Electron ping-pong in the nanoworld

For the first time, an international research team has succeeded in controlling and observing strongly accelerated electrons on nanospheres with extremely short and intense laser pulses.

When strong laser light encounters electrons in nanoparticles that are made up of many millions of atoms, electrons can be released and greatly accelerated. Such an effect in quartz nanospheres has now been recorded by an international research team in the Laboratory for Attosecond Physics (LAP) at the Max Planck Institute of Quantum Optics. The researchers observed how strong electric fields (near fields) in the vicinity of the nanoparticles were built up in the laser light and released electrons - the nanoparticles are ionized in the laser light.

Fig. 1: Mechanism of acceleration of electrons on glass nanospheres. The laser field (red wave) leads to the release of electrons (green particles), which are then moved away from the nanoparticle by the laser field and then accelerated back again.
After an elastic collision with the surface of the nanosphere, very high energies are finally achieved for the released electrons. The picture shows three snapshots of the acceleration (from left to right): 1) the electrons are stopped and return to the surface, 2) the electrons collide elastically with the surface and bounce off and 3) the electrons become very strong accelerated away from the nanosphere.
Christian Hackenberger / LMU

With the help of the near fields and collective interactions of the resulting charges, released electrons could be accelerated with light far enough that they far exceeded the limits of the acceleration observed at individual atoms. The precise movements of the electrons can be precisely controlled via the electric field of the laser light. The new findings of this light-controlled process could help to generate very energetic extreme ultraviolet (XUV) radiation. The experiments and their theoretical modeling, which the scientists describe in the journal Nature Physics, also offer new perspectives for the development of ultrafast, light-controlled nanoelectronics, which could work up to a million times faster than today's electronics.

The process of electron acceleration is reminiscent of a short rally in table tennis. Serve, return and another quick hit, which leads to the point win. It is just as similar when electrons in nanoparticles come into contact with light pulses. An international team, in which Prof. Marc Vrakking from the Max Born Institute (MBI) is involved, has now succeeded in observing the mechanisms and their effects of such a ping-pong game of electrons in nanoparticles under the influence of strong laser light fields.

Fig. 2: Reinforced near fields on a nanosphere of glass. The near fields on the polar axis of the particle are time-dependent, with the time running from bottom right to top left as in the illustrated wave. Along the polarization axis of the laser (along the crests and valleys) the fields show a clear asymmetry in their amplitude. This asymmetry leads to a higher energy gain of the electrons on one side of the nanoparticle compared to the other. In the case shown, the fastest electrons are formed by the maximum field elevation on the back of the particle. The energy of the electrons and their directions of emission are determined in the experiment.
Christian Hackenberger / LMU

For the first time, scientists were able to observe and record in detail the phenomenon of this direct elastic recoil in a collective nanocomposite. The researchers used polarized light for their experiments. In polarized light, the light waves oscillate only along one axis and not in all directions, as in normal light. "Intense radiation pulses can alter or destroy the nanoparticles. Therefore, we prepared isolated nanoparticles in one beam so that fresh nanoparticles were used for each laser pulse. This is crucial for the observation of high-energy electrons, "explains Prof. Eckart Rühl of the Freie Universität Berlin.

The accelerated electrons left the atoms in different directions and with different energies. These trajectories recorded the scientists in a three-dimensional image, with which they determined the energies and the directions of emission of the electrons. "The electrons are accelerated not only by the laser-induced near field, which itself is already significantly stronger than the laser field, but also by interactions with other electrons, which are released from the nanoparticle," describes Prof. Matthias Kling from the Max Planck Institute for Quantum optics in Garching the experiment. Finally, the positive charge of the nanoparticle surface also plays a role. Since all contributions add up, the energy of the electrons can be very high. "The process is complex, but it shows that there is still a lot to discover in the interaction of nanoparticles with strong laser fields," adds Kling.

The movements of electrons can also produce pulses of extreme ultraviolet light, namely whenever the electrons strike the surface again, but instead of being ricocheted, absorbed and emitting light. Extreme ultraviolet light is especially interesting for biological and medical research.

"According to our findings, the recombination of the electrons on the nanoparticles can reach energies of the emitted photons that are up to seven times the limit that was previously observed for individual atoms," explains Prof. Thomas Fennel from the University of Rostock. The proof of the collective acceleration of the electrons with the nanoparticles offers great potential. "This results in promising new applications in future, light-controlled, ultra-fast electronics, which could work up to a million times faster than conventional electronics," Matthias Kling is convinced.

Original publication

Controlled near-field enhanced electron acceleration from dielectric nanospheres with intense few-cycle laser fields

S. Zherebtsov, T. Fennel, J. Plenge, E. Antonsson, I. Znakovskaya, A. Wirth, O. Herrwerth, F. Süßmann, C. Peltz, I. Ahmad, S. A. Trushin, V. Pervak, S. Karsch, M. J. J. Vrakking, G. Langer, C. Graf, M. I. Stockman, F. Krausz, E. Rühl, M. F. Kling

Nature Physics 7 (2011) 656-662