The human fascination to understand, control and manipulate light is as old as the man itself. First humans may have made campfire shadow art by obstructing the light with their hands to make shadows of the animals they’ve seen. Today, we create practical holograms and project them in to the thin air to make objects appear almost real with clever optical techniques. Tomorrow, a scientist would bend the light around a material to make an object completely invisible as if it is not there anymore. Widening of our understanding about the matter and our abilities to control it at the minute of the scale has given us the power to control light in ways that we never imagined possible.
Among all the things people have imagined to do with light, cloak of invisibility; a material that can wrap something with to make that object disappear, is indisputably the Holy Grail. Today thousands of scientists all over the world work towards this goal. They know that the ability to make real invisibility cloak would be a paradigm shift in our way of doing things, and would have immediate practical applicability in many areas including, telecommunications, military and photonics. But how close are we to make a real invisibility cloak. This would be the question of the day!
How to make something disappear?
In contrast to the common belief, we see things with our brain. Our eyes only does the role of capturing light waves that are coming from different directions, make it in to an electronic signal and feed it consistently in to the brain. Our brain, analyzes the constant flow of signals, filter out most relevant to the situation, and makes a “translation” of the world around us. This means that, we do not see a real representation of external reality, but only a “perception” formed by our eyes and brains. This brings up an interesting point. If we can trick our eyes to see the waves of light in a special way we can simply convince our brain to see an object appear or disappear. Our brains always see an object relative to a background. It’s usually a difference of contrast or depth or both that usually triggers the brain differentiate an object from the background.
It’s not uncommon for few people to knock their heads on a glass door in a hotel. This is a one form of invisibility. Glass door-although hard solid material-doesn’t distort or absorb light waves coming from the background. However, if the glass door is made from a cylindrical block of glass, light waves will be subjected to distortion due to refraction, and immediately the door will be more noticeable. This essentially means that as long as an object doesn’t distort the light waves coming from the background, it will remain invisible, despite of its physical existence.
Unfortunately, the world we live in is not a one made of transparent, sheet like Items. Objects in the environment would, absorb, reflect or refract light coming from the environment, making the object clearly visible. The only way to make these kind of materials invisible is to bend the light around an object. But there’s a small problem. No any conventional material can bend light in this way. It turns out that, to bend the light like this the refractive index of the material need to be negative. But all the conventional materials have positive refractive indexes. The refractive index of air is “1”, and all the other conventional materials have refractive indices higher than 1. Then there’s only one way to make an object invisible. That is to wrap it with a material which can bend the light, thus giving the name; invisibility cloak.
These special type of materials are known as “metamaterials”. By their same definition, these are man-made materials that can transcends the optical properties of the conventional materials. Unlike the conventional materials these materials can bend the light in the wrong way and force it to curve around the object that need to be made invisible. These materials would behave in very bizarre ways that if we somehow make a metamaterial in a liquid form, a straw in that liquid would look as shown in the below picture.
However, the making of a metamaterial that can bend electromagnetic radiation in the visible range is a quite challenging task. To do this, one would have to play with electromagnetic waves and light, and their wavefronts and directions of propagation by introducing appropriate geometries. This require quite small yet complex assemblies of micro and nanostructures, that require intricate manufacturing techniques which still remains as a challenge.
Therefore, the first actual metamaterials were experimented in microwave range in electromagnetic spectrum which has wavelengths in centimeter level. This allowed, scientists to cope with the complexity of the metamaterial manufacturing. The metamaterial was constructured as a series of unit cells with split ring resonators and conductive wires. The unit cells were in the size range of 10 milimeters which is smaller than the wavelength of the waves tested. Still most of the research is on metamaterial cloaking in microwave or radiowave dimensions.
The metamaterial cloak that works for visible light
With the invent of technologies to manipulate matter at the nanoscale, scientists can now build structures with more complicated assemblies. This ability has given the metamaterial research a new boost, as with these techniques, structures that can actually bend electromagnetic wavelength in the visible region is possible.
The closest, most influential work on metamaterial cloaking in visible scale was carried out by group of scientists from University of Central Florida. They have fabricated a fishnet like structure that composed of metal and dielectric composite films. More interestingly, the material is fabricated using a nanotransfer printing method, a readily scalable technique for nanomaterial production. The three dimensional metamaterial fabricated at UCF lab allowed the control of the electromagnetic radiation in the visible region, a feat that was not achieved before.
Many researchers agree that invisible cloaks are to stay at the lab benches for a long time. They have numerous practical implications that needed to be solved before implementation in any useful applications. So if you were waiting for an invisible cloak soon to see what your employees do when their boss is away. It’s probably a good idea to find some other way.