Nanoparticles in Camouflage

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A 3-D skin cloak made of metamaterial and comprised of nanoantennas covering an arbitrary shaped 3-D object, the red arrows depict incident light and light reflecting off the surface [4]. 

(A and B) Images of the skin cloak on/polarized (B) and off (A) covering 3D bump-shaped object. (C) SEM image of bump-shaped object with polarized skin cloak [2].

Physical observation is made possible through the light scattering (reflection) of incident light off an object at an angle. The waves of the reflected light are then returned to our eyes to interact with photoreceptor cells in the retina, which then sends the signal to our brains to convert it into an image. However, if the angle of the incident light equals the angle of the reflected light it will produce specular reflection, or mirror-like reflection [1]. This type of reflection is the scientific basis for invisibility cloaks, which reflect similarly to a flat mirror rather than reflect to produce an image of the object you are attempting to conceal.

These invisibility cloaks may be made of metamaterials in the future. Metamaterials are organized systems of nanomaterials whose wavelengths are smaller than the light they interact with, and whose unique properties are derived from their shape and size rather than their chemical composition [2]. One of these unique properties is its Negative Refraction Index, which produces refractions of incident light differently than materials with a normal refraction index. This Negative Refraction Index can be adjusted with the size and geometry of the nanoparticles of the metamaterial to produce reflections of identical wavefront and phase (wave position), creating a mirror-like reflection to mask the 3D shape of an object [3]. Original “carpet cloaks” were bulky and created a phase change, caused by the reflection off a surface within a higher refraction index than the material by which the incident light travelled (air), which made the cloaked object detectable to phase-sensitive detection. Metamaterials with a Negative Refraction Index generate a reflection in the same phase, making it invisible. Recent innovations in invisibility cloaks use rectangular, ultrathin gold nanoantennas as the metamaterial, which has yielded 84% reflectivity of objects at 730nm [4]. At their current scale, these invisibility skin cloaks can primarily be used in security encryption purposes, like hiding the layout of microelectronic components.

References

  1. "Time and Frequency from A to Z". "Phase". National Institute of Standards and Technology (NIST). Retrieved 30 August 2016.
  2. Krishnamoorthy, Harish, You Zhou, Shriram Ramanathan, Vinod Menon, and Evgenii Narimanov. "Tunable Hyperbolic Metamaterials Utilizing Phase Change Heterostructures." Applied Physics Letters, 2014. Web. 31 Aug. 2016.
  3. Shelby, R. A.; Smith D.R; Shultz S. (2001). "Experimental Verification of a Negative Index of Refraction". Science. 292 (5514): 77–79. doi:10.1126/science.1058847.PMID 11292865.
  4. Ni, Xingjie, Yuan Wang, Zi Jing Wong, Michael Mrejen, and Xiang Zhang. "An Ultrathin Invisibility Skin Cloak for Visible Light." Applied Optics 349.6254 (2015): n. pag. 

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Benefit Summary: 

Through the use of metamaterials, gold nanoantenna cloaks with a Negative Refraction Index are able to reflect light with a wavefront and phase identical to the incident light, rendering an object it is covering invisible. At only ~80nm in thickness, these invisibility skin cloaks can easily be scaled to larger sizes with meter-scale nanofabrication. Additional use of a dielectric spacer layer powered by a gap plasmon resonance can further increase reflectivity [4]. 

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Much of the knowledge on metamaterials is still confined to academic papers, and the risks to human health and the environment have not as of yet been thoroughly explored. 

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