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| Fig. 1: Photonic crystal. - (Source: Wikimedia Commons) |
From cloaking devices on the Starship Enterprise to Harry Potters invisibility cloak, humanity has longed for a way to mask objects and ourselves. Up until recently, the capability of making a material thing invisible has been the stuff of science fiction and magic. While most people still may only find themselves transcended into this invisibility realm by watching Wonder Woman in her plane or some other imaginative film, the idea of invisibility had been taking form even before the 1990s and John Pendry's work at the Imperial College London working with transformation optics, a discipline that looks at how we can transform grids of space. As early as 1898, Indian scientist Jagadish Chandra Bose penned a paper that dealt with the effect of a man-made twisted structure on the polarization of an electromagnetic wave. [1] Bose was a pioneer in electromagnetic theory with research in what we would call microwaves and millimeter waves and a forerunner, therefore, in transformation optics. But in order to attain invisibility a new science had to emerge with metamaterials. The problem is that with anything that seems too good to be true and magical, there is hype associated with it. First we will look at the advent of the field and then at the metamaterials that have actually been created. Metamaterials, unnatural materials are not especially easy to create and so far the work done has been crude.
Metamaterials are a class of material that has been engineered (on a microscopic scale in the hopes of progressing to larger scales) to produce properties that don't occur naturally. In the case of optic transformation, metamaterials allow us to manipulate electromagnetic fields and light by bending around the object. The concept is analogous to water flowing through a stream when faced with a tree or a rock in its path. The water, a naturally occurring material, flows around the tree and the streamlines that ran around the tree become parallel after the tree, so the observer on the other side only sees the stream. Metamaterials allow us to do to light was water does naturally - bend around an object so the observer behind the object does not see it. Metamaterials allow the light rays to bend smoothly around an object placed in the center of its shell and recombine on the other side. In order to do this, a negative index of fraction is needed. An index of refraction of a material is a number that conveys how light propagates through the material. For example, the index of refraction of water is 1.333. This means that light passes through a vacuum 1.333 times faster than in water. If the index of refraction of a material was negative, then light would not pass through it at all but rather would be redirected. This redirection of light is theoretically how metamaterials can make invisibility possible. [2]
The introduction of metamaterials caused the field of electromagnetism to be shaken to its core - and built up again into an exciting science that is hoping to develop into areas as diverse as military and medicine in helping mankind. The beginning of the insurgence regarding metamaterials occurred in 1996, when Pendry et al. realized an artificial electric plasma using a wire medium with negative permittivity. By 1999, the researchers discovered the artificially magnetic plasma with negative permeability, whereupon split-ring resonators (SRR) were used to gain magnetic response. [2,3] The first artificial metamaterial of its kind was created by Smith et al. in 2001 with wires and SRRs. [2,4] Thus, the negative refraction phenomenon resulted from this experiment. Because, however, this first incarnation of a metamaterial suffered from a narrow bandwidth, the physics community sought other elements or features beyond negative refraction. These arrived in 2005 and 2006, respectively, with the gradient refractive index medium bending electromagnetic waves and optical transformation in invisibility cloaking by using metamaterials to control propagation of electromagnetic waves. [2] From here on out, there was an incredible interest in this new element, this new field of metamaterials with different disciplines asking how they might utilize metamaterials.
But exactly what are metamaterials? Rodger M. Walser, University of Texas at Austin, in 1999, originally defined this new material as "... macroscopic composites having a synthetic, three-dimensional, periodic cellular architecture designed to produce an optimized combination, not available in nature, of two or more responses to specific excitation." [2] The birth of the term coincides with the advent, therefore, of the discipline and first appeared in a paper by Smith et al.), that explained the existence of man-made/artificial materials that consisted of negative permeability and permittivity but can also be described as artificial media with unusual electromagnetic properties [1.4] To really understand a metamaterial, we must first look at natural materials. Natural materials are made up of small elements - atoms and molecules - that are amorphous (random) or crystalline (patterned). With artificial materials, we can replace the building blocks with macroscopic, engineered elements, allowing a freedom to choose the size and the patterning. Essentially, 1. Metamaterials are engineered composites that exhibit superior properties not found in nature and not observed in the constituent materials. 2. A metamaterial is an artificial material in which the electromagnetic properties, as represented by the permittivity and permeability, can be controlled. It is made up of periodic arrays of metallic resonant elements. Both the size of the element and the unit cell are small relative to the wavelength. [1] Another definition more clearly expresses this unique material: Metamaterial artificial media structured on a size scale smaller than the wavelength of external stimuli. Whereas conventional materials derive their electromagnetic characteristics from the properties of atoms and molecules, metamaterials enable us to design our own atoms and thus access new functionalities, such as invisibility and imaging, with unlimited resolution. [5]
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| Fig. 2: Refraction in a left-handed metamaterial. - (Source: Wikimedia Commons) |
As previously mentioned, metamaterials arent easy to make. The properties of metamaterials determine the electromagnetic field. An example of this is shown in Fig. 2, which illustrates refraction in a left-handed metamaterial to that in a normal material.
Metamaterials are classified in four groups on the basis of permittivity and permeability. There are Double Positive (DPS) materials with both permittivity and permeability greater than zero; Epsilon Negative (ENG) Material with permittivity less than zero and permeability greater than zero (seen in many plasmas); Mu Negative (MNG) Material where permittivity is greater than zero but permeability is less than zero (some gyro tropic materials); and finally, Double Negative (DNG) Material where both permittivity and permeability are less than zero and only can be produced artificially. [6]
After classifications come types. There are several metamaterial types including: Electromagnetic (EM), Chiral, Terahertz, Photonic (see Fig. 1), Tunable, Frequency Selective Surface (FSS) and Non-Linear.
