Invisibility Cloak – every Harry Potter’s fan dream. Until very recently invisibility was possible only to imagine, but science shows us that our dream might come true. How? The answer below!

How to take control of the light?

One way to achieve invisibility is… to make a ring road for light around us. Then, as in Fig. 1, light rays will be guided around the red sphere [1]. If light rays don’t reach the red sphere at all, then we don’t see the sphere. Light rays are the same before and after passing the object. On the paper it seems very easy, so what’s the catch?

The problem is that it is very hard to make a material with such properties that will guide light as presented in Fig. 1. . Firstly, the material itself shouldn’t be visible. It should be colorless. But that’s not sufficient, because e.g. glass is also colorless and still visible. How? When we look through a window in the night and we have our lights on, then we clearly see that the window reflects that light. Therefore Invisibility Cloak must be made of a material which doesn’t reflect light. However the most important feature of the cloak is that it should bend the light around the cloaked object.

Bending the light.

We need now to consider one of optics law – Fermat principle. It states that a light ray moves through a path, which is the fastest one. We have thus said correctly, that we need a ring road for light rays. We need to make such material, for which the way around the object is faster than the way through the object.

There is a quantity in physics called refraction index. It informs us, how tough is a road for light. In the air the refraction index equals 1. It means that light moves with typical speed of light in vacuum (300 000 km/s). The bigger is a refraction index, the slower is the light. We can imagine that refraction index 1 is a straight road with no difficulties, whereas refraction index 1,6 is a walk through marshes with 1,6 times lower speed. Therefore interesting materials for us are materials with refraction index less than 1, where the speed of light is higher than… speed of light in vacuum.

It is not sufficient to use just any material with refraction index less than 1. That’s because if we would like the light not to be reflected, we need to handle it with kid gloves. In other words, we need to cautiously and precisely control the refraction index at each point in the material. It is possible since the invention of metamaterials.

Metamaterials

What is light? Light is an electromagnetic wave. It means that it has an electric part and a magnetic part. So let’s play as electrician. Let’s see whether we can with well-designed circuits affect those electric and magnetic parts of light.

• 

• 

Indeed, it is possible and it is how metamaterials work. Examples of the first metamaterials are presented in Fig. 2. [2] [3]. They are nothing more than arrays of electric circuits. For such materials it is possible to get refraction index less than 1 or even negative!

By the way, what is the meaning of a negative refraction index? Does it mean that light has negative velocity? Does it go backwards? It’s not that simple, but luckily it is relatively easy to imagine. Light does something close to moonwalk: It moves forward all the times and only it seems to move backwards.

Coming back to invisibility cloak, a prototype from 2006 is presented od Fig. 3. Unfortunately it doesn’t apply to basic rule of invisibility – it is visible. However it is designed to be invisible for microwaves. In fact even for microwaves it is partially visible, nonetheless it is a first step towards true invisibility.

What differs microwaves from visible light? Both are electromagnetic waves, but microwaves are much longer waves than visible light. So if circuits measuring a few millimeters affect microwaves, then what affects much shorter visible light should be much smaller circuits – circuits in the nanoscale.

Here comes nanotechnology, which is capable of making such nanometric circuits. In effect now we can make metamaterials affecting visible light. Maybe in future it will be possible to be invisible using such highway for light?

Any other solution?

This isn’t the only suggestion from scientific world on invisibility. Science proposes at least 3 other possibilities. Those ideas will be presented in next posts. We plan to publish next post in 2 weeks! Let us know whether you like this post in comments and don’t hesitate to ask a question! We encourage you to discuss. And if you’d like to see our everyday life, see our Instagram (zakrecone_loczki_dwa).

Bibliography

[1] Pendry, J. B., Schurig, D. & Smith, R. Controlling electromagnetic fields. Science, 312, 1780–1782 (2006)
[2] Shelby RA, Smith, DR, and Schultz, S, Experimental Verification of a Negative Index of Refraction. Science, 2001, 292, 77- 79.
[3] Smith D.R. et al. (2001) Left-Handed Metamaterials. In: Soukoulis C.M. (eds) Photonic Crystals and Light Localization in the 21st Century. NATO Science Series (Series C: Mathematical and Physical Sciences), vol 563. Springer, Dordrecht
[4] D. Schurig, et al., Metamaterial Electromagnetic Cloak at Microwave Frequencies, Science 314, 977 (2006)