An illustration of a metamaterial designed by Daraio's team. Though each individual square is identical, their layout and the metamaterial's overall design cause waves to propagate in a specific pattern through the material, as indicated by the squares' different colors. Image: Chiara Daraio/Caltech.
An illustration of a metamaterial designed by Daraio's team. Though each individual square is identical, their layout and the metamaterial's overall design cause waves to propagate in a specific pattern through the material, as indicated by the squares' different colors. Image: Chiara Daraio/Caltech.

Engineers at the California Institute of Technology (Caltech) and ETH Zürich in Switzerland have created a method for systematically designing metamaterials using the principles of quantum mechanics. Their work could pave the way for wider use of metamaterials in more mainstream applications by creating a purpose-driven framework for their design.

Metamaterials are engineered materials that exploit the geometry of their internal structure to manipulate incoming waves. For example, a metamaterial that manipulates electromagnetic waves might bend light in an unusual way to create a cloaking device. Meanwhile, a wafer-thin acoustic metamaterial might reflect incoming sound waves to soundproof a room.

This ability to control waves derives from how the material is structured, often on a microscopic scale. In 2010, Caltech researchers developed an optical metamaterial that uses a surface coated with three-dimensional structures to redirect light as desired. More recently, engineers at Caltech showed that flat surfaces covered with tiny pillars of silicon could focus light like a lens.

Picture a crystal – a solid whose physical properties are determined by the way it is built from a repeating series of atomic structures. Carbon atoms structured in flat plates create crumbly graphite, while carbon atoms structured in tetrahedra create ultra-hard diamonds. Similarly, metamaterials are constructed from a repeating series of nano- and micro-scale structures that give them their unique properties.

Despite their promise and wide array of possible applications, metamaterials will not become widely used unless engineers can design them to have particular desired properties. Up to now, much progress has been made in the design of metamaterials that interact with electromagnetic waves. Overall, however, the design of mechanical metamaterials – those that influence mechanical waves such as sound waves or seismic waves – remains a scattershot affair, says Chiara Daraio, a professor of mechanical engineering and applied physics at Caltech.

"Before our work, there was no single, systematic way to design metamaterials that control mechanical waves for different applications," she says. "Instead, people often optimized a design to fulfil a specific purpose, or tried out new designs based on something they saw in nature, and then studied what properties would arise from repeated patterns."

To address this, a team led by Daraio and consisting of graduate students Marc Serra Garcia and Antonio Palermo, postdoctoral scholar Katie Matlack, and Sebastian Huber at ETH Zürich, turned to the field of quantum mechanics. On the surface, the choice was an unlikely one. Quantum mechanics governs the often counterintuitive behavior of subatomic particles, and would seem to have no bearing on the micro- and macro-scale designs of the metamaterials studied by Daraio's team.

Quantum mechanics predicts the existence of certain exotic types of matter: among them, a ‘topological insulator’ that conducts electricity across its surface while acting as an insulator in its interior. Daraio's team realized that they could build macro-scale versions of these exotic systems, which could conduct and insulate against vibrations, rather than electricity, by using the principles of quantum mechanics.

In quantum mechanics, materials can sometimes be described as an ensemble of interacting particles. "Imagine that each particle is a tiny mass, connected to its neighbors by springs," Daraio explains. "Each particle reacts to incoming waves in a unique way that is determined, in part, by the reaction of its neighbors. In our approach, we apply this mass-and-spring model to macroscopic, elastic materials, maintaining their characteristic properties."

Because metamaterials are built from arrays of geometrical structures (with building blocks at the nano-, micro- or macro-scale) connected in repeating patterns, Daraio and her colleagues realized that, by representing each repeating structure as an ensemble of particles, it would be possible to design many different types of metamaterials. These could include waveguides, acoustic lenses and vibration insulators.

When struck by an incoming wave, each repeating structure in a metamaterial has the potential to deform in a number of different ways. That deformation is governed not only by the geometry of the structure, but also by how the structures are connected and how other structures around them are reacting. Treating this as a system of masses and springs, Daraio's team was able to predict how these systems would react, and then engineer them to react in desired ways.

It is complicated, but also predictable – which is the important part.

As a theoretical proof of concept, Daraio's team designed metamaterials made from a series of rectangular millimeter-scale plates, each loosely connected to one another like a piece of a puzzle. By tuning the design of the plates and how well-connected the plates were, the team created a perfect acoustic lens that focuses sound without loss of signal. The plates also act as a waveguide that directs and slows the propagation of sound. The method could be used to design many other devices or sensors where high sensitivity, precision or control are necessary, Daraio says. She and her team report their work in a paper in Nature Materials.

Though Daraio's work is theoretical, validated using computer simulations, her co-authors at ETH used the method to design and build a 10cm by 10cm silicon wafer that consists of 100 small plates connected to each other via thin beams. When the wafer is stimulated using ultrasound, only the plates in the corners vibrate; the other plates remain still, despite their connections. This device, which is reported in a paper in Nature, could be used as a precise waveguide in a communications network.

The design process described can also be used to design optical metamaterials, antennas and optical signal processing devices, says Daraio.

This story is adapted from material from Caltech, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.