When physicists speak of the idea of having cloaking capabilities, the elusive invisibility cloak, the metamaterial type falls under EM. Scientists are exploring optical and microwave applications. Some of the areas include microwave couplers and antenna radomes. Chiral metamaterials are created by arrays of dielectric gammadions or planar metallic on a substrate, while Terahertz metamaterials are the combination of artificially created materials interacting with THz. Here the exploration revolves around desired magnetic responses. Tunable metamaterials can randomly change the frequency of a refractive index, making it possible to reconfigure a device while operating it. FSS based metamaterials were developed to control the transmission and reflection characteristics of an incident radiation wave. Non-Linear metamaterials permeability and permittivity showcases the response of electromagnetic radiation; they are artificial and nonlinearity exists. Finally, photonic metamaterials are where active research is taking place regarding zero index refraction (ZIMs) and negative values for index refraction (NIMs). Photonics are electromagnetic metamaterials that are created to interact with Optical metamaterials. [6]
In 2006, a team from Duke University led by David Smith, working with Pendry, manipulated light using microwaves and created the first microwave invisibility shield but just two years later, Pendry designed a carpet cloak, that sits on a surface and hides objects under it. In order to do this without microwaves, Pendry looked at shrinking the metamaterial to a size needed for visible light. Essentially, Pendry came up with the idea that a transformation of the positions of the electric and magnetic fields where they would never actually interact with the object. And that was the key. They would instead flow around the object, bending the light, creating an electromagnetic cloak. [7] Through metamaterials we have possibilities regarding the applications for manipulating electromagnetic fields, so its only natural that arrays of applications are at the forefront of all research. But we must remain skeptical because of the sheer differences in metamaterial classifications and types so as not to rush to the idea that we can create an Invisible Man.
Indeed, the scientific community believes that the next stage permutation of the metamaterial explosion will be the development of active, controllable, and nonlinear metamaterials surpassing natural media as platforms for optical data processing and quantum information applications. [5] From technologies utilizing electromagnetic radiation to cosmology, from biotechnology to computers, metamaterials are expected to impact an array of fields, by providing a flexible platform for modeling and mimicking fundamental physical effects as diverse as superconductivity and cosmology and for templating electromagnetic landscapes to facilitate observations of phenomena that would be otherwise difficult to detect. [5]
To that end, one interesting way in which metamaterials may ultimately improve lives isnt through cloaking but in the realm of neurosurgery. A 2014 study looked at graphene-based metamaterials and their possible uses in neurosurgery because of the research that had already been accomplished with it in disciplines as diverse as quantum physics, molecular biology, and energy management, and the necessity for neurosurgery to rely heavily on innovations like this. The study found that Neuro-oncology, in particular might benefit from the use of graphene nanoparticles for tumor-targeted imaging, selective photothermal therapies, and anti-cancer electrical field stimulation. [8] In the area of neuroregeneration research, development is moving forward on a brainmachine interface device with graphene metamaterials that could control and enhance neuroregeneration with electric field stimulation. [8] Electroactive graphene-coated scaffolds are being developed to stimulate neuronal growth in peripheral nerves. Perhaps most exciting is the use of metamaterials for use in spinal situations with the metamaterial being used in instrumentation hardware. [8]
Aviation is also looking at ways to use metamaterials and research is being done with acoustics and aviation. Metamaterials may allow for the creation of sub-wavelength acoustic absorbers, acoustic invisibility, perfect acoustic mirrors and acoustic lenses for hyper focusing. With the ability of zero refractive sound index with the use of metamaterials, scientists can control acoustic patterns and sounds at sub-wavelength scales. The aeronautical community is exploring how metamaterials might help mitigate the noise pollution on communities. [9]
And, of course, the military is looking at how metamaterials may be used to manipulate everything from weapons of mass destruction detection to bending sound with submarines and sonar, while Silicon Valley explores the viability of photonic chips for advanced speed in technology. [6]
Ultimately, however, we remain cautious as the study of metamaterials is truly a new field. However, research is accumulating because of the unique electromagnetic properties of these man-made materials, which means there may be promise for the future.
© Josh Orrick. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] L. Solymar and E. Shamonina, Waves in Metamaterials (Oxford University Press, 2014).
[2] T. J. Cui, D. R. Smith, and R. Liu, Metamaterials: Theory, Design and Applications (Springer, 2010).
[3] J. B. Pendry et al., "Low Frequency Plasmons for Thin- Wire Structure," J. Phys. Condens. Matter 10, 4785 (1998).
[4] D. R. Smith et al., "Composite Medium with Simultaneously Negative Permeability and Permittivity," Phys. Rev. Lett. 84, 4184 (2000).
[5] N. I. Zheludev, "The Road Ahead for Metamaterials," Science 328, 582 (2010).
[6] G. Singh, Rajni, and A. Marwaha, "A Review of Metamaterials and its Applications," Int. J. Eng. Trends Technol. 19, No. 6 ,305 (2015).
[7] S. Tretyakov, A. Urbas, and N. Zheludev, "The Century of Metamaterials," J. Optics 19, 080404 (2017).
[8] T. A. Mattei and A. A. Rehman, "Technological Developments and Future Perspectives on Graphene-Based Metamaterials: A Primer for Neurosurgeons," Neurosurgery 74, 499 (2014),
[9] G. Palma et al., "Acoustic Metamaterials in Aeronautics," Appl. Sci. 8, 971 (2018).
