New Application Of Laser Technology: Boosting Research On Metamaterials

Metamaterials, though made from everyday polymers, ceramics and metals, have extraordinary properties due to their intricate and complex precision microstructures.
With the help of computer simulations, engineers can combine any number of microstructures and observe how certain materials transform, for example, to see how certain materials can be transformed into sound-focusing acoustic lenses or lightweight bulletproof membranes.
But simulation design can only go so far. Physical testing of metamaterials is necessary to determine whether they will achieve the desired results. But there has been no reliable way to push and pull on metamaterials at the microscopic scale and see how they will react without touching and physically damaging the metamaterial structure in the process.
To solve this problem, MIT researchers have developed a technique to probe metamaterials using a two-beam laser system-one laser beam rapidly illuminates the structure, and the other laser beam measures the way the structure responds to vibrations, much like striking a bell with a mallet and recording its reverberation. In contrast to a mallet, lasers make no physical contact. Yet they create vibrations in the tiny beams and struts of the metamaterial, just as if the structure had been physically struck, stretched or sheared.
Image This optical micrograph shows an array of microscopic metamaterial samples on a reflective substrate.
Engineers can then use the resulting vibrations to calculate a variety of dynamic properties of the material, such as how it responds to impacts and how it absorbs or scatters sound. Using ultrafast laser pulses, they can excite and measure hundreds of microstructures in a matter of minutes. This technique provides, for the first time, a safe, reliable and high-throughput method for dynamically characterizing microscale metamaterials.
"With this approach, we can accelerate the discovery of the best materials based on the desired properties." Professor Carlos Portela, a researcher at MIT's School of Mechanical Engineering, said. The research team calls this method LIRAS (Laser Induced Resonance Acoustic Spectroscopy).
Portela used metamaterials made from common polymers, which he 3D printed into tiny scaffold-like towers made of microscopic struts and miniature beams. Each tower is patterned by repeating and layering individual geometric units, such as an octagonal configuration of connecting beams. When stacked end-to-end, the tower arrangement can impart properties to the entire polymer that it would not otherwise have.
But engineers are severely limited in their options for physically testing and verifying these metamaterial properties. Nanoindentation is the quintessential way to probe such microstructures, albeit in a very careful and controlled manner. The method uses a micron-sized tip to slowly push down on the structure while measuring the tiny displacements and forces as the structure compresses.
But this technique can only be carried out very quickly and can damage the structure," says Portela. We wanted to find a way to measure the dynamic behavior of these structures in the initial response to a strong impact without destroying them."
The team came up with laser ultrasound - a non-destructive method that uses short laser pulses tuned to ultrasonic frequencies to excite very thin materials (such as gold films) without contact. Laser excitation produces ultrasound waves over a range of frequencies that can cause the film to vibrate at a certain frequency, which scientists can use to determine the precise thickness of the film with nanometer accuracy. The technique can also be used to determine whether a film has defects.
The team realized that ultrasonic lasers could also safely induce their 3D metamaterial towers to vibrate; these towers, which range in height from 50 micrometers to 200 micrometers, are similar to thin films on a microscopic scale.
To test this idea, the researchers built a tabletop device consisting of two ultrasonic lasers-a "pulse" laser to excite the metamaterial sample and a "probe" laser to measure the resulting vibrations. A "probe" laser to measure the resulting vibrations.
The researchers then printed hundreds of microscopic towers, each with a specific height and structure, on a chip smaller than a fingernail. They placed this microscopic structure of metamaterials in two laser units and then excited the towers with repeated ultrashort pulses. The second laser then measured the vibrations of each tower. From there, the team collected data and looked for patterns in the vibrations.
Image 3D printed tower. The MIT researchers used a laser to safely scan the metamaterial microtowers, which triggered the vibrations, which were then captured with a second laser and analyzed to infer the structure's dynamic properties, such as stiffness in response to shocks.
We excited all these structures with the laser as if we were hitting them with a hammer," Portela said. We captured the oscillations of hundreds of towers, which oscillated in slightly different ways. From this we can analyze these oscillations and extract the dynamic properties of each structure, such as their stiffness to impact and the speed at which the ultrasound waves propagate through them."
The researchers used the same technique to scan the pylons for defects. They 3D printed several defect-free towers and then printed the same structures with varying degrees of defects, such as missing struts and beams (which are even smaller than red blood cells).
Portela says, "Since each tower has a vibrational signature, we found that the more defects we put into the same structure, the more this signature changes. If you detect a structure with a slightly different signature, you know it's not perfect."
Scientists could easily recreate the laser device in their own labs, he said. The discovery of practical, real-world metamaterials would then be accelerated. In Portela's case, he's passionate about creating and testing metamaterials for focusing ultrasound waves, for example to increase the sensitivity of ultrasound probes. He is also exploring impact-resistant metamaterials, for example for the design of liner arrangements inside bicycle helmets.
Characterizing the dynamic behavior of metamaterials through this research will help explore the extremes of metamaterials, the researchers said. The study was published in the journal Nature.