In working to resolve a twenty-year-old debate about what fate holds for neutral atoms blasted by a powerful laser, researchers have revealed a surprising twist. A four-strong team led by Ulli Eichmann at Berlin's Max Born Institute used lasers to accelerate neutral atoms to one of the fastest rates known in the Universe — without tearing the atoms apart.

Physicists often apply laser pulses lasting just a split second to tiny objects such as atoms. The laser's energy is transferred to the atoms, which typically become ionized, forming ions and electrons. These charged particles shoot away from the position of the laser beam. Laser-accelerated ions and electrons have already found various important applications in biology and physics, for example as optical tweasers. But the hunt is still on for techniques that can accelerate neutral particles in powerful and precise ways.

The idea that a neutral atom — so named because its equal positive and negative charges cancel each other out — might be able to survive a strong laser field intact was first mooted in the 1980s. However, it was not until 2008 that a paper provided evidence of how this might actually happen. The work suggested that the neutral atom would survive in an excited state, leading Eichmann and his colleagues to wonder whether it might be possible to accelerate neutral atoms by applying a strong laser pulse to them. “I was curious to see if we could detect this acceleration in excited states”, says Eichmann.

The researchers decided to pursue the experiment using helium atoms. As an atomic featherweight, helium is easy to 'push around' with the forces supplied by the lasers. In the particle accelerator, “you apply hell to these atoms, and then you see the details”, explains Eichmann. These details — revealed by a detector that measures the positions of accelerated particles — showed that excited neutral atoms had indeed survived the laser (see page 1261). Even more surprisingly, the acceleration on these particles had been ultra-strong — much higher than anything previously observed in the laboratory.

So why was this? Using a quantitative theoretical model, Eichmann and his team explain that helium's electron, which oscillates vigorously near its ionic core in response to the laser field, still lacks enough energy after the pulse to escape the core. As a result, the electron is recaptured by the core in a bound excited state. During the oscillation, however, the strongly focused laser beam induces a specific non-linear force, or ponderomotive force, on the electron, pushing it towards areas of lower laser intensity. Caught in a tug-of-war between the atom's core and the ponderomotive force, the electron drags the core with it, accelerating the whole atom at an ultra-high rate.

According to Eichmann, examples of how such super-acceleration could be used include more precise manipulation of atom motion — much like the way in which light can be manipulated with lenses — and the deposition of atoms on the surfaces of nanostructures.

Eichmann remains most excited about the fundamental insight into strong-field and particle physics. “Twenty years ago, there was a big discussion on the stabilization of atoms in strong laser fields — but there was only indirect evidence that this was true,” he says. “Our study is a strong indication that this type of configuration is as was predicted. All the people who saw this result were astonished that this worked.”