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Metamaterials are artificially structured materials designed to control and manipulate physical phenomena such as light and other electromagnetic waves, sound waves and seismic waves in unconventional ways, resulting in exotic behavior that’s not found in nature. This class of micro- and nano-structured artificial media are predicted to be able to protect the building from earthquakes by bending seismic waves around it. Similarly, tsunami waves could be bent around towns, and sound waves could be bent around a room to make it soundproof.
The core concept of metamaterials is to craft materials by using artificially designed and fabricated structural units to achieve the desired properties and functionalities. These structural units – the constituent artificial ‘atoms’ and ‘molecules’ of the metamaterial – can be tailored in shape and size, the lattice constant and interatomic interaction can be artificially tuned, and ‘defects’ can be designed and placed at desired locations.
An optical metamaterial is composed of elements that are smaller than the wavelength of light, but that can interact with light in interesting ways. Optical metamaterials use carefully controlled nanostructures to manipulate visible light, enabling lighter and thinner optics for everything from eyeglasses to mobile phone cameras.
Negative refraction
Among the most sought-after properties of metamaterials is the negative index of refraction of light and other radiation. Negative refraction is based on the equations developed in 1861 by Scottish physicist James Maxwell. All known natural materials have a positive refractive index so that light that crosses from one medium to another gets slightly bent in the direction of propagation. For example, air at standard conditions has the lowest refractive index in nature, hovering just above 1. The refractive index of water is 1.33. That of diamond is about 2.4. The higher a material’s refractive index, the more it distorts light from its original path.
Metamaterials can be endowed with properties and functionalities unattainable in natural materials. By engineering the arrangement of these nanoscale unit cells into a desired architecture or geometry, one can tune the refractive index of the metamaterial to positive, near-zero or negative values.
In some metamaterials, negative refraction occurs such that light and other radiation gets bent backward as it enters the structure. The existence of substances with a negative refractive index was predicted as early as the middle of the 20th century. Metamaterials with a negative refractive index could be used in super-resolution imaging or cloaking, but they are very hard to make because the structural elements must be much smaller than the wavelength of light. For visible wavelengths, that means that the elements must be 100 nm or smaller.
But only in the early 2000s have researchers figured out how to create materials of any type that can achieve negative refraction. The first samples of metamaterials were made from arrays of thin wires and only worked with microwave radiation.
With such negative refraction materials, many applications become possible in electronics manufacturing, lithography, biomedicine, insulating coatings, heat transfer, space applications, and perhaps new approaches to optical computing and energy harvesting.
Optical Metamaterials
Structures such as split-ring resonators (SRRs), sub-wavelength wires, and fishnets can manipulate electromagnetic waves at optical frequencies. These artificial structures can induce electric or magnetic coupling, leading to exotic properties such as negative refractive index, perfect absorption, and hyperbolic dispersion.
The most basic optical metamaterials, such as metalenses, enable production of optics and waveguides with lower weight, smaller size, superior imaging and sensing, and greater design freedom. Others, such as photonic metamaterials, provide unique capabilities for applications from medicine to optical computing.
For instance, in order for invisibility cloak technology to obscure an object or, conversely, for a ‘perfect lens’ to inhibit refraction and allow direct observation of an individual protein in a light microscope, the material must be able to precisely control the path of light in a similar manner. Metamaterials offer this potential.
Optical metamaterial features:
Lightweight and thin: By combining multiple functionalities in a single layer, and making each layer much thinner, metamaterial optics can have less than half as many components as conventional optics
Digital design: Commercially relevant metamaterials require highly complex patterns that are typically designed in software, much of which is proprietary to each supplier based on their own expertise, materials, processes, and IP; this feature in turn enables very fast design times
compared to conventional optics
Wavelength specificity: Metamaterials are highly wavelength-specific by default, though it is increasingly feasible to design and manufacture broadband materials that cover most or all of the visible range
Device design freedom: Because metamaterials can guide light at angles and along trajectories unattainable with any conventional optics, it they enable device manufacturers greater freedom of design
Applications
Over the last two decades, optical metamaterials have enabled possibilities of invisibility cloaking, super-resolution imaging, and efficient energy harvesting.
Optical invisibility camouflage (or invisibility cloaking) is a technology to make an object seem invisible by causing incident light to avoid the object, flow around the object, and return undisturbed to its original trajectory. To date several research institutes have carried out the theoretical and experimental study of invisibility camouflage devices, using the extraordinary optical properties of metamaterials and the technique of transformation optics.
Optical camouflage devices designed using transformation optics have a closed region that incident light from every direction avoids. A person hiding in this region therefore seems invisible to external onlookers. However, no light can enter the cloaked region, and consequently the person hiding therein cannot be able to see outside. This is quite inconvenient for practical use. A practical camouflage device must have unidirectional transparency such that a person inside cannot be seen from the outside but can see the outside.
A superlens allows to view objects much smaller than the roughly 200 nanometers that a regular optical lens with visible light would permit. This theoretical resolution limit (diffraction limit) of conventional optical imaging methodology was the primary factor motivating the development of higher-resolution scanning probe techniques. Though scanning electron microscopes can capture objects that are much smaller, down to the single nanometer range, they are expensive, heavy, and, at the size of a large desk, not very portable.
