Mmm, invisibility donut… researchers try new takes on the “invisibility cloak”

Duke University researchers who previously demonstrated invisibility cloaking in the lab have employed 3D printing to build their latest “cloak” – a disk that can block microwaves.

The thickness of the donut-like disk roughly matches one wavelength, and its combination of air and dielectric (insulating, nonconducting) composite material deflects microwaves. An object placed in the center effectively “disappears” when microwaves are aimed at it. Because of the properties of transformation optics (physics of electromagnetic radiation that behaves similarly to relativistic warping of space-time), the shell of the disk eliminates any backward reflection that a viewer or detector would use to see the object, and also suppresses shadows and scattering.

The cloaking disk is made of plastic, but another transparent polymer or glass would work equally well, the researchers say. Simulations shows that the cloak could be made thinner and larger in area, and could potentially work for shorter wavelengths, like visible light.


Transformation optics also underlies another advance towards invisibility with metamaterials. These are engineered materials with new kinds of properties that don’t normally exist in nature. The key development by the researchers from Stanford and Spain is tailoring the new metamaterial’s refractive index, or the degree to which it can bend light. Only positive refractive indices (like 1.33 for water) exist in nature, but using transformation optics the investigators were able to design constituents of the new material that have a negative refractive index.

In order for metamaterials to have the interesting properties they were designed for, they need to interact with both magnetic and electric fields. The constituent “atoms” of the new material can do both, which means their interactions with light over broader wavelengths can be controlled. The visible spectrum of light extends from 400-700 nanometers, but previous invisibility efforts have only been able to cover about 50 nm of this range.

In their theoretical analysis, the researchers started with an infinite sheet of material that they fold into a crescent-shape on the nanometer scale. This is their constituent “atom,” which is placed into an array with other identical ones in a background material. The result is a structure that has negative refractive index, i.e. “invisibility” over much of the visible spectrum, in a band over 200 nm wide. Engineering a material from the bottom up opens up new optical potentials, like precisely controlling the light path, and changing the geometry of the nano-crescents or shrinking them could help the invisibility band grow to cover the whole visible spectrum. The material’s negative refraction is shown in the video below.

Image of Yaroslav Urzhumov via Duke University