaterial – the “active ingredient” in an invisibility device.

The fluid proposed by Ji-Ping Huang of Fudan University in Shanghai, China, and colleagues, contains magnetite balls 10 nanometres in diameter, coated with a 5-nanometre-thick layer of silver, possibly with polymer chains attached to keep them from clumping.

In the absence of a magnetic field, such nanoparticles would simply float around in the water, but if a field were introduced, the particles would self-assemble into chains whose lengths depend on the strength of the field, and which can also attract one another to form thicker columns.


The chains and columns would lie along the direction of the magnetic field. If they were oriented vertically in a pool of water, light striking the surface would refract negatively – bent in way that no natural material can manage.

This property could be exploited for invisibility devices, directing light around an object so that it appears as if nothing is there, or be put to use in lenses that could capture finer details than any optical microscope.

Dark spots

The team simulated three wavelengths to confirm negative refraction for much of the red portion of the visible spectrum and into infrared wavelengths.

Ulf Leonhardt at the University of St Andrews in the UK finds the work fascinating and the analysis sound. “There are a few caveats,” he says. “I doubt whether the useful frequency band of the material can be extended to the entire visible spectrum”, because it would be difficult to prevent some light being absorbed or sent elsewhere, resulting in dark spots.

A solid metamaterial that negatively refracts the long wavelengths of red light has already been developed, but it has proven tricky to get further into the visible spectrum: the smaller the wavelengths, the smaller the structures needed to control them, and the more difficult they are to build.

Convincing simulations

Huang holds that in principle the fluid could negatively refract all wavelengths in the visible spectrum, provided the nanoparticles had the right coating.

“One can reduce the thickness of the [fluid] sample in order to reduce the loss,” he says. “As a result, the dark spots can disappear.”

“The simulations look convincing,” says Igor Smolyaninov of the University of Maryland in College Park, but he stresses that the set-up is still only theoretical. Huang says one of his co-authors, Xiang Zhang of the University of California, Berkeley, currently has a team at work creating a fluid metamaterial.

Journal reference: Physical Review Letters (in press)