Electron ping pong in the nano-world

Apr 25, 2011 by Thorsten Naeser
Figure 1: 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. Credit: Christian Hackenberger/LMU

(PhysOrg.com) -- An international team of researchers succeeded at the Max Planck Institute of Quantum Optics to control and monitor strongly accelerated electrons from nano-spheres with extremely short and intense laser pulses. (Nature Physics, 24. April 2011).

When intense laser light interacts with electrons in that consist of many million individual atoms, these electrons can be released and strongly accelerated.

Such an effect in nano-spheres of silica was recently observed by an international team of researchers in the Laboratory for Attosecond Physics (LAP) at the Max Planck Institute of . The researchers report 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 by the laser light, the released electrons are accelerated, such that they can by far exceed the limits in acceleration that were observed so far for single atoms. 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, which are described by the scientists in the journal , open up new perspectives for the development of ultrafast, light-controlled nano-electronics, which could potentially operate up to one million times faster than current electronics.
 
Electron acceleration in a laser field is similar to a short rally in a ping pongmatch:  a serve, a return and a smash  securing the point. A similar scenario occurs when electrons in nanoparticles are hit by light pulses. An international team, led by three German groups including Prof. Matthias Kling from the Laboratory for Attosecond Physics (LAP) at the Max Planck Institute of Quantum Optics in Garching and the Ludwig-Maximilian University Munich, Prof. Eckart Rühl from the Free University of Berlin and Prof. Thomas Fennel from the University of Rostock, was now successful in observing the mechanisms and aftermath of such a ping pong play of electrons in nanoparticles interacting with strong laser light-fields.

Figure 2: Amplified near-fields at the poles of a silica nanosphere. The local field on the polar axis is plotted as 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 backside, where the highest peak field is reached. The energies of the electrons and their emission directions are determined from the experiment. Credit: Christian Hackenberger/LMU

The researchers illuminated nanoparticles with a size of around 100 nm with very intense light pulses, lasting around five femtoseconds (one femtosecond is a millionth of a billionth of a second). Such short 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 travelling less than one nanometer away from the surface of the nano-spheres, some of the electrons can be returned to the surface 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 ca. 60 times the energy of a 700 nm wavelength laser photon (in the red spectral region of light) have been found.

For the first time, the researchers could observe and record the direct elastic recollision phenomenon from a nanosystem in detail. The scientists used polarized light for their experiments. With polarized light, the light waves are oscillating only along one axis and not, as with normal light, in all directions. “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.”, explains Eckart Rühl.

The accelerated electrons left the atoms with different directions and different 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,” describes Matthias Kling about the experiment. 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,” adds Kling.

The electron movements can also produce pulses of extreme ultraviolet 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,” explains Thomas Fennel. The evidence of collective acceleration of with nanoparticles offers great potential. Matthias Kling believes that “From this may arise new, promising applications in future, light-controlled ultrafast electronics, which may work up to one million times faster than conventional electronics.”

Explore further: From light into matter, nothing seems to stop quantum teleportation

More information: Sergey Zherebtsov, et al. Controlled near-field enhanced electron acceleration from dielectric nanospheres with intense few-cycle laser fields, Nature Physics, 24. April 2011, doi: 10.1038/NPHYS1983

Provided by Max Planck Institute

5 /5 (2 votes)

Related Stories

Laser pulses control single electrons in complex molecules

Sep 01, 2009

Predatory fish are well aware of the problem: In a swarm of small fish it is hard to isolate prey. A similar situation can be found in the microcosm of atoms and molecules, whose behavior is influenced by "swarms" of electrons. ...

Light oscillations become visible

Aug 28, 2004

The human eye can detect changes in the intensity of light, not however the wavelength because light oscillates too fast (approximately 1000 trillion times per second). An international collaboration led by Ferenc Krausz an ...

Scientists track electrons in molecules

Jun 13, 2010

(PhysOrg.com) -- Physicists in Europe have successfully glimpsed the motion of electrons in molecules. The results are a major boon for the research world. Knowing how electrons move within molecules will ...

A Bunch of Electron Chicanery

Nov 21, 2006

As the Linac Coherent Light Source (LCLS) takes shape over the next few years, one of the key issues occupying the minds of physicists is controlling the size and shape of the electron pulses used to generate ...

Recommended for you

Quantum mechanics to charge your laptop?

Sep 18, 2014

Top scientists from UC Berkeley and MIT found the expertise they lacked at FIU. They invited Sakhrat Khizroev, a professor with appointments in both medicine and engineering, to help them conduct research ...

Physicists design zero-friction quantum engine

Sep 16, 2014

(Phys.org) —In real physical processes, some energy is always lost any time work is produced. The lost energy almost always occurs due to friction, especially in processes that involve mechanical motion. ...

User comments : 0