Wednesday, January 06, 2016

Invisible Light Flashes Ignite “Nano-Fireworks”

As people around the world prepared for the excitement of New Years Eve and the fireworks that often accompany the stroke of midnight, fireworks of a different kind were taking place at a laboratory in Berlin.

Nanoparticles, clusters of atoms roughly a billionth of a meter in size, are rapidly finding applications in everything from highly-targeted drug delivery to the fabrication of metamaterials—substances that display properties unseen in nature. To utilize these tiny tools, scientists need to be able to manipulate nanoparticles in a variety of ways. Sometimes that means etching away atoms one at a time, sometimes it means laying down atoms in such a way that they self-organize into highly ordered structures, building infinitesimal carbon cages the same way water molecules latch together to form snowflakes.

Once in a while, it means blowing them to smithereens.

A quick, high-intensity pulse of infrared photons is often enough to do the trick—electromagnetic oscillations will toss atoms within the nanoparticle back and forth, causing wild collisions that destroy the covalent bonds between atoms, creating highly-charged ions and causing the particle's constituents to fly apart. In some cases, that's not enough.

Argon nanoparticles, the subject of a new study from the Max Born Institute's Extreme Ultraviolet (XUV) Lab, are ordinarily transparent to the infrared frequencies that lead to this rapid ionization and decoherence. Thanks to argon's noble nature, it doesn't need to form inter-atomic bonds; each atom's electron shell is already full. As a result, the electrons in the nanoparticle can't be dragged around by the comparatively large waves of infrared light, limiting their absorption of its energy. It's a bit like how a buoy, tethered to a rock away from the shore, can't be swept on to the sand by the waves, even if the tether has some stretch to it.

To circumvent this limitation, the scientists at the XUV lab do what they do best, setting up the reaction by applying pulses of photons in the extreme ultraviolet range, just on the near side of x-ray territory. This first XUV pulse "primes" the nanoparticle, ionizing just a few atoms by tearing off some of their electrons. Before those electrons have a chance to settle down or fly off, a second light pulse is initiated—this one in the near-infrared (NIR) range.

The one-two punch works spectacularly. In the commotion caused by the XUV pulse, electrons find themselves without a home—free to surf the sudden barrage of NIR waves. As they absorb energy from the second pulse, those initial free electrons slam into other electrons with enough energy to free them. Those electrons start absorbing the NIR light, setting off a chain reaction of energy absorption known as an ionization cascade, disintegrating the nanoparticle into a fireworks display a billion times smaller and more than a trillion times faster than the real thing—all the more breathtaking for its small stature.

A visualization of an argon nanoparticle, showing the trajectories its atoms might take as it disintegrates.
Image Credit: Mathias Arbeiter & Thomas Fennel, University of Rostock

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