The superlens concept relies on the generation of surface plasmon polaritons enhancing the evanescent fields to restore the near-field components of the Fourier decomposition of the source object, hence breaking the diffraction limit.
Since superlenses have demonstrated the capability of subdiffraction-limit imaging, they have been envisioned as a promising technology for potential nanophotolithography. Already, superlens lithography is able to demonstrate the required sub-diffraction-limit resolution and high contrast performance required for cost-effective and high throughput nanopatterning mass production
Since these metalenses are ultrathin, flat surfaces, they have attracted tremendous attention because they can overcome limitations of conventional bulky, curved and heavy refractive optical lenses and they are poised to revolutionize everything from microscopy to cameras, sensors, and displays
Although metamaterials already have revolutionized optics, their performance has been limited by their inability to function over broad bandwidths of light. Designing a metamaterial that works across the entire visible spectrum remains a considerable challenge. The fascinating functionalities of metamaterials typically require multiple stacks of material layers, which not only leads to extensive losses but also brings a lot of challenges in nanofabrication. Many metamaterials consist of complex metallic wires and other structures that require sophisticated fabrication technology and are difficult to assemble. The unusual optical effects do not necessarily imply the use of volumetric (3D) metamaterials.
Besides, they are generally not compatible with biological environments, hindering their potential applications. These issues can be addressed by the incorporation of soft materials, or the sophisticated design of metamaterials that are globally deformable.
Manufacturing
The surfaces of metamaterials — that is, metasurfaces — feature high-precision topographies that typically require specialized fabrication processes and equipment, such as those used to manufacture ICs. With no dedicated glass fabs in existence today, metamaterial designers are forced to turn to semiconductor fabs designed to work with silicon substrates.
To harness the capabilities of an IC fab, metamaterial developers must ensure that their optical material behaves like a silicon wafer. Corning is currently devising specialty glass materials and wafers that it hopes will work seamlessly in a typical silicon fab. This will enable the fab to make metasurface products on glass when needed, said Xavier Lafosse, commercial technology director of Advanced Optics at Corning Inc.
“Glass has very different optical, electrical, chemical, and mechanical properties from silicon,” he said. “Therefore, the key challenge is to make glass behave similarly to silicon using existing process and equipment infrastructure. Corning has been working in this area for the last few years with established fabs and foundries, and we have made significant progress. We are currently engaging customers and will be prepared to ramp up to production volume when the metamaterial community requires such glass wafers.”
The marriage of silicon foundries with metamaterial production could be a stroke of genius — if it can be perfected. Harnessing established top-down fabrication techniques, such as nanoimprinting or nanomasking, will give makers the flexibility to pattern a substrate with engineered nanostructures on a variety of optical materials with almost any geometrical shape. Importantly, these metamaterials could be produced in highly scalable volumes.
Bottom-up approach
Most ways of making optical metamaterial structures require the same kind of “top-down” fabrication techniques that are used in making microchips, but these techniques aren’t very good at making 3D materials.
One simple way to make a 3D optical metamaterial is to create a suspension of nanoscale electromagnetic resonators, each of which responds isotropically to incoming light. “Resonator” means that when light of a certain frequency range hits the element, it is strongly scattered, and “isotropic” means that the resonance doesn’t depend on the orientation of the resonator or the direction of the incoming light. These isotropic resonators don’t need to be arranged in an orderly way to yield a negative refractive index. A disordered arrangement of such resonators is called a “metafluid.” This idea was first proposed by Urzhumov, Shvets, Fan, Capasso, Brandl, and Nordlander in Optics Express in 2007.
The challenge is figuring out how to make the resonators. They have to be smaller than the wavelength, and highly symmetric, so that their response is isotropic. Urzhumov and coworkers proposed that gold nanoparticles arranged in tetrahedral clusters with gaps of a few nanometers between the particles might work well. The resulting metafluid might look something like this:
In 2010, Jonathan Fan, working with Federico Capasso and collaborating with our group and others, was able to make electromagnetic resonators by assembling gold nanoshells in a drying colloidal droplet. Each resonator was a triangle of three gold nanoshells. Jon and colleagues measured the electromagnetic response of individual clusters and showed that they have both electric and magnetic dipole resonances (see Fan, Wu, Bao, Bao, Bardhan, Halas, Manoharan, Nordlander, Shvets, Capasso. Science, 2010). The frequencies of the two resonances can be tuned relative to one another by changing the separation gaps between the nanoshells.
Jon explained the resonance of the clusters by analogy to an LC circuit. Each particle has a plasmonic resonance, because incident light can excite a surface wave. When the particles are close enough to each other, the fields from neighboring particles can interact, producing a magnetic resonance. In the circuit analogy, the gaps between the nanoparticles act like capacitors, and the nanoparticles themselves like inductors.
The gaps between the particles are only a few nanometers, so small that you can’t see them in the electron microscope image. If the gaps are too large, the coupling between the nanoparticles is too weak to give a strong magnetic response. If -they are too close together, the particles could touch and short the circuit. We were able to make such precise gaps by putting a self-assembled monolayer of polymer on each particle. This “bottom-up” method can control the gap thickness much more precisely than top-down methods such as lithography.
