The dream of invisibility lives in human imagination. Invisible beings, invisibility cloaks, and unseen devices inhabit many fiction books and movies. In real life, not only magicians can make things disappear right before our eyes, using mirrors, scientists also have some hidden secrets. Learn about the real invisibility cloaks.

A matter of perception
We know our eyes deceive us all the time (See the article Eyes: Those pretty little liars ). The visible spectrum for the human eye comprises a wavelength range of 390 – 700 nm, which excludes ultraviolet and infrared light. Unfortunately, we cannot admire the elaborate flower patterns that show bees where plants store nectar, as they are only visible in ultraviolet light. Neither are we able to sense infrared, as snakes can. Even walking in the dark would be a tough task for us, whereas it is a piece of cake for cats and dogs. They bear a structure called tapetum lucidum that allows them to see in the dark. Hence, as far as we can see, part of our world is already invisible to us.

Within the visible spectrum, we only perceive the wavelength each object reflects, which can be subject to manipulation. Invisibility is simply a manipulation of perception. The same light that makes objects visible to human eyes can also make them disappear. Several scientists from different parts of the world are currently working on cloaking technology using three different scopes: ray optics, transformation optics, and a mixture of both.

Transformation optics
In transformation optics, the light is bended around a cloaking device, so anything covered by it becomes invisible at certain wavelengths. The cloak is composed of metamaterials: artificially produced structures arranged in repeated microscopic patterns that bear reflexive properties. At first, scientists created a cloak for electromagnetic waves and only later adapted it to light waves.

Cloaking devices can assume different sizes and shapes, but none so far resembles the invisible cloaks of stories. Some devices are cylindrical, flat or even wedge-shaped.

A flat cloak carpet, for example, mimics a flat ground plane. It conceals the object under it by restoring the wavefront as if it were reflected from a flat surface. However, some cloak carpets could make the background shift making them visible.

Most cloaks are so tiny that experiments must occur under the microscope, and the objects they hide are even smaller, with sizes ranging from one to one hundred wavelengths. Step by step, this technology is improving to produce from bulky to ultrathin cloaks able to conceal not only 2D but also 3D objects.

In 2015, a research group composed of scientists from the USA and Saudi Arabia published in Science a paper about the development of an ultrathin invisibility cloak. This cloak is made of nanoantennas produced with gold blocks and can make 3D objects invisible. Its thickness is only 80 nanometers, which is equivalent to one-ninth of a wavelength. Although these scientists could only hide an object measuring 36µm in their experiment, they claim this technology have the potential to hide macroscopic objects.

Ray optics
Ray optics uses another cloaking principle: an optical device made of a series of lenses that images the background. This technique has no need of metamaterials and promises to cloak large visible objects, from centimeter to meter-long. In 2014, scientists from the University of Rochester, in New York, USA, produced a perfect cloaking device using four conventional lenses. They state that that a perfect cloaking device should be invisible to the observer. Therefore, they used achromatic doublets that combine two lenses together to correct chromatic and other aberrations. In their experiment, any object placed in the cylindrical region between the first and last lenses becomes invisible, as rays do not pass there. The device also works with three lenses but shows its best performance using four.

Combination of transformation and ray optics
A third approach incorporates the principles of transformation optics using lenses instead of metamaterials. Similarly to ray optics, this method also uses low-cost materials and simple techniques. A research group, which gathers scientists from the Singapore and the USA, the Singapore – MIT Alliance for Reasearch and Technology, created a macroscopic invisibility cloak for visible light made of two pieces of calcite and a mirror as a ground plane. Calcite is a common anisotropic optical material that allows seeing through it with the naked eye. Their cloak can conceal objects as big as 2-mm long in a transparent liquid medium. In this approach, the object behind the cloak becomes invisible as the anisotropic cloak acts as an electromagnetic space. Hence, the light behaves as if there were no object on top of the ground plane, i.e., as if it is propagating in an empty space. The current model only works in 2D geometry, but the researchers claim it is possible to adapt it to a 3D geometry.

Potential applications
Apart from obvious military application in camouflage outfits and hiding plane, ships, and submarines, this technology has other potential uses. In the medical field, for example, this technology could be used in diagnosis and surgery, by eliminating from view parts of the body and focusing on the target organ. Also, in the transport area, it could eliminate blind spots from vehicles, which would make driving safer.

Choi, J.S. and Howell, J.C., 2014. Paraxial ray optics cloaking. Optics express, 22(24), pp.29465-29478.
Li, J. and Pendry, J.B., 2008. Hiding under the carpet: a new strategy for cloaking. Physical Review Letters, 101(20), p.203901.
Ni, X., Wong, Z.J., Mrejen, M., Wang, Y. and Zhang, X., 2015. An ultrathin invisibility skin cloak for visible light. Science, 349(6254), pp.1310-1314.
Zhang, B., Luo, Y., Liu, X. and Barbastathis, G., 2011. Macroscopic invisibility cloak for visible light. Physical Review Letters, 106(3), p.033901.

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