The field of metamaterials has come a long way and will soon be standard design technology in many of the devices that we use every day. This review covers the early history of the field, a simplified illustration of the basic science, the various techniques being researched currently, summaries of some close to market technologies, and finally some blue sky thinking that truly captures the imagination.
Background and History
Metamaterials are a new class of designer electromagnetic materials with properties that cannot be found in nature. In 2000, Professor John Pendry at Imperial College London, published the theoretical basis of an “invisibility cloak” that could bend light by Negative Refraction.
At the time it sounded like something out of the Harry Potter franchise and was even covered on the BBC. Since then, this technology has moved into the mainstream: we now know that a material can overcome some of the basic laws of physics such as refraction and conductance if it is reengineered at a scale smaller than the wavelength of the electromagnetic wave.
The Basic Science
The optical analogy is the easiest way to understand the fundamentals of this field. As we all know light is not a ray but an electromagnetic wave. All EM waves are composed of electrical fields that oscillate up and down and magnetic fields that oscillate at 90 degrees to the electrical field in a circular fashion. The wave spectrum comprises radio waves, microwaves, infrared waves, optical waves, ultraviolet waves, X-rays and gamma waves in decreasing order of wavelength. This is summarised in Figure 1 where we can see that lower energy waves have longer wavelengths. This is an important concept and underpins the science of metamaterials.
Now, let us examine the concept of “Negative Refraction”. Refraction itself is the manipulation of EM waves as they travel from one object to another, changing its direction . The relationship between electrical and magnetic fields is elegantly encapsulated by the simplified formula:
Here ‘n’ is the index of refraction and ‘ε’ and ‘µ’ are properties of the material through which the wave passes. Epsilon (ε) represents the electric response or permittivity and Mu (µ) the magnetic response or permeability. In most naturally transparent materials, both of these are positive. If these properties of permittivity and permeability can be modified, a negative refractive index material can be created. This concept was first described by the Russian scientist, Victor Veselago in 1968. A material with ε and µ < 0 does not exist in nature, mainly because the resonance frequencies of electrical waves and magnetic waves are so different and do not usually correspond. However, it can be synthesised by applying a specific electromagnetic field with a series of circuits across the waveform so long as the circuits are smaller than the EM wavelength. This has to be very small, indeed, but with modern nanotechnology, is now achievable. Figure 1 demonstrates that this process would be easiest with longer wavelengths.
The early days
The first prototype metamaterial worked primarily with microwaves. They were easier to manage since the wavelengths ,positioned between infrared and radio waves , were relatively large and well within engineering capabilities in 2000. In a ground-breaking experiment, Professor David Smith created the first metamaterial. Using tiny electrical circuits embedded in a conductive material, he was able to modify the Epsilon and Muin real time and demonstrate that such a material was conductive. Figure 2 is a recent example of such an electromagnetic metamaterial. The adjacent diagram demonstrates the difference between normal refraction (represented by red lines) and negative refraction (represented by the green line).
The metamaterial revolution then moved onto the world of optics. Figure 3 demonstrates how an engineered material can have optical capabilities similar to a glass lens and yet be many orders of magnitude smaller than a conventional lens. Recently, a team from Harvard University created a “Metalens” with titanium dioxide nano pillars and many experts believe that this “perfect lens” may have valuable applications in the near future . The visible spectrum of light has many wavelengths and hence each wavelength will need its own corresponding nanopillar as shown by Figure 3 bottom right image, making it a highly complex fabrication process. In comparison, monochromatic lenses are simpler and depicted in Figure 3 bottom left image.
In Figure 4, we see examples of engineered nanomaterials contrasted with the scale of day-to-day objects. A single nanopillar for the optical metalens would be the size of a bacterium and the circuits in a metamaterial for microwaves would be thinner than a strand of human hair. Such tiny objects are created by methods such as electron beam lithography, a technique that uses focused electron beams to etch 3-D structures into a solid material.
