“Ah, your father happened to leave it in my possession and I thought you may like it.” Dumbledore’s eyes twinkled. “Useful things… your father used it mainly for sneaking off to the kitchens to steal food when he was here.”

-Harry Potter and the Philosopher’s Stone

These lines are taken from J.K. Rowling’s popular series of novels, Harry Potter, to introduce the invisibility cloak, which generated curiosity among readers regarding the existence of such a material. As we know, the quest to develop something apparently unreal is one of the amazing features of the human brain. Invisibility did not remain unexplored, and thuscame the idea of “Metamaterials”. Metamaterials are artificial structures. They are composed of basic unit cells which comprise of metallic resonators, unlike natural crystals in which the atoms are found at lattice points [1]. The presence of these metallic resonators at lattice points gives the materials some characteristic properties. Generally, the metallic resonators are made up of split ring resonators (SRR) and thin wire structures. The materials from which SRR and thin wire structures are made have no special properties individually (i.e. their permeability and permittivity are greater than zero), butas a whole it is the arrangement of the SRR in the lattice of a metamaterial that gives rise to its unique properties. In other words the metamaterial gains its properties from its structure rather than inheriting them directly from the material it is composed of. Usually optical metamaterials are artificial periodic structures with lattice constants (edge length of unit cell) that are much smaller than the wavelength of the incident radiation. [1,2]. Why are these materials special? The concept of a negative refractive index comes into play, which is very uncommon, and hence makes these materials special. Snell’s law gives the definition of refractive index, which depends on the permittivity and permeability of the material medium. Thus, if we can somehow control these factors, then we can prepare materials with different refractive indices. This is possible by manipulating the basic unit cell structure of the materials so that their permittivity and permeability change, which in turn changes the interaction of the EM (electromagnetic) radiation with the unit cell [2]. Metamaterials can be characterised using the modified Maxwell’s equations [2] (where the symbols carry their usual meanings);

∇ × E = -j⍵μH ∇ × H = J + j⍵εE ∇ . D = ⍴ ∇ . B = 0

Figure 2 - The effect of Snell’s law on positive (normal materials) and negative refractive index (metamaterials). Note that the angle of refraction is negative in case of the metamaterials Applications of metamaterials include: [2] As cloaks Cloaking is the process of shielding something from view by controlling electromagnetic radiation. Cloaking can be achieved by making the electromagnetic field zero in that particular region to be cloaked. This can be acheived if we can somehow transform the coordinate system which would guide the wave around the region (transformation optics). Due to their peculiar optical properties, metamaterials can be used to effectively guide the waves and thus achieve cloaking. As antenna The zero refractive index metamaterials can be used to make high directivity antennas. By using metamaterials as antenna we can increase the bandwidth, reduce the antenna size as well as increase the antenna radiation efficiency. As absorbers In the fast growing field of electronics, metamaterials absorbers are preferred because they are thin, lightweight and do not require the use of expensive materials. They can also be used over a wide frequency range. Presently, metamaterials are under intense investigation. They have a wide range of technological implications. However, we can surely say that these materials can have a good impact wherever electromagnetic radiation is used, and they can play a key role in the next technological revolution.

References

1. Marek S. Wartak, Kosmas L. Tsakmakidis and Ortwin Hess, Introduction to Metamaterials by , 30 C PHYSICS IN CANADA / VOL. 67, NO. 1 (Jan.-Mar. 2011)

2. Kaushal Gangwar, Dr. Paras and Dr. R.P.S. Gangwar Metamaterials: Characteristics, Process and Applications, Advance in Electronic and Electric Engineering. ISSN 2231-1297, Volume 4, Number 1 (2014), pp. 97-106.