(Image: Jose-Luis Olivares, MIT)

A new laser-based technique could speed up the discovery of promising metamaterials for real-world applications.

The technique, developed by MIT engineers, probes metamaterials with a system of two lasers — one to quickly zap a structure and the other to measure 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, the lasers make no physical contact; yet they can produce vibrations throughout a metamaterial’s tiny beams and struts, as if the structure were being physically struck, stretched, or sheared.

The engineers can then 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. The new technique offers a safe, reliable, and high-throughput way to dynamically characterize microscale metamaterials, for the first time.

“We need to find quicker ways of testing, optimizing, and tweaking these materials,” said Professor Carlos Portela. “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 appearing in Nature.

Here is an exclusive Tech Briefs interview with Portela and First Author Yun Kai.

Tech Briefs: I’m sure there were too many to count, but what was the biggest technical challenge you faced a while A) developing the laser setup, B) while printing the microscopic towers, and C) while scanning?

Portela: The hardest part was to be able to get significant signals out of nanometer-scale vibrations. I think that’s the hardest challenge because the amplitudes of these vibrations are so small. That took a lot of knowledge from optics with the right interferometry technique. What typically would be noise was actually a signal for us.

Kai: I want to echo what Carlos just said. The challenge is kind of a combination of all three, but the immediate challenge, at least for me, was to get great nanosecond temporal resolution. That’s why we designed this complicated optical arrangement.

Portela: For fabrication, the hard part was that these are tall towers; one of the challenges was to make sure that they’re straight and they don’t have any defects — printing them at these small scales, that's obviously one of the risks. It took a lot of optimization.

Also, a lot of the analysis that we do relies on them being a tower. So, the other question was, ‘OK, when do we stop?’ in terms of how short they can be so that we can still think of them as a tower.

Kai: One analogy I’ll try to use is a pianist: Those towers are like the piano keys. You can first make a nice, perfect piano so that you can have the right pitches and the right wave factors. Then the pianist is the optic — the guy that must play well so that you can get the frequency and the resonance. So, it’s the merging of two things.

The scanning part I don’t think was too challenging because for microscale samples it is inherently fast. We are not scanning millimeter- or centimeter-scale samples. So, the third aspect is kind of like, for me, the natural consequence of aspect one and two.

Portela: One thing that we can’t forget is aiming the laser in the right spot. Because being able to aim it off-center gives us more information than doing it on-center or right in the center.

This image shows electron microscope micrographs of polymeric metamaterial samples, approximately 50 micrometers wide and as tall as about twice the width of human hair, whose properties were determined via the LIRAS technique. Pump and probe lasers were aimed at the flat tops to induce vibrations throughout the samples. (Image: Courtesy of the researchers)

Tech Briefs: Would you mind explaining in simple terms how the scanning works?

Portela: So, the scanning, how it works, basically, is we have an ultra-fast pump of a laser. Think of it as a single burst that’s on the order of 300 picoseconds long. And the analogy there is that it’s like a hammer where you’re hitting the material very abruptly. At the macro scale, this means that you have a lot of vibrations, you're exciting the sample. So that's literally all this laser is doing — basically starting all these broadband vibrations.

The analogy breaks down because we are not making physical contact, but rather what’s happening is that we have a very thin metallic coating at the top. The laser heats up this coating very quickly so it expands and it contracts back again. And that is what exerts the mechanical stimulus.

Then, as we’re doing this, there’s another laser that is pointing at the top of the sample, which is our interferometer — using the constructive and structured interference of a laser to give you measurements of displacement, for instance. So, this interferometer was looking at the top of the sample or at the side of the sample, depending on where we were measuring, and that was giving us a displacement versus time signal.

Tech Briefs: You’ve said that scientists can easily recreate the laser setup in their own labs, and you predict that discovery of practical, real-world metamaterials will then take off. My question is, how soon do you think ‘then’ is?

Portela: Besides the hard part of thinking about how we could get useful information out of these measurements, anyone that has a laser that fits these capabilities can already do experiments like this. It really is just a laser that you can buy commercially, and then the interferometry, which is, again, another commercial laser that you just use in an interesting manner.

That part isn’t hard, but what’s hard is understanding and analyzing the data in a way that is useful. In terms of applicability of metamaterials, one of the applications that I find that this technique can start enabling is acoustic metamaterials. So, megahertz vibrations and sound, right? Ultrasound.

So, I foresee in the next five years we can potentially start seeing some metamaterials that can interface with medical ultrasound applications.

Kai: It’s like playing with Legos — you have all the components available; it’s the way you arrange them.

Tech Briefs: Do either of you gentlemen have any advice for engineers aiming to bring their ideas to fruition?

Portela: Don’t be afraid to try things. There are ways of using existing methods but to extract a lot more information that previously has either not been measurable or has been discarded because it’s noise. So, I personally think that, especially when it comes to experimentation, if there are engineers who are interested in experiments and real validation, what we need to think of is how can we do a single experiment that gives us 10 answers instead of just one property?