Invisibility Cloaks: Now you see it, Now you don’t.

Invisibility is something we often see on TV shows and in comic books. We’ve always thought it’s unattainable, at least not in the foreseeable future. However, two groups of scientist have shown this past year two promising methods of achieving “invisibility” using some advanced materials.

We can see an object because the light reflected off it bounces into our eyes to help us form an image. So if we want to achieve invisibility, we simply need to prevent light from bouncing off an object.

Sounds easy enough ヾ(・_・;). One way would be to bend light around an object, and the object would disappear like magic. In my opinion a true invisibility cloak should allow us to see through an object, and see the environment behind it.

In my opinion, a true invisibility cloak should work like this (please excuse me for my old-school pen and paper drawing):

Now you see it, and now you don't
My opinion of how an ‘ideal’ invisibility should work.

i.e. allow us to see the environment from behind the object, i.e. see through the object.

Bending light is not hard, however bending light in a controlled manner can be very difficult. Light rays bend when they pass from one medium to another and their speed changes. This is like in the illusion which I’m sure we have all seen before, of a “bent” pencil in a glass of water.

At least two journal papers published this year have found different ways to manipulate the refraction phenomenon to create an optical cloak, an “invisibility cloak” that works within the visible light spectrum.

Let’s take a look at how they do it.

Method 1 – Artificial Mirage
A group of scientist at the University of Texas in Dallas have created a cloak that hides an object behind it by creating an artificial mirage.

A mirage is essentially the result of a refractive index gradient due to a temperature gradient present.

Aliev et al. [1] used a heat source to create a mirage effect used to conceal an object behind it. The heat source is their “invisibility cloak” made up of highly aligned multi-wall carbon nano-tubes. They claim their cloak will work for wavelengths ranging from the Infrared to the UV range.

Light begins to deflect more and more as it gets closer to the heat source due to the decreasing refractive index. The light doesn’t actually interact (i.e. reflect, absorb,…) with the object behind the cloak but just conceals the object from plain view. It works like this:

The working principle of the artificial mirage presented by Alieve et al.

However, the angle of deflection is usually low because the variation in refractive index caused by temperature changes is relatively small.This is true for the case of air. Why use air as an example? Well it’s all around us and one of the most likely places where this cloak might be used. Using air as an example also make sense because the data can be readily estimated using the “Refractive Index of Air Calculator” in [2].

Refractive index for a red laser pointer in air at -20°C is approximately 1.00031486
“              “      “    “  “      “         “       in air at 80 °C is approximately  1.00022263
This is only a 0.009 % change even through there is a 100°C change.

Aliev et al. [1] have estimated that their cloak with a length of 1 m will only deflect light off the plane of the cloak by 3+°, i.e. the illusion will only work at a glancing angle of less than 3° from the plane of the cloak. Water on the other hand has a larger refractive index variation with temperature changes, so the cloak works for a wider range of angles [1]. By using the cloak in water, Aliev et al. have shown it is possible for their cloak to deflect light at a glancing angle of ~5° from the plane of their 5 cm long cloak.

Notice that the cloak by Aliev et al. [1] turns on and off relatively quickly to hide the words “invisibility cloak” printed on the other side of the tank. The fast on/off time is due to the extremely low thermal capacitance (the ability to store heat) and the high heat transfer nature of their highly aligned multi-walled carbon nano-tubes used make the cloak [1]. This means the sheet of carbon nano-tubes can heat up and create a temperature gradient very quickly and also cool down and dissipate the temperature gradient very quickly, allowing an almost instantaneously on/off response time.

Method 2 – Metamaterial Approach
To learn about metamaterial, refer to Cath’s post on the topic.

Another group at UC Berkeley has taken advantage of metamaterials, and the level of control we have on their properties, to make an “invisible carpet”. The basic idea is that it will give the illusion of a flat surface, even with bumps or objects hidden underneath it.

To do this, Gharghi et al. [3] at UC Berkeley first begin with a rigid form of known geometry, under which they hide objects of any shape and of size that can fit in the rigid form. They then apply optical transformation (a complicated mathematical transformation based on optics) to determine the refractive index necessary at different locations around the rigid form in order for light to bounce off as if the surface was flat.

The varying refractive index necessary is facilitated by the layer of metamaterial formed on top of the rigid form [3]. To change the refractive index along the metamaterial layer, Gharghi et al. [3] introduced very small holes (much smaller than the wavelength of light, i.e. less than 0.0004 mm) in the range of 0.000065 mm in the metamaterial layer made of silicon nitride (SiN). The holes are arranged in various patterns and spacings so the refractive index at specific locations is the same as what’s calculated from the optical transformation. The refractive index can be changed by varying the ratio of SiN to air (in the holes) at specific locations [3].

The result of the Invisibility Carpet, image borrowed from ref. 3.

Gharghi et al. [3] have only made a very small scale version of the carpet where the rigid form is ~0.005 mm, but they have shown its effectiveness with blue, green, and red lasers (i.e. light at wavelengths that span the visible spectrum), shown above. However, we should keep in mind that Gharghi et al. [3] have intended the carpet to be used on a chip photonic device, hence the small size [3].

Two other groups, at MIT [4] and the University of Birmingham in the UK [5] respectively, have also demonstrated earlier this year a similar design working at a much larger and more practical scale. The variation of refractive index necessary, in the case of [4] and [5] is facilitated by the special properties of calcite crystals under a certain polarization of light.  Unfortunately, this also means the cloaking ability only works for certain polarization of light, unlike the metamaterial cloak developed by Gharghi et al. [3]. However, the large scale version presented in [4,5] demonstrate the potential of “invisibility cloaks” at a practical scale.

Final Thoughts
Both of these cloaks are an amazing feat in science and engineering. However, would we consider them to be “real” invisibility cloaks? My answer is yes and no.

Both can certainly conceal objects, but they both have drawbacks.

The artificial mirage by Aliev et al. [1] only works at glancing angles. For the reader with keen eyes, you’ll notice in the video above that the image on the cloak is not of the environment behind the object (the environment from behind the tank) but rather a reflection from the far end of the cloak onto the front of the cloak.

The invisibility carpet by Gharghi et al. [3] does give an illusion that the rigid form, and hence anything underneath the form, to be the same as the flat surface it is lying on. However, the object that can be hidden is limited to the design of the rigid form. The size of the cloak also seems to be quite a bit larger than the rigid form itself from [3], [4], and [5].

Neither of these “invisible” cloaks really project an image of the environment behind an object onto the cloak, so we can’t really “see through” an object. But the cloaks create a pretty darn good illusion of seeing through an object.

References

[1] Aliev, A., Gartstein, Y., & Baughman, R. (2011). Mirage effect from thermally modulated transparent carbon nanotube sheets Nanotechnology, 22 (43) DOI: 10.1088/0957-4484/22/43/435704

[2] Engineering Metrology Toolbox [online], Available: http://emtoolbox.nist.gov/Wavelength/Edlen.asp

[3] Gharghi M, Gladden C, Zentgraf T, Liu Y, Yin X, Valentine J, & Zhang X (2011). A carpet cloak for visible light. Nano letters, 11 (7), 2825-8 PMID: 21619019

[4]  Zhang B, Luo Y, Liu X, & Barbastathis G (2011). Macroscopic invisibility cloak for visible light. Physical review letters, 106 (3) PMID: 21405275

[5] Chen X, Luo Y, Zhang J, Jiang K, Pendry JB, & Zhang S (2011). Macroscopic invisibility cloaking of visible light. Nature communications, 2 PMID: 21285954

//featured image: screen capture from the video from University of Texas

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