Pulses Control Electrons from Nanospheres

Extremely short and intense laser pulses can now control and monitor strongly accelerated electrons from nanospheres.

When intense laser light interacts with electrons in nanoparticles that consist of many million individual atoms, these electrons can be released and strongly accelerated. Such an effect in nanospheres of silica was recently observed by an international team of researchers, led by three German groups headed by Professor Matthias Kling from the Laboratory for Attosecond Physics (LAP) at the Max Planck Institute of Quantum Optics and the Ludwig-Maximilian University Munich, Professor Eckart Rühl from the Free University of Berlin and Professor Thomas Fennel from the University of Rostock.

The researchers observed how strong electrical fields (near fields) build up in the vicinity of the nanoparticles and release electrons. Driven by the near fields and collective interactions of the charges resulting from ionization by the laser light, the released electrons are accelerated, such that they can by far exceed the limits in acceleration that to date have been observed for single atoms.

Mechanism of the acceleration of electrons near silica nanospheres. Electrons (depicted as green particles) are released by the laser field (red wave). These electrons are first accelerated away from the particle surface and then driven back to it by the laser field. After an elastic collision with the surface, they are accelerated away again and reach very high kinetic energies. The figure shows three snapshots of the acceleration (from left to right): 1) The electrons are stopped and forced to return to the surface; 2) When reaching the surface, they elastically bounce right back; 3) The electrons are accelerated away from the surface of the particle, reaching high kinetic energies. (Images: Christian Hackenberger/LMU)

The exact movement of the electrons can be precisely controlled via the electric field of the laser light. The new insights into this light-controlled process can help to generate energetic extreme-ultraviolet (XUV) radiation. The experiments and their theoretical modeling open up new perspectives for the development of ultrafast light-controlled nanoelectronics, which potentially could operate up to 1 million times faster than current electronics.

Electron acceleration in a laser field is similar to a short rally in a ping pong match: a serve, a return and a smash securing the point. A similar scenario occurs when electrons in nanoparticles are hit by light pulses. The team has now been successful in observing the mechanisms and aftermath of such a ping pong play of electrons in nanoparticles interacting with strong laser light fields.

The researchers illuminated silica nanoparticles with a size of around 100 nm with very intense 5-fs light pulses. Such short laser pulses consist of only a few wave cycles. The nanoparticles contained around 50 million atoms each. The electrons are ionized within a fraction of a femtosecond and accelerated by the electric field of the remaining laser pulse. After traveling less than 1 nm away from the surface of the nanospheres, some of the electrons can be returned by the laser field to the surface, where they were smashed right back (such as the ping pong ball being hit by the paddle). The resulting energy gain of the electrons can reach very high values. In the experiment, electron energies of around 60 times the energy of a 700-nm wavelength laser photon (in the red spectral region) have been found.

Amplified near fields at the poles of a silica nanosphere. The local field on the polar axis is plotted as a function of time, where time within the few-cycle wave runs from the lower right to the upper left. The fields show a pronounced asymmetry along the polarization axis of the laser (i.e., along the rims and valleys of the wave). This asymmetry leads to higher energies gained by electrons on one side of the nanoparticle as compared to the other side. For the given example, the most energetic electrons are emitted from the back, where the highest peak field is reached. The energies of the electrons and their emission directions are determined from the experiment.

For the first time, the researchers could observe and record in detail the direct elastic recollision phenomenon from a nanosystem. The scientists used polarized light for their experiments.

“Intense radiation pulses can deform or destroy nanoparticles. We have thus prepared the nanoparticles in a beam, such that fresh nanoparticles were used for every laser pulse. This was of paramount importance for the observation of the highly energetic electrons,” said Rühl.

The accelerated electrons left the atoms in different directions and with varying energies. The flight trajectories were recorded by the scientists in a three-dimensional picture, which they used to determine the energies and emission directions of the electrons.

“The electrons were not only accelerated by the laser-induced near field, which by itself was already stronger than the laser field, but also by the interactions with other electrons, which were released from the nanoparticles,” explained Kling, adding that finally, the positive charging 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 shows that there is much to explore in the interaction of nanoparticles with strong laser fields.”

The electron movements can also produce pulses of XUV light when electrons that hit the surface do not bounce back but are absorbed, releasing photons with wavelengths in the XUV. XUV light is of particular interest for biological and medical research.

“According to our findings, the recombination of electrons on the nanoparticles can lead to energies of the generated photons, which are up to seven times higher than the limit that was so far observed for single atoms,” said Fennel.

The team said these findings may generate light-controlled ultrafast electronic applications that are up to 1 million times faster than conventional electronics.

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