Metamaterials can have a negative index of refraction, something never seen in nature. Can these un-natural materials lighten our environmental footprint and improve the planet?

Metamaterials combine micro- or nano structural features rather than relying on composition alone to achieve the desired properties. They have sparked the imagination of the scientific community with their unusual properties. Researchers are developing unique metamaterials for their potential as invisibility cloaks, high-efficiency photovoltaics, super-antennas, and ultrabright LEDs.

These metamaterials impact widely diverse industries, with the most notable advances in the optical and photonic domain. The global market for metamaterials is projected to top $1.9 billion US by 2021, with the electromagnetic sector representing the largest growth.

Sustainable materials are designed to be cost-effective, have a light environmental footprint, and benefit mankind today and into the future. Thus, they garner significant interest in many industries, with developers minimizing composition though improving structures, substituting alternatives to toxic or environmentally polluting constituents, using recycled or recyclable components, and reducing energy requirements throughout the product lifecycle.

Material scientists historically have described their materials in terms of chemical composition. Brass is an alloy of copper and zinc. NdFeB magnets, the strongest magnetic material known, are defined by their composition of neodymium, iron, and boron.

With metamaterials, it is not so simple. A left-handed metamaterial is better described as an assemblage of metallic rods and rings, for instance, and not defined only by its composition.

In the case of NdFeB magnets, assemblages of metamaterials are being investigated as environmentally friendly substitutes for neodymium and other elements.

Consumer preference for greater miniaturization has resulted in increased demand for these permanent magnets for micromotors in hard drives and smart phones and for amplifiers for loudspeakers and in-ear headphones. The magnets are critical in the clean tech industry too, being essential components of wind turbines.

Unfortunately, the uses for neodymium, along with four other rare-earth elements, have far outstripped the supply. Almost all the rare-earth element production (more than 97%) is from China, and China is looking to keep more of it for its own use.

In addition, the process of neodymium extraction leads to millions of tons of waste, including some that is radioactive.

Supported by ARPA-E, the U.S. Department of Energy’s innovation arm, researchers are looking at constructing nanomagnet assemblages with soft and hard magnetic components that use less neodymium, or none at all, yet maintain the strength and permanence of the traditional neo-magnet. If successful, this will make today’s electronics, acoustical devices, and wind turbines more environmentally friendly and less dependent on a sole source.

Eco-metamaterials are also touted for solar technologies. These materials are able to capture light for solar energy collection, and natural photovoltaic materials provide discrete but limited bandgap options. By engineering metamaterials, nanostructures more suitable for solar energy conversion can be realized. For instance, man-made lattices can be optimized for a given spectrum.

The best metamaterials most recently demonstrated for photovoltaics are embedded quantum dots. Researchers at the University of Tokyo and Sharp Corp. report impressive efficiencies of over 18% by manipulating InGaAs/GaAs structures.

Development of sustainable materials is a process that generally begins with the reduction in the amount of materials required. For metamaterials, their constructs minimize material requirements while simultaneously improving properties.

That isn’t the case with traditional materials where developers must change several manufacturing processes. Polyethylene-terephthalate (PET) soda water bottles for instance, now contain less plastic through a combined structural redesign and a reduction of plastic thickness. Reducing the amount of plastic used in the bottles led to a reduction in weight, which reduced the energy needed to produce and ship the products as well as the tons of material headed to landfills.

Reducing the amount of materials can also reduce the size of a device or product. For example, metamaterials can improve gain and bandwidth of conventional communication antennas while also allowing the antennas to be shrunk in size without sacrificing the desired performance. This makes metamaterial-based antennas a far more sustainable option.

LG Electronics was the first manufacturer to use metamaterial antennas in cell phones. Remember the bulky “bag phones” of the 1990s with their 4-inch antennas? Today, we would be hard pressed to identify the antenna in our smart phones. (Your smart phone antenna, by the way, is only a few millimeters long and as thin as a piece of paper.)

