An international team of researchers succeeded to
control and monitor strongly accelerated electrons from nano-spheres with
extremely short and intense laser pulses. This result is published in the
journal “Nature Physics”.
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 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 Quantum Optics. 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 ionization 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
“Nature Physics,” 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,
in which Prof. Marc Vrakking from the Max-Born-Instiute in Berlin participates,
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.
The
researchers illuminated silica 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 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
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 Prof. Eckart Rühl from
the Free University of Berlin.
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 Prof. Matthias Kling from the Max Planck Institute of
Quantum Optics in Garching 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 Prof. Thomas Fennel from the University of
Rostock. The evidence of collective acceleration of electrons 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.”
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.
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.
Courtesy of
Christian Hackenberger/LMU
Original Publication:
Controlled near-field enhanced
electron acceleration from dielectric nanospheres with intense few-cycle laser fields
Sergey Zherebtsov, Thomas Fennel,
Jürgen Plenge, Egill Antonsson, Irina Znakovskaya, Adrian Wirth, Oliver
Herrwerth, Frederik Süßmann, Christian Peltz, Izhar Ahmad, Sergei A. Trushin,
Vladimir Pervak, Stefan Karsch1, Marc J.J.
Vrakking, Burkhard Langer, Christina Graf, Mark I. Stockman, Ferenc Krausz,
Eckart Rühl, Matthias F. Kling
Nature Physics,
24. April, doi: 10.1038/NPHYS1983
Kontakt:
Prof. Dr. Marc Vrakking |
