This optical micrograph shows an array of microscopic metamaterial samples on a reflective substrate. Laser pulses have been digitally added, depicting pump (red) and probe (green) pulses characterizing a sample in the center. Image courtesy of Carlos Portela, Yun Kai, et al.
This optical micrograph shows an array of microscopic metamaterial samples on a reflective substrate. Laser pulses have been digitally added, depicting pump (red) and probe (green) pulses characterizing a sample in the center. Image courtesy of Carlos Portela, Yun Kai, et al.

Metamaterials are products of engineering wizardry. They are made from everyday polymers, ceramics and metals. And when constructed precisely at the microscale, in intricate architectures, these ordinary materials can take on extraordinary properties.

With the help of computer simulations, engineers can play with any combination of microstructures to see how certain materials can transform, for instance, into sound-focusing acoustic lenses or lightweight, bulletproof films.

But simulations can only take a design so far. To know for sure whether a metamaterial will meet expectations, physically testing them is a must. But there’s been no reliable way to push and pull on metamaterials at the microscale, and to know how they will respond, without contacting and physically damaging the structures in the process.

Now, a new laser-based technique offers a safe and fast solution that could speed up the discovery of promising metamaterials for real-world applications.

The technique, developed by engineers at Massachusetts Institute of Technology (MIT), probes metamaterials with a system of two lasers. One laser quickly zaps a structure while the other measures the ways in which it vibrates in response, much like striking a bell with a mallet and recording its reverb. In contrast to a mallet, however, the lasers make no physical contact with the metamaterials. Yet they can still produce vibrations throughout a metamaterial’s tiny beams and struts, as if the structure were being physically struck, stretched or sheared.

The engineers can use the resulting vibrations to calculate various dynamic properties of the material, such as how it would respond to impacts and how it would absorb or scatter sound. With an ultrafast laser pulse, they can excite and measure hundreds of miniature structures within minutes. For the first time, the new technique offers a safe, reliable and high-throughput way to dynamically characterize microscale metamaterials.

“We need to find quicker ways of testing, optimizing and tweaking these materials,” says Carlos Portela, professor in mechanical engineering at MIT. “With this approach, we can accelerate the discovery of optimal materials, depending on the properties you want.”

Portela and his colleagues detail their new system, which they’ve named LIRAS (laser-induced resonant acoustic spectroscopy), in a paper in Nature. His MIT co-authors include first author Yun Kai, Somayajulu Dhulipala, Rachel Sun, Jet Lem and Thomas Pezeril, along with Washington DeLima at the US Department of Energy’s Kansas City National Security Campus.

The metamaterials that Portela works with are made from common polymers. Via 3D printing, these polymers are formed into tiny, scaffold-like towers made from microscopic struts and beams. Each tower is patterned by repeating and layering a single geometric unit, such as an eight-pointed configuration of connecting beams. When stacked end to end, the tower arrangement can give the whole polymer properties that it would not otherwise have.

But engineers are severely limited in their options for physically testing and validating these metamaterial properties. Nanoindentation is the typical way in which such microstructures are probed, though in a very deliberate and controlled fashion. This method uses a micrometer-scale tip to slowly push down on a structure while measuring the tiny displacement and forces on the structure as it’s compressed.

“But this technique can only go so fast, while also damaging the structure,” Portela notes. “We wanted to find a way to measure how these structures would behave dynamically – for instance, in the initial response to a strong impact – but in a way that would not destroy them.”

The team turned to laser ultrasonics – a nondestructive method that uses a short laser pulse tuned to ultrasound frequencies to excite very thin materials such as gold films without physically touching them. The ultrasound waves created by the laser excitation are within a range that can cause a thin film to vibrate at a frequency that scientists then use to determine the film’s exact thickness down to nanometer precision. The technique can also be used to determine whether a thin film holds any defects.

Portela and his colleagues realized that ultrasonic lasers might also safely induce their 3D metamaterial towers to vibrate. The height of the towers – which range from 50µm to 200µm — is on a similar microscopic scale to the thin films.

To test this idea, Yun Kai, who joined Portela’s group with expertise in laser optics, built a tabletop setup comprising two ultrasonic lasers – a ‘pulse’ laser to excite metamaterial samples and a ‘probe’ laser to measure the resulting vibrations.

On a single chip no bigger than a fingernail, the team printed hundreds of microscopic towers, each with a specific height and architecture. They placed this miniature city of metamaterials in the two-laser setup, then excited the towers with repeated ultrashort pulses. The second laser measured the vibrations from each individual tower. The team then gathered the data and looked for patterns in the vibrations.

“We excite all these structures with a laser, which is like hitting them with a hammer,” Portela explains. “And then we capture all the wiggles from hundreds of towers, and they all wobble in slightly different ways. Then we can analyze these wiggles and extract the dynamic properties of each structure, such as their stiffness in response to impact and how fast ultrasound travels through them.”

The team used the same technique to scan the towers for defects. They printed several defect-free towers and then printed the same architectures with varying degrees of defects, such as missing struts and beams, each smaller than the size of a red blood cell.

“Since each tower has a vibrational signature, we saw that the more defects we put into that same structure, the more this signature shifted,” Portela says. “You could imagine scanning an assembly line of structures. If you detect one with a slightly different signature, you know it’s not perfect.”

He says scientists can easily recreate the laser setup in their own labs, and predicts this would cause the discovery of practical, real-world metamaterials to take off. For his part, Portela is keen to fabricate and test metamaterials that focus ultrasound waves, which could be used to boost the sensitivity of ultrasound probes. He’s also exploring impact-resistant metamaterials, which could be used to line the inside of bike helmets.

“We know how important it is to make materials to mitigate shock and impacts,” Kai says. “Now with our study, for the first time we can characterize the dynamic behavior of metamaterials and explore them to the extreme.”

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