Metamaterials. Engineering the structure of materials to discover the unexplored and unexpected

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1 Metamaterials Engineering the structure of materials to discover the unexplored and unexpected First theorised by Victor Veselago in 1968[1], metamaterials are about fine-tuning the structural make-up of materials to give these materials properties not usually found in nature. Common examples of such characteristics include: negative refractive index and negative mass density. This article gives a brief overview of this topic with some examples. Setting the scene From the start, it should be noted that with metamaterials, the focus is on engineering the macro structure of a material to exhibit certain properties, usually not found in nature. Particularly, the focus is on two keynote properties: permittivity ε and permeability µ, properties which are used to divide metamaterials into 4 categories[2]: Double Positive (DPS) - both ε and µ are positive. Examples of such materials are dielectrics Epsilon Negative (EPS) - ε is negative but µ is positive. Plasmas occurring at certain frequencies can fall under this category Mu Negative (MNG) - ε is positive, but µ is negative this time. Sometimes DPS and EPS are referred to under Single Negative (SNG) metamaterials. Double Negative (DNG) - finally, both ε and µ are negative. Materials with these parameters have not yet been discovered in nature and hence can only be achieved artificially. With regards to the type of metamaterials, they can also be divided into different groups, where their use for a certain discipline is more obvious and examples include; electromagnetic (mostly for optical uses), acoustic and mechanical. Since it is their substructure component which gives them their distinguished properties, the substructure can be made to reveal the properties at certain wavelength and hence, certain frequencies. As such, some metamaterials may only work for certain frequency ranges, for example terahertz frequencies or tunable metamaterials, are capable of adjusting performance depending upon the background. There is also a category of metamaterials which exhibit negative mass density or bulk modulus. Aircraft invisibility cloak - a future 'must' for accessories in technology?

$Our secondary school physics tells us that light entering a denser transparent medium will be refracted off its trajectory towards the normal by a given amount, depending on the medium in question$

2 Our secondary school physics tells us that light entering a denser transparent medium will be refracted off its trajectory towards the normal by a given amount, depending on the medium in question and its refractive index (see the glass on the left hand side in the picture below, which exhibits a positive refractive index). Most materials in nature have a positive refractive index, but metamarials that fall under the DPS category have a negative refractive index. For this reason, metamaterials are heralded as the next big thing in the field of optics and photonics. For those who know their optics and theory of magnetism well, they can see that it all gets turned on its head; Snells law still applies, but the wave is refracted on the same side of the normal as it entered, Cherenkov radiation points in the opposite direction and electromagnetic waves obey a Left hand rule. Figure 1 For illustration purposes only, the glasses on the left hand side and right hand sight represent a positive refractive index and a negative refractive index respectively. To make the analogy easier, the straw can be thought of 'a wave of light' However, in metamaterials, a lot of attention is paid to the micro scale design and manufacture. Treating light as a wave which passes around an object, the object's ability to propagate a magnetic and electric field around it has an effect on the refractive index. The sub-wavelength structure will have special capacitive and inductive properties which, when light will pass through, will 'bend the light waves' as seen in the glass on the right hand side in the picture below. The ultimate goal of many areas of research is to develop an invisibility cloak capable of fooling the human eye after proof of principle was achieved in 2006 by Duke University North Carolina [3]. The question was then naturally extended whether it would be possible to use this technology for advanced stealth technology; to make combat and reconnaissance aircraft invisible. It would be possible to coat the aircraft with a layer of metamaterial possessing negative refractive index.

