By now, just about everyone has seen the headlines announcing the invention of invisibility cloaks, or at least some simple prototypes. Many research groups have been working on this for years with a wide range in success and cloaking method. These news articles always include three things. First, they throw out great-sounding words like metamaterials. Then, they show a few colorful pictures, and if you are lucky, they might even be of something related to the experiment. Lastly, it seems they are required to make at least one Harry Potter reference.

Now I have no problem with Harry, but what most curious people (which includes you if you are taking the time to read this) want to know from the articles is what these devices are made of and how they work. Luckily for you, a few years ago, I happen to have done some theoretical research on the very materials that are now making invisibility real, and am going to tell you a bit about them.

Before you can understand what is new, you must first understand what was known before it. I will start with a small about of background and I promise no math!

When light travels from one medium, or substance, to another it will likely change directions. The amount the light bends is based on the color (wavelength) of the light and the physical properties (index of refraction) of the 2 mediums. The bending of light in this way is called refraction. The first picture shows a laser bending as it passes from air to glass and back out of the glass into the air. The second image is a diagram showing how light is bent as it passes from one regular material to the next. There is a simple formula that you can use to calculate how much the light will bend called Snell’s Law, but I promised no math so I provided the link instead.

[1] 

Scientist have measured the index of refraction of everything they could get their hands on, and it has always been found to be positive. Everyone thought they knew what was going on and everyone was happy up until about 45 years ago. Victor Veselago (below) hypothesized in 1968 that a material with a negative index of refraction could exist. He referred to these materials as “left-handed” because the wave vector is antiparalell to the usual right-handed system. You can’t explain the left vs right handed without math so you can either just accept it (boring) or do a bit of reading on the subject and you might just learn something (awesome).

So what could really be changed by just changing one number from positive to negative? It turns out, a whole bunch! Now this article isn’t about how you could use one of these left handed materials (LHM for short) to make a magnifying glass so powerful and clear that you could see individual atoms or that you use LHM to allow you to pull things with light like a tractor beam. They could even allow a ship with a solar sail to return to its point of origin, but we are talking about cloaking devices so I will leave the others to you to look up or future blog posts.

In the above diagrams you can see how light behaves when moving between right handed materials (RHM) and left handed materials (LFM). Notice how in the RHM to RHM the light travels up and to the right in both blocks of material. When light passes from RHM to LHM the light starts going up and right but then turns to travel down and right. Being able to turn the light in this fashion is what allows scientist to build a devise that can bend light around an object.

The above diagram shows how bending light around an object makes in invisible. As the rays of light (red lines) from the red light source come in contact with the blue cloaking device the rays of light are bent around the object and come out the other side. On the back side of the device the rays of light are moving in the exact same path as they would have been traveling if nothing had been there at all. This means that anything inside the gray area in the diagram would be invisible to any outside observer. Below is a picture of actual data from a working cloaking device. The light source in this case is an antenna giving off microwaves (light with a wavelength too long for you to see with your naked eye). The white circle is a metal disk. In image (c) you can see the strong shadow made by the disk. In image (d) they added the cloaking device (white circle) around the metal disk and the shadow is almost completely eliminated, making it almost invisible.

The reason we have not heard about cloaking devices before now is that LHM do not exist in nature. You can not even make them by mixing chemicals, you must build them. LHM are just one type of a class of material called metamaterials. Metamaterials are a structure built (usually of mulitple elements) so that their shape and physical design changes how they react to different things. In LHM they are generally made by imbedding metal rings or strips inside a insulating material in very precise patterns.  Below left is a picture of a cloak that is similar to the one used in the data shown above. The picture to the lower right is a cube of metamaterial. You can see in both that they consist of very small metal wires that are head by an insulating support structure.

             [4]

We still have a long way to go before we will be able to walk down the street unseen. It is very difficult to make any metamaterial that will work on visible light, as it requires the metal designs in the material to be very very small. The materials built so far are also stiff and need to keep a fixed shape in order to work. This means that we will likely have invisible buildings and vehicles before we have something we can wear.  Before you ask why we need invisible buildings, we are not too far from making buildings invisible to cell phone signals, which would mean that you could get cell reception when downtown. We could even use the technology to make sure you keep cell coverage inside elevator or tunnels.

I hope this shines a bit of light on (or around) this fascinating subject.

[1] “Chemical Paradigms – Unit 2 Refraction of Light.” Chemical Paradigms – Unit 2 Refraction of Light. N.p., n.d. Web. 20 June 2012. http://chemicalparadigms.wikispaces.com/Unit 2 Refraction of light.

[2]M. Selvanayagam and G.V. Eleftheriades, “Experimental verification of the effective medium properties of a transmission-line metamaterial on a skewed lattice”, IEEE Antennas and Wireless Propagat. Letters; Special Cluster Issue on Metamaterials,                       vol. 10, pp. 1495-1498, 2011

[3]M. Zedler and G.V. Eleftheriades, “Anisotropic transmission-line metamaterials for 2D transformation optics applications “,

Proceedings of the IEEE, vol. 99, pp. 1634-1645, Oct. 2011 (invited).

[4]Shadrivov, I. V., and Yu S. Kivshar. “Left-Handed Metamaterials.” Left-Handed Metamaterials. Nonlinear Physics Centre, n.d. Web. 20 June 2012. <http://wwwrsphysse.anu.edu.au/nonlinear/research/lhm/&gt;.

 

–Jay Murphree

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