Cloaking Device Theory Update

This is a review article onthe current status of thinking about ‘cloaking devices’.  Rather interesting and some of itpromising. 

The potential for actual cloaking is probably better thanone would think since it is plausible to design an object in such a way as to possiblyhide tell tales.  One could do that todaywithout an actual cloaking device.  A cloakingdevice could expand possibilities though.

It is worth the read, even if we suspect that this is allgoing nowhere anytime soon.

Introductory Summary of Invisibility, Metamaterials andTransformative Optics

JANUARY 11, 2011

llustration of the idea of a "Pendry cloak". Light raysilluminating the cloak are bent around the central region and allowed tocontinue on their original path. Figure from BBC News. 

In 1988, a mathematician named Nachman provided a rigorous proof that invisibleobjects do not exist: if one shines enough light on an object from enoughdirections, it will be detectable.

Nachman's theorem, though rigorous, had two big "loopholes" in itthat were overlooked by researchers of the time but were caught by the 2006researchers. Leonhardt correctly noted that Nachman's theorem only precludedperfectly invisible objects; a cloak that is 99.9% invisible, however, mightvery well be possible. Pendry, Schurig and Smith observed that Nachman'stheorem only applies to isotropic materials, in which light travels at the samespeed regardless of its direction and polarization. Anisotropic materials, suchas calcite crystals, behave differently depending on the nature of the lighttraveling through them, and give rise to phenomena such as double refraction.

The Pendry, Schurig and Smith cloak is an anisotropic cloak, and not subject toNachman's impossibility theorem. In 2007, other researchers showed through morerigorous calculations that this design is, in principle, perfectly invisible.

A few points are worth making about these early cloaks.

1. They require the fabrication of materials with a wide range of refractiveindices and spatial variations that are not found in nature. The constructionof a cloak that would work for visible light therefore requires the use ofso-called metamaterials, materials that derive their properties frommodification of their structure on the scale of a billionth of a meter. As itstands, nobody really knows how to make such materials reliably andefficiently.

2. These cloaks work only for a single wavelength (color) of light, or a small range ofcolors. Note: There is work to expand the rangse. Looking at the image of thePendry cloak, light that intersects the middle of the cloak has to travelfarther than light that hits the edge of the cloak. If the cloak is designed tomake all of the light "synch up" when it reemerges at one wavelength,it will in general not be synched at another wavelength; there is no goodsolution to this problem as yet either.

3. The behavior of light inside these cloaks is in many ways analogous to thebehavior of light in a gravitational field under Einstein's general theory of relativity. A new subfield of optics knownas transformation optics has been developed that applies the mathematical toolsof general relativity to design new cloaks and other unusualoptical devices.

So what other kind of optical devices have been imagined? It seems that it ispossible to make light do almost anything these days -- at least theoretically

In June of 2009, however, Alu and Engheta proposed a technique for cloaking asensor that allows the sensor to detect, but not to be seen. Idea behind"cloaking a sensor". The light scattered by the cloak is out of phasewith the light scattered by the sensor, resulting in a partial cancellation ofthe total field scattered by the object.

The existence of metamaterial invisibility devices implies that we can alsoconstruct a cloak that makes one object look like a completely differentobject. This is potentially more useful than a true invisibility cloak: animperfectly invisible object would likely draw much more attention than animperfectly imaged mundane object. A group of researchers in Hong Kong in 2009 had the idea of making "optical illusion"using transformation optics and metamaterials

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