We've previously seen - or should that be "not seen" - invisibility cloaks in the laboratory that are able to render two-dimensional objects invisible to microwaves. Such feats relies on the use of metamaterials - man-made materials that exhibit optical properties not found in nature and have the ability to guide light around an object. Now researchers at the University of Texas at Austin (UT) claim to have brought invisibility cloaks that operate at visible light frequencies one step closer by cloaking a three-dimensional object standing in free space with the use of plasmonic metamaterials.
Although, like previous studies, the UT team was only able to cloak an object to microwaves, they claim the technique enabled by the use of plasmonic metamaterials could, in principle, be applied to visible light. Additionally, unlike previous flat, "carpet cloaks" that had to be placed on top of the object being cloaked, the new plasmonic metamaterial-based technology can cloak an object positioned away from the cloak, in free space.
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While we see everyday objects because light hitting the object bounces off the material at the same angle it struck it at - angle of incidence equals angle of reflection - before reaching our eyes, plasmonic metamaterials have the opposite scattering effect.
"When the scattered fields from the cloak and the object interfere, they cancel each other out and the overall effect is transparency and invisibility at all angles of observation," says study co-author Professor Andrea Alù. "One of the advantages of the plasmonic cloaking technique is its robustness and moderately broad bandwidth of operation, superior to conventional cloaks based on transformation metamaterials. This made our experiment more robust to possible imperfections, which is particularly important when cloaking a 3D object in free-space."
To cloak an 18 cm (7 in) long, 2.5 cm (0.9 in) diameter cylindrical tube from microwaves, the team shelled it in a plasmonic metamaterial. The team says the cloak hid the 3D object for all angles of incidence and observation. They tested it by directing microwaves towards the cloaked cylinder and mapping the resulting scattering both around the object and in the far-field. The cloak worked best when the microwaves were at a frequency of 3.1 GHz, reducing the scattering of polarized microwaves by more than 9 dB for a 60-degree range of angles.
But the team says demonstrating the cloaking on a 3D object using visible light is their key challenge.
"In principle, this technique could be used to cloak light; in fact, some plasmonic materials are naturally available at optical frequencies. However, the size of the objects that can be efficiently cloaked with this method scales with the wavelength of operation, so when applied to optical frequencies we may be able to efficiently stop the scattering of micrometre-sized objects," said Professor Alù.
"Still, cloaking small objects may be exciting for a variety of applications. For instance, we are currently investigating the application of these concepts to cloak a microscope tip at optical frequencies. This may greatly benefit biomedical and optical near-field measurements," Professor Alù added.