Active and Tunable Metamaterials
The metamaterials covered so far are all examples of passive metamaterials which have limitations on the strength of the nanoscale structures and bandwidth. Since each wavelength requires its own complex nanostructure, multiwavelength applications are complicated and bulky as seen in Figure 4. For instance, many experts think that larger objects like a human body cannot be cloaked from multiple wavelengths of electromagnetic radiation with passive metamaterials such as external plating. Tunable metamaterials have electromagnetic properties that can be actively reconfigured and may be a solution to this problem. Here, the unit cells’ functions can be altered by thermal, optical, electrical or geometric tuning. Despite the higher fabrication costs, these products are much more compact. For example, Germanium antimony telluride is a compound capable of thermal tuning. It changes from crystalline to amorphous states at different temperatures and has been used to create a composite metamaterial with a quartz base exhibiting unique optical properties . Figure 5 is a diagram of a liquid crystal-based metamaterial that incorporates Frequency Selective Surfaces (FSS) to tune specific wavelengths.
Just around the corner?
The metamaterial industry is likely to grow to $10.7 billion by 2030 and we are very likely to see the first applications in microwave frequencies such as 5G mobile communication devices, autonomous driving vehicles and drones.
An innovative example of this is the use of metamaterials in the enhancement of Short Backfire Antennas (SBFA). Lightweight and compact, the enhanced SBFA is ideal for satellite-based antenna arrays with nearly 100 % efficiency from the use of foam walls lined with metasurfaces. Another example is Light Detection and Ranging (LIDAR) , a form of laser scanning to create detailed area maps that are used in autonomous driving vehicles. Previous examples were bulky and slow but metamaterial technology has helped make such components much lighter and cheaper in comparison and will quicken the introduction of self-driving technology into mainstream automobiles .
Solar panels are at the forefront of the global drive for renewable and sustainable energy. However, their large-scale implementation is limited by a shortage of key materials such as indium and gallium. Another significant limitation is the drop in efficiency due to varying incidence angles of sunlight onto the panel surface, unfortunately a routine occurrence as the earth rotates on its own axis during the day. With tunable metamaterial technology the impact of the incident angle can be overcome and furthermore, a wider spectrum of wavelengths can be used for energy generation. In addition, with dielectric metamaterial technology, this can be achieved with materials that are much more easily available;thereby minimising the use of those that are likely to run out in the next few years.
Terahertz (THz) radiation includes the wavelengths positioned between infrared and microwave radiation (300 GHz to 10 THz) and has a number of advantages over ionising radiation. In the last decade, the generation and detection of THz radiation has been heavily influenced by metamaterial technology and day to day application now seem tantalisingly close.
THz spectroscopy can detect a range of substances with a single sensor. It can penetrate clothing and cardboard and other solid objects and can detect water content and porosity and even distinguish between different forms of polymorphic materials- materials that are chemically identical but with varying crystalline forms, that are pivotal in healthcare. For example, an effective drug can turn into an ineffective one while it sits on the shelf. Ordinarily, this could only be detected by powdering the tablets for analysis but with THz spectroscopy, this can be performed on intact tablets and repeated as many times as required giving real time information. This technology will probably see widespread application in the pharmaceutical industry very soon.
Even more exciting is its potential in medicine. Design improvements in optoelectronic sensors with metamaterials can enable efficient telemedicine to remotely monitor a patient’s condition. Since it is non ionising it does not damage living tissue unlike x rays and Gamma rays and yet it provides exquisite molecular surface detail. Much like the applications in pharmaceuticals, a single detector could be used to scan a viable biological sample and detect a large number of molecular signatures. The first applications may be in diagnosing skin conditions such as cancer at your own home but other technologies are set to follow.
Blue sky thinking
The scope for metamaterials is not limited to microwave and terahertz although these will probably be first to the post. Metamaterials are attractive options for sound proofing such as a metamaterials “cage”, a ring of solid structures interspersed with vacant channels surrounding the sound source. The sound waves are limited to the cage and yet air flows freely through it – this is known as Helmholtz resonance. This technology could eventually be seen in a diverse range of fields, from ship building and military vehicles to the design of concert halls. Taking acoustic cloaks even further, it may be possible to create a so-called “earthquake cloak” that can be embedded in the structure of buildings. These rings could compress seismic waves from earthquakes directing them away from inhabited areas. Such a device could work across a number of frequencies. The rings themselves could form the building’s foundation, with added efficiency of scale but the stiffness and elasticity of these large structures will have to be carefully controlled. This could revolutionise the field of seismology and benefit millions.
