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"Nano-urchins" Could Save Soldiers' Eyes

Novel materials darken quickly when hit by intense light, such as from a stray laser beam.

One danger of modern warfare is taking a laser beam to the eye. It's not just enemies deliberately trying to blind soldiers but also accidents involving a laser rangefinder. The brief but powerful burst of light can burn eye tissue or damage the optical nerve, and even if the soldier sees the flash of light, the damage is done before he can blink.

Now a Lincoln Laboratory physicist and a materials scientist from the MIT campus are trying to develop a material that can block such bursts of light and protect the eye from damage. They're working with nanoparticles that can be embedded in the lenses of a pair of goggles and can absorb dangerous laser beams.

"For something like a high-peak-power pulse, it doesn't take long to damage an eye, says Vladimir Liberman, a member of the Laboratory's Submicrometer Technology Group. "A brief flash of light can do a lot of damage."

"The sort of tricks used in off-the-shelf self-darkening sunglasses wouldn't work with laser light," he explains. The photochromic dyes embedded in those lenses undergo a chemical change in response to bright light, but take many seconds to do so, and respond only to ultraviolet light. The pulse from a laser rangefinder, which uses visible or infrared light, may last only about 10 nanoseconds, so any material that would block it would have to react that quickly.

Nanoscale particles with spikes like those of a sea urchin— “nano-urchins”— rapidly become opaque when exposed to intense light.

Liberman's proposal is to use an interestingly shaped nanoparticle accidentally created by Francesco Stellacci, the Paul M. Cook Career Development Associate Professor of Materials Science at MIT. He was making gold nanoparticles, but instead of the expected smooth spheres, he got particles with spines sticking out of them. Stellacci named the particles nano-urchins for the sea creatures they resembled.

Liberman wants to study nano-urchins made of gold or silver. Each of these metals is a so-called plasmonic material, meaning that its physics is dominated by free-electron oscillations. That fact coupled with the small particle sizes—the nanoparticles are 20 to 30 nanometers in diameter, and the spines are 5 to 10 nanometers long—results in the odd effect of concentrating electrical fields at the tips of the spines. When light, an electromagnetic wave, hits the nano-urchin, the strong field at the tip of the spine interacts with incident photons, causing the particle to absorb the light. If you were to make lenses out of a polymer embedded with these nanoparticles, the high-intensity light would never make it to the eye, and wouldn't do any damage. At least that's the hope.

Among existing optically limiting materials, Liberman says, the best performers are able to limit incoming light with an intensity of 10 millijoules over a square centimeter delivered in pulses a few nanoseconds long. If the light has higher intensity, it triggers the reaction that blocks it. To protect eyes from laser flashes, the materials will need to be three or four orders of magnitude more sensitive—that is, triggered to turn dark at intensities as low as 1 to 10 microjoules per square centimeter in 10 nanoseconds. As this project is still in its early stages, researchers would be happy to get down to 100 µJ/cm² in 10 ns pulses, or even 1 mJ/cm² in 10 ns. Their hope is that demonstrating these initial results would generate ideas for lowering that threshold further.

Another issue is which wavelengths to block. Laser rangefinders tend to emit at visible or near-infrared wavelengths. But other existing lasers work from the ultraviolet to the far-infrared, so the researchers might want to cover as wide a range as seems practical. The best approach, Liberman suggests, would probably be to use a mix of particles of different shapes and materials that would work at different wavelengths.

To test what works best, Stellacci will synthesize nanoparticles of different core sizes and different materials, which should change what wavelength the particles respond to. He may also be able to alter their characteristics by changing the size of the spines and how densely packed they are. He may even try attaching different molecules to the nano-urchins. Once the samples are delivered to Lincoln Laboratory, Liberman will use a laser that can be tuned from ultraviolet to infrared wavelengths and will measure how the different particles react.

But the researchers aren't limiting themselves to a shotgun approach. Liberman has been running electromagnetic simulations to perform what he calls "pretty intense calculations" of the distribution of electrical fields around the nanoparticles. After running these simulations on his computer, he can suggest to Stellacci what shapes and sizes might best accomplish their goals.

There may be other applications for optical-limiting materials in addition to goggles for soldiers on the ground. For instance, there are reports of pilots being targeted by ground-based lasers. "This problem, however, is much more challenging," Liberman says, "because while it's important pilots not be blinded or dazzled by laser light, they also can't afford to have a windshield that goes dark for any period of time."

The one-year project was just gearing up this summer and will consist of six months of computer simulations, followed by six months of actual measurements. By the fall of 2009, Liberman may be looking for a sponsor who is interested in taking the research further. But first he needs to show whether this idea will work. "We need to understand some fundamental things," he says.