3 Not just to be unseen, but to be unheard... Another application of metamaterials is in aeroacoustics, by mathematical analogy from electromagnetic waves. Noise cancellation or concealment is not a new idea, but an example of yet another potential application of metamaterials. Considering the following 1D infinite length chain of mass-inmass system connected by linear springs: Figure 2 - Huang and Sun study on negative mass density occurring at micro scale in materials [7] Here, the external mass is ring shaped and has a solid internal mass attached through the spring. Near the resonant frequency of the internal mass, the wave decay and hence oscillation, exhibits special behaviour because the effective mass becomes negative. By Newton's second law (force is equal to the mass of an object multiplied by acceleration) a negative mass means opposite acceleration, and so a decrease in the amplitude of the wave oscillation. Consequently, it would be possible to use this approach for acoustic shielding by breaking the mass density law, allowing sound attenuation at sub wavelength scale. Nonetheless, there is still work to be done to mature the technology as a lot of tests have been done for only 1 frequency [4] or a small bundle of frequencies and of course, the frequency of noise can have wider ranges. Secondly, the concept of acoustic concealment through such mass absorbers is dependent upon the medium considered; treating airborne noise as opposed to noise propagating through aqueous media is more difficult, studies done under water are still helpful [4]. There are examples of studies which tests for larger frequencies and where the so-called 'acoustic cloak' can be fine-tuned to match the frequencies of noise better. Such an example is a 2D cloak made of 16 concentric rings of acoustics circuits, structured to guide sound waves. Each individual ring has its own index of refraction so when the sound wave travels through, the speed of the wave sound is gradually decreased through each ring [5]. Whilst this example from the University of Illinois is encouraging, and the demonstration was done under water, it shows nonetheless it is possible to adapt the acoustic cloak to a wider range of frequencies. As an additional bit of information, a problem arose when trying to conceal the noise behind underwater propeller blades, as the water bubbles could affect the performance of the cloak. Finally, for airborne noise, a study at Duke University, North Carolina, in the United Stated, proposes the use of actively controlled acoustic concealer [6]. An active device, as opposed to a passive device, requires extra power to be functioning whereas the passive device needs additional power. But the advantage of an active one is its flexibility in response to different background conditions, through the use of a feedback loop. Embedding electronic circuits in metamaterials for acoustic shielding is advantageous because the sound waves are slower than the response time of the circuit, which guarantees fast response.

4 Some applications considered here may have solutions or alternatices which do not involve the use of metamaterials, such as tuned mass absorbers or use for structural health monitoring (SHM) where a careful consideration of the lay-up method of the composites sheets can yield nonmetamaterials based solutions for the same problem. However, this article seeks to inform the reader on a general level about what metamaterials are and what the future holds for this fields. The concepts are not new, but it is acknowledged that a lot of work is still needed to mature the present technologies, especially on the manufacturing challenges. Once this aspect has been developed in greater detail, one could see these technologies incorporated in real life complex applications, such as aircraft. Undoubtedly, the idea of making an aircraft invisible (not just radar invisible through stealth technology) greatly excites the most of us. Of course, there are other applicative contexts in which metamaterials can be used, such as seismic, development of solar cells, quantum computing and super lenses. Furthermore, the focus on the substructure of the metamaterials will advance to finer detail i.e. nanometamaterials. One thing is for certain, this is a field in which advancements will grow exponentially in the future and where your career can make a significant contribution, which definitely will not be unnoticeable! Article was compiled and edited by Alexandra Stefanescu, RAeS YPC Newsletter editor. Many thanks for the help of Prof. Fabrizio Scarpa of the University of Bristol with providing references and information. The contribution of James Wood-Fisher, student at the University of Surrey, to writing the optics part of metamaterials is also acknowledged. REFERENCES: [1] V. G. Veselago, The electrodynamics of substances with simultaneously negative values of epsilon and nu, Sov. Phys. Uspekhi, vol. 10, no. 4, [2] S. E. Mendhe and Y. P. Kosta, Metamaterial properties and applications, Int. J. Inf. Technol. Knowl. Manag., vol. 4, no. 1, pp , [3] D. Schurig, J. J. Mock, B. J. Justice, S. A. Cummer, J. B. Pendry, A. F. Starr, and D. R. Smith, Metamaterial electromagnetic cloak at microwave frequencies., Nov [4] L. Zhao, B. Liu, Y. Gao, Y. Zhao, and J. Huang, Enhanced scattering of acoustic waves at interfaces, Front. Phys., vol. 7, no. 3, pp , Jul [5] Newly Developed Cloak Hides Underwater Objects From Sonar - US News and World Report. [Online]. Available:

5 [6] B.-I. Popa, L. Zigoneanu, and S. a. Cummer, Tunable active acoustic metamaterials, Phys. Rev. B, vol. 88, no. 2, p , Jul [7] H. H. Huang, C. T. Sun, and G. L. Huang, On the negative effective mass density in acoustic metamaterials, Int. J. Eng. Sci., vol. 47, no. 4, pp , Apr

Electromagnetic Absorption by Metamaterial Grating System

PIERS ONLINE, VOL. 4, NO. 1, 2008 91 Electromagnetic Absorption by Metamaterial Grating System Xiaobing Cai and Gengkai Hu School of Science, Beijing Institute of Technology, Beijing 100081, China Abstract