Mass diffusion metamaterials may revolutionise the world of chemical and biological separations; they can cloak compounds while simultaneously concentrating others, helping to separate out different compounds. Generally, we assume that the concentration gradient is the most important factor in mass transport but in fact the chemical potential gradient is more influential . Metamaterials can influence the chemical potential and with minimal energy use separate out the constituents of a compound such as C02 into carbon and oxygen. This type of metamaterial can prove very useful in the chemical engineering and molecular biology sector .
Metamaterials have accelerated from theoretical conceptions in the late 20th century, to proof-of-concept experiments in the early 2000’s and now in the words of Prof. John Pendry : “well on its way to being embedded as a standard design technique” in electromagnetic wave devices. It is very likely to have its first widespread commercial applications in 5G communication devices where metamaterials will enhance ultra low profile antenna designs. Autonomous driving vehicles with compact antennae and laser assisted real time mapping devices are probably set to follow closely behind as Google enters this highly competitive field with significant R&D investment.
Healthcare applications, such as metamaterial optical diagnostic devices for skin and ophthalmic conditions will probably soon see widespread usage in the pharmaceutical industry. Post marketing quality control with metamaterial handheld tablet scanning devices might soon be seen in high street pharmacies. Exciting as these early adopters might appear, they will be no more than the tip of the iceberg. We cannot be sure where this transformative technology will take us in 10 years’ time, but it will certainly be surprising.
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- Figure 1: An image representing the Electromagnetic spectrum. EM_spectrum_compare_level1_lg.Jpg (750×281).” n.d. Accessed June 1, 2020. https://imagine.gsfc.nasa.gov/Images/science/EM_spectrum_compare_level1_lg.jpg “FIO117: Figure 8.1” by Rosenfeld Media is licensed under CC BY 2.0
- Figure 2: Image a) A diagrammatic representation of a tunable metamaterial, ): Adapted from Wikimedia : Created by [email protected] (point of contact)/Public domain https://commons.wikimedia.org/wiki/File:Lefthanded_metamaterial_array_configuration.jpg Image b): Adapted from Wikimedia: Metarefraction.svg: created by NASAderivative work: Antilope / Public domain https://commons.wikimedia.org/wiki/File:Metarefraction_fi.svg
- Figure 3: Monochromatic lens Adapted from National Institute of Standards and Technology / Public domain https://commons.wikimedia.org/wiki/File:Metamaterial_%27Flat_Lens%27_(8889691315).jpg and Achromatic lens (https://creativecommons.org/licenses/by-sa/4.0/0) Adapted from Comparison between monochromatic metasurface lenses and achromatic… | Download Scientific Diagram [Internet]. [cited 2020 Oct 13];Available from: https://www.researchgate.net/figure/Comparison-between-monochromatic-metasurface-lenses-and-achromatic-lenses-a-Schematic-of_fig1_328784727
- Figure 4: A composite figure made with images from various sources representing a comparison between nano scale objects Figure 4 a) (adapted from Sureshbup / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0) Figure 4 b) (By Nanoink1 – Own work, CC BY-SA 3.0) (https://commons.wikimedia.org/wiki/File:Large_meta_SEM.png) Figure 4 c) Puentes G. Spin-Orbit Angular Momentum Conversion in Metamaterials and Metasurfaces. Quantum Reports [Internet] 2019 [cited 2020 Oct 12];1(1):91–106. Available from: https://www.mdpi.com/2624-960X/1/1/10 with https://creativecommons.org/licenses/by/4.0/ Figure 4 d) Pimpin A, Srituravanich W. Reviews on micro- and nanolithography techniques and their applications. Engineering Journal2012;16(1):37–55. (https://creativecommons.org/licenses/by-sa/4.0/0)
- Figure 5: Adapted from “overall layers of absorber” by pennstatenews is licensed under CC BY-NC-ND 2.0 A diagram of a tunable metamaterial for Terahertz (Thz) https://www.nanotech-now.com/news.cgi?story_id=49452
- Figure 6: Author’s impression of a metamaterial atomic separator loosely based on a description from Restrepo-Flórez J-M, Maldovan M. Breaking separation limits in membrane technology. Journal of Membrane Science [Internet] 2018;566:301–6. Available from: http://www.sciencedirect.com/science/article/pii/S0376738818308561