The concept of an invisibility cloak, that is a device which can completely conceal whatever is under it, has been present in science fiction for many decades, but will invisibility cloaking ever be a reality? Teams of researchers at various institutions such as the University of Birmingham have brought this concept to a reality through the principles of transformation optics. The most common approach is known as carpet cloaking and this is the method that will primarily be discussed. There is both theoretical and experimental evidence for the possibility of rendering an object invisible to an observer. Whilst there is substantial research being done in this field, many developments are still being made. This could mean that the invisibility cloaks we see in science fiction and popular culture, such as the cloak in Harry Potter, could one day be a reality.
Transformation optics uses the fundamentals of electromagnetism and light (i.e. Maxwell’s equation) but uses a set of coordinate transformations which show that a region of space can be rendered undetectable by electrical impedance topography . These transformations can be modelled by imagining a grid representing the potential paths of light rays, if an isotropic distortion in this grid was produced then this would leave voids underneath these distortions. Light would therefore be unable to travel along a path containing a distortion of this type and could be rerouted around any object within this region. It is metamaterials that allow these transformations to be applied to a physical cloak .
The most common method of cloaking is known as carpet cloaking. This conceals the object as a bulging surface which is made to appear flat. The carpet cloak reduces the amount of light scattered by the ‘bulge’ which is what allows this region of space to appear flat. A wide range of incident angles on the cloak will produce total internal diffraction which limits the number of rays that are able to pass through the cloak; this makes it much easier to conceal the rays passing over the object. This therefore leaves the object as an apparent flat surface from the perspective of an observer. Cloaking is more successful when light is polarised as unpolarized light requires the use of anisotropic materials; these materials have a varying refractive index through out and are very difficult to create .
The theory and experimental data support the possibility of cloaks of this kind existing, yet there is no object of this kind available commercially. This perhaps brings into question the credibility of the claims that there has been success in producing a carpet cloak. It is worth noting here that these cloaks are still in relatively early stages of experimental design. So far most successful attempts have not been able to produce cloaking for broadband wavelengths. Only a small range of wavelengths are able to remain hidden from the observer, and often these wavelengths are below the visible range . However, more research is being done on the materials used to produce these cloaks which is improving the range of wavelengths and the ratio of cloak to concealable size of object .
While the principles of cloaking are pretty much the same in all designs the methods of carrying them out do vary. Researchers at the University of Birmingham (and other institutions) designed a very promising cloak using calcite crystals. These crystals were able to reflect light beams in such a way that a raised surface appeared to be flat. This was shown to be effective at a better range of wavelengths than previous attempts, however there are still issues with the design. Imperfections in the alignment of the crystals causes a narrow beam of un-reflected light to pass through the cloak producing a small band of light in the centre. To an observer this appears as small angle distortions (less than 1°) in the surface. Despite these imperfections the cloak was proven to be successful over ranges of 100nm wavelengths in the visible light range of the electromagnetic spectrum .
A team of researchers from multiple institutions came up with a slightly different approach. This team used a silicon on insulator platform (SOI platform) allowing for better cloaking due to the anisotropic property of the material. This cloak is manufactured by etching silicon dioxide onto the surface of a silicon grating which has a reflective surface coated with gold. A cloak of this design interacts differently with electromagnetic waves than a normal object of similar size; this is demonstrated in figure 1 .
This simulated data shows the differences in interactions between uncloaked objects (a) and cloaked objects (b). Not only is this an effective method of cloaking objects it also reduces the amount of energy that needs to be absorbed by the material. The cloaking effect was observed to be effective for a range of 1000nm .
In 2017, another team came up with a new approach to carpet cloaking; they used doped semiconductors or metals at plasma frequencies as nanoparticles to be placed in the cloak. These nanoparticles alter the refractive index of the segment of material that they are in. The cloak is made up of many segments each containing a random distribution of nanoparticles. This study showed that optimal cloaking is achieved when a high number of nanoparticles is used. It was found that cloaks with a refractive index less than 1 will only operate at a single wavelength, however when the refractive index was greater than 1 a cloak that operated in the range 3-5μm was achieved. It was also noted that the use of plasma frequency nanoparticles was no more effective than a typical dielectric at refractive indexes greater than 1.333 .
Carpet cloaking does not only apply to the classic concept of an object making light invisible. Whilst there have been successes in optics and across the electromagnetic spectrum a similar theory can also be applied to acoustic waves. This has been used to investigate underwater acoustic carpet cloaks. Here the aim is to alter the paths of sound waves and hide the sound emitting object’s signal. The principle here however is much simpler, incident waves will be backscattered which will cancel out the signal via destructive interference .
The concept of invisibility cloaking is a very interesting topic that encompasses a range of disciplines within the physical sciences. The theory is very much founded in pure physics using concepts of electromagnetism and optical interactions such as diffraction, however there is a great overlap with materials science as it has been established that the best way to realise an invisibility cloak is through the use of metamaterials. The applications of the carpet cloak are numerous as we have already seen that this concept is not exclusive to optics. This type of cloaking could be very useful outside of its applications within physics. In particular both acoustic and optical carpet cloaks could have very useful applications for the military as they could provide a means to hide a source of both acoustic and optical signals.
To conclude it is fairly clear that invisibility cloaking is not entirely science fiction. Successful cloaks have been produced by multiple independent teams using the carpet cloak method. While the methods vary these cloaks are widely based on diffraction and reflection of incident waves in order to conceal the source. It has been demonstrated that carpet cloaking is possible using crystal structures, silicon on insulator platforms and plasma frequency nanoparticles. At present these cloaks have only been successful at small ranges of optical frequencies however researchers continue to investigate the possibility of a broadband range invisibility cloak. It is worth noting here that these invisibility cloaks are not quite the same as those in science fiction, whilst carpet cloaks are able to conceal objects under a surface which is observed as flat, current research does not indicate the production of a cloak that will make the wearer seem to disappear. Nevertheless, this is a very exciting concept that continues to be researched by large collaborations of scientists. Small range carpet cloaks are not science fiction, although difficult to manufacture, they are very much a reality. Whilst a broadband range cloak is yet to be developed this research looks very promising. It is therefore apparent that invisibility cloaks are not merely science fiction; in the near future they may well be a widely known reality.
Bibliography and references
(1) Chen X, Luo Y, Zhang J, Jiang K, Pendry JB, Zhang S. Macroscopic invisibility cloaking of visible light. Nature Communications 201 0;2(1):176.
(2) Xu L, Chen H. Conformal transformation optics. Nature Photonics 2015 Jan;9(1):15-23.
(3) McCall M, Pendry JB, Galdi V, Lai Y, Horsley SAR, Li J, et al. Roadmap on transformation optics. Roadmap on transformation optics 601 0;20(6):37-38.
(4) Zhang J, Liu L, Luo Y, Zhang S, Mortensen NA. Homogeneous optical cloak constructed with uniform layered structures. Optics Express 425 0;19(9):8625-8631.
(5) Khosravi S, Rostami A, Dolatyari M, Rostami G. Broadband Carpet Cloak Designed Using Nanocomposite Metamaterials for 3–5-μm Wavelength Range. Nanotechnology, IEEE Transactions on 1701;16(1):44-48.
(6) Bi Y, Jia H, Sun Z, Yang Y, Zhao H, Yang J. Experimental demonstration of three-dimensional broadband underwater acoustic carpet cloak. Appl Phys Lett 528 0;112(22).