Market
While metamaterials have already started to make an impact in the radio and microwave spectrum – aided by the emergence of applications in 5G networks – the additional complexity of designs needed for the higher-frequency operation has held back their visible-range counterparts thus far.
Lux’s new report, “Innovation Digest: The Lux Take on the Future of Optical Metamaterials,” explores the market readiness of optical metamaterials while identifying opportunities and challenges. The technology to design and manufacture optical metamaterials is rapidly maturing, making commercial adoption likely soon. The potential $50 billion market ranges from corrective lenses and consumer devices to industrial, medical, aerospace, automotive, and military equipment.
Optical metamaterials are important to the market over incumbents for these key reasons
• Greater control over direction, transmission, and focusing of light on all major performance axes
• Ability to access novel capabilities including negative, tunable, and complex refractive indices
• Ability to combine multiple optical functions, such as higher-order image corrections, in a single device layer
Metamaterial products are now emerging. From damage-resistant antireflective optics to tunable flat lenses, metamaterials have already made commercial inroads. Now they are finding employment in smartphones, cameras, cars, and gaming consoles. And the most immediate consumer application appears to be depth-sensing in mobile devices, driverless cars, and augmented and virtual reality headsets.
Aiming for the smartphone market, Metalenz has just unveiled its “Orion” family of dot pattern projectors for 3D sensing applications. Optical metamaterials are ready for initial commercial deployment and will command a market worth several billion dollars by 2030.
“Optical metamaterials will provide a performance advantage and competitive edge to early adopters that will drive accelerating growth as they substitute and supplement conventional optics,” write Vicari and Holman. They see the most valuable markets appearing in cell phone cameras and corrective lenses, and say that although it will take time for optical metamaterials to scale up to the volumes demanded by such applications, a wide range of relatively niche applications will provide plenty of demand in the meantime.
Attention initially focused on exotic ideas such as “invisibility cloaks” in the optical spectrum, but there is enormous market potential in more prosaic applications taking advantage of the ability to manipulate light with greater control than is possible with conventional optics. With greater control over the direction, transmission, and focusing of light on all major performance axes, metamaterial devices are able to deliver novel capabilities including negative, tunable, and complex refractive indices. They can also combine multiple optical functions, such as higher-order image corrections, in a single device layer, making for thinner and lighter products.
Industry
A growing number of startups are forming, and large corporations are showing significant interest, including partnerships, investments, and product launches from Lockheed Martin, Intel, 3M, Edmund Optics, Airbus, Applied Materials, and TDK.
“A growing number of startups are forming, and large corporations are showing significant interest, including partnerships, investments, and product launches from Lockheed Martin, Intel, 3M, Edmund Optics, Airbus, Applied Materials, and TDK,” they suggest.
“Optical metamaterials will impact niches within the lens market in the next year,” added lead author Vicari. “Lack of production infrastructure and of device designers familiar with the technology have held back progress so far, but design and production technologies have matured rapidly in the past few years.”
The Lux Research report identifies four key features that define the new technology. These include the ability to make optical components much thinner and lighter; the use of digital patterning for much faster product design; wavelength-specific devices; and much greater design freedom.
“Though production costs are falling rapidly, they are still too high, and production scale too small, for many applications,” states the report. “In addition, there are only a handful of leading developers of this technology, which may become a bottleneck for innovation and adoption in the near term.”
Among that handful of companies directly developing optical metamaterials right now, Vicari and Holman identify Canadian firm Meta (Metamaterials Inc.), the Harvard spin-out Metalenz, and gradient index (GRIN) lens developer Vadient Optics as three of the leading protagonists in the sector. The Boston firm, which raised $10 million in venture funding earlier this year, says that its design – based around a single, flat meta-optical component – is “bound” to replace refractive lens and diffractive element optical stacks.
“We’re able to improve the system-level performance while replacing as many as six optics in the current modules with a single meta-optic,” claims company co-founder and CEO Robert Devlin. Evaluation kits for Orion should be available from Metalenz later this month. Unusually for such disruption, regulations are not expected to impact the trajectory of the new technology. “These [optical metamaterials] replace or augment conventional optics, usually using combinations of existing, known materials that are already in use,” explains the report. “In addition, most applications are in unregulated or lightly regulated market segments, other than aerospace.”
In the longer term, metamaterials could be completely revolutionary, disrupting and displacing conventional optics entirely. “While metamaterials are fundamentally a design technology, optimized formulations and equipment can improve reliability, scalability, cost, and ease of design, and startups can address these needs better with large company partners,” write the authors. “As adoption increases, the equipment and materials markets for conventional optics will shift and likely decline.”
Some of the companies in this area are Applied Metamaterials, Kymeta, Metamagnetics, Metamaterial Inc., Xi’an Tianhe, Metaboards Limited
References and Resources also include:
https://www.nanowerk.com/what-are-metamaterials.php
https://manoharan.seas.harvard.edu/self-assembled-metamaterials
