January 26, 2012

Three-dimensional plasmonic cloak hides a cylinder from microwaves

New Journal of Physics - Experimental verification of three-dimensional plasmonic cloaking in free-space (14 pages) They optimized the cloak design for the 3 GHz range. They have hidden a cylinder from microwaves, demonstrating cloaking of an object in free space, rather than a two-dimensional image. The group has not been able to scatter visible light, but it expects that cloaking small objects is possible. The results pave the way to realistic, practical applications of 3D stand-alone cloaks for radar evasion and non-invasive radio frequency probing.

BBC News - The approach used is unlikely to work at the visible light part of the spectrum. Prof Alu explained that the approach could be applied to the tips of scanning microscopes - the most high-resolution microscopes science has - to yield an improved view of even smaller wavelengths of light.

In future applications, plasmonic materials could be combined with the structured metamaterials idea already in development elsewhere. Light can be channelled where it needs to go, or its effects undone, as need be.

Prof Apu said that if he had to bet in five years what kind of cloaking technique might be used for applications, for practical purposes, then he would say plasmonic cloaking is a good bet.

We report the experimental verification of metamaterial cloaking for a 3D object in free space. We apply the plasmonic cloaking technique, based on scattering cancellation, to suppress microwave scattering from a finite-length dielectric cylinder. We verify that scattering suppression is obtained all around the object in the near- and far-field and for different incidence angles, validating our measurements with analytical results and full-wave simulations. Our nearfield and far-field measurements confirm that realistic and robust plasmonic metamaterial cloaks may be realized for elongated 3D objects with moderate transverse cross-section at microwave frequencies.
Photographs of: (top) the assembled cloak on the test cylinder with end caps; (bottom left) a cross-sectional view of the assembly with end cap removed; (bottom right) a shell segment edge with copper tape used to form the metallic strip for the metamaterial cloak.

We have reported here what we believe to be the first experimental verification of a 3D standalone cloak in free space, achieved by applying the plasmonic cloaking technique to a finite cylinder approximately two wavelengths long illuminated by microwave radiation. Our results show that robust and strong scattering suppression can be obtained over a moderately broad frequency range, weakly dependent on illumination and observation positions. Experimental measurements follow quite closely our theoretical predictions and numerical simulations validating our results. Scattering is strongly reduced even for large incidence angles and neargrazing incidence for various forms of excitation, and even in the near-field. Moreover, it is evident that the extensive near-field and far-field experimental measurements are in full agreement with the total SCS predicted from our full-wave numerical simulations of the realized plasmonic cloak reported in below.

Total SCS, as predicted by full-wave simulations, of the cloaked and uncloaked cylinders around the design frequency for normal incidence illumination.

The design chosen here limited the thinness of our cloak, and its functionality to one polarization.We are currently exploring alternative realizations using the mantle-cloaking technique which may further reduce the overall cloak thickness and achieve scattering suppression for all polarizations.

Near-field mapping of the electric field distribution (snapshot in time) around and on top of the object under test. The first and fourth rows (2.7 and 3.8 GHz, respectively) correspond to frequencies at which the cloak is expected to perform poorly. The second row (3.1 GHz) shows the lack of near-field scattering at the design frequency. The third row (3.3 GHz) corresponds to the upper-band edge of the cloak’s performance.

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