Physicists in the US claim to have created the first 3D invisibility cloak that can operate alone in free space. The cloak, based on a “plasmonic” shell, can hide a cigar-sized cylinder from microwaves – although it currently only operates for one microwave polarization.
We report the first experimental verification of a metamaterial cloak for a 3D object in free space. We apply the plasmonic cloaking technique, based on scattering cancellation, to suppress microwave scattering from a finite dielectric cylinder. We verify that scattering suppression is obtained all around the object and for different incidence angles, validating our measurements with analytical results and full-wave simulations. Our experiment confirms that realistic and robust plasmonic metamaterial cloaks may be realized for elongated 3D objects at microwave frequencies.
To conclude, we have presented the rst experimental demonstration of a 3D stand-alone cloak in free space, applying the plasmonic cloaking technique to a nite circular cylinder approximately two wavelengths long, illuminated by microwave radiation. Our results show that robust and strong scattering suppression can be obtained at the frequency of interest and over a moderate frequency range, weakly dependent on the excitation and observer positions. Experimental measurements closely match theoretical predictions and numerical simulations. Scattering may be strongly reduced even for large incidence angles and near-grazing incidence. These concepts may be extended to infrared and optical wavelengths using alternative realizations of plasmonic metamaterials. The design chosen here limited the ultimate thinness of our cloak. We are currently exploring an alternative realization using the mantle-cloaking technique, which may further reduce the overall cloak thickness.
Last year there was a big step towards this goal when Martin Wegener and colleagues at the Karlsruhe Institute of Technology in Germany developed the first 3D cloak, operating in the near-infrared. But this was a flat, “carpet” cloak, whereby the hidden object had to be placed on a surface, with the cloak itself laid on top. Ideally, a 3D cloak would allow an object to be positioned away from a surface, in free space.
Now Andrea Alù and colleagues at the University of Texas at Austin claim to have made just such a cloak. Unlike previous metamaterial designs, the device is based on a plasmonic cloaking concept, in which the light scattered by an object is cancelled precisely by an exterior shell. Plasmonic materials have special properties at certain frequencies where the electromagnetic radiation can excite electron oscillations called plasmons. The shell works because it has a very low permittivity, providing it with a polarization opposite to that of the object. Any light scattering off the object is therefore cancelled out, and the object appears transparent.
Alù’s research group achieved this with a hollow dielectric cylinder 18 cm long and 2.5 cm in diameter constructed from eight segments. At a frequency of 3 GHz, the scattering of polarized microwaves was reduced by more than 9 dB for a 60° range of angles.
Isotropic response required
Alù suggests there is a way to make a similar cloak for unpolarized light, however. “For ease of realization, we have picked a metamaterial design that is anisotropic, and therefore works only for one polarization,” he explains. “[But] one can, in principle, come up with other metamaterial designs, such as 3D wire media or isotropic arrays of inclusions, that would provide an isotropic response independent of the impinging polarization.”
Martin McCall, a theorist specializing in invisibility cloaks at Imperial College London, thinks the experiment is still a long way from the “dream” of a cloak that works in 3D over the broad range of visible light frequencies. “I would say this is an interesting development, but brings us only a small step closer,” he says.