An international team of scientists successfully concentrated the
energy of infrared laser pulses using a nano funnel enabling them to
generate extreme ultraviolet light pulses, which repeated 75 million
times per second.
Who wants to decant liquids in the kitchen without spilling knows to
value a funnel. Funnels are not only useful tools in the kitchen. Light
can also be efficiently concentrated with funnels. In this case, the
funnels have to be about 10.000-times smaller. An international team of
scientists from the Korea Advanced Institute of Science and Technology
(KAIST) in Daejeon (South Korea), the Max Planck Institute of Quantum
Optics (MPQ) in Garching (Germany), and the Georgia State University
(GSU) in Atlanta (USA) has now managed to concentrate the energy of
infrared light pulses with a nano funnel and use the concentrated
energy to generate extreme ultraviolet light flashes. These flashes,
which repeated 75 million times per second, lasted only a few
femtoseconds. The new technology can help in the future to measure the
movement of electrons with the highest spatial and temporal resolution
(Nature Photonics, 16.10.2011).
Light is convertible. The wavelengths composing the light can change
through interactions with matter, where both the type of material and
shape of the material are important for the frequency conversion. An
international team of scientists from the Korea Advanced Institute of
Science and Technology (KAIST), the Max Planck Institute of Quantum
Optics (MPQ), and the Georgia State University (GSU) has now modified
light waves with a nano funnel made out of silver. The scientists
converted femtosecond laser pulses in the infrared spectral range to
femtosecond light flashes in the extreme ultraviolet (EUV). Ultrashort,
pulsed EUV light is used in laser physics to explore the inside of
atoms and molecules. A femtosecond lasts only a millionth of a
billionth of a second.
Light in the infrared (IR) can be converted to the EUV by a process
known as high-harmonic generation, whereby the atoms are exposed to a
strong electric field from the IR laser pulses. These fields have to be
as strong as the fields holding the atom together. With these fields
electrons can be extracted from the atoms and accelerated with full
force back onto the atoms. Upon impact highly energetic radiation in
the EUV is generated.
To reach the necessary strong electric fields for the production of EUV
light, the team of scientists has now combined this scheme with a nano
funnel in order to concentrate the electric field of the light. With
their new technology, they were able to create a powerful EUV light
source with wavelengths down to 20 nanometers. The light source
exhibits a so far unreached high repetition rate: the few femtoseconds
lasting EUV light flashes are repeated 75 million times per second.
The core of the experiment was a small, only a few micrometers long,
slightly elliptical funnel made out of silver and filled with xenon gas
(see Fig. 1). The tip of the funnel was only ca. 100 nanometers wide.
The infrared light pulses were sent into the funnel entrance where they
travel through towards the small exit. The electromagnetic forces of
the light result in density fluctuations of the electrons on the inside
of the funnel. Here, a small patch of the metal surface was positively
charged, the next one negative and so on, resulting in new
electromagnetic fields on the inside of the funnel, which are called
surface plasmon polaritons. The