A new and potentially sustainable advancement in metamaterials is occurring with lasers. The ability to tailor structure and locate elements in precise arrangements in metamaterials may lie in the exactness afforded by laser technology. Bolstered by research done at Oak Ridge National Laboratory (USA), the National Institute for Laser, Plasma and Radiation Physics (Romania), and elsewhere, lasers have been shown to be effective in depositing thin films of elements, creating nanostructures efficiently.

Sadly, lasing exacts a high energy cost. However, laser researchers are now pushing the scale of lasers to new lows (nano) while maintaining or improving power.

At University of California, San Diego (USA) lasers are being downsized so that they require very little power to operate.

The UC team in San Diego has built the smallest room-temperature nanolaser to date, as well as a highly efficient, “thresholdless” laser that funnels all its photons into lasing, without any waste.

The two new lasers require very low power to operate, an important breakthrough since lasers usually require greater and greater pump power to begin lasing as they shrink to nano sizes.

The small size and extremely low power of these nanolasers could make them very useful components for future optical circuits for computer chips, according to Mercedeh Khajavikhan and her colleagues.

These nanolasers also have the potential to allow metamaterial manufacturers to produce better, cheaper, and smaller energy-footprint components — a more sustainable manufacturing process.

Developing sustainable materials is a start down the path toward the ultimate goal of a cleaner, greener planet.

Eco-metamaterials may not yet be ‘green’ based on their composition. But the reduced quantities of toxic materials and metamaterials’ inherent potential for exotic properties allow technologists to improve outcomes well beyond what is found in nature, using less material and less energy.

Interestingly, the field of metamaterials has yet to embrace sustainability as a metric.

A survey of the literature (including a Google search) produced no reports of life-cycle assessments for metamaterials.

But increased sustainability may actually be the byproduct of the development work in metamaterials, as their inherent properties allow for increased performance per unit mass.

The next challenges may include using recycled content and even fewer toxic materials. However the ultimate prize in the development of metamaterial-based systems may be with a reduced waste footprint. Metamaterial systems that can be deconstructed and the components or elements recycled and reused should be considered as a design option.

It would certainly be more sustainable if electro-optical devices could be deconstructed and the components recycled rather than dumped in a landfill — or Harry Potter’s invisibility cloak washed and reused. Sustainability isn’t just about recycling and not polluting the atmosphere; the basic premise is touching the planet lightly while improving lives. Metamaterials are allowing product developers to ‘right size’ their products where that ‘right size’ is smaller, cheaper, and faster than is possible with natural materials.

There are many technical challenges surrounding the next generation of metamaterials, with success limited only by the imagination of the technologists involved in their development. But these challenges have the potential to become opportunities for this unique material.

Isn’t it ironic that these un-natural materials may help make the world far more sustainable than what nature can provide?

Rosemarie Szostak is a senior analyst with Nerac Inc. where she advises clients in the areas of innovation, specialty chemicals, materials, renewable energy, and sustainability. She has a PhD in chemistry from University of California, Los Angeles.

Nerac is a global research and advisory firm for companies developing innovative products and technologies.

Nerac analysts are frequent contributors to SPIE Professional.

Michael Kapralos is an analyst intern at Nerac and an electrical engineering graduate student at University of Connecticut.

The quest for sustainable optical technologies is what makes the annual SPIE Optics + Photonics such a huge green photonics event.

In addition to hundreds of presentations on solar technologies and solid-state lighting at SPIE Optics + Photonics, 12-16 August in San Diego, CA, metamaterials will be the subject of a symposium-wide plenary talk and a full conference.

Nearly 100 technical presentations are scheduled for the Metamaterials: Fundamentals and Applications conference within the NanoScience + Engineering symposium.

SPIE Fellow Allan Boardman of University of Salford (UK) will chair the conference, which will cover nanoantennas, cloaking, and related topics such as:

At the symposium-wide plenary session on Sunday, 12 August, Vladimir M. Shalaev, scientific director for nanophotonics at Purdue University (USA) will review this growing field in a talk titled “The Exciting Science of Light with Metamaterials.” Shalaev, an SPIE Fellow, is expected to also discuss recent developments in such areas as tunable metamaterials, artificial optical magnetism, and nanolasers.

For more information: spie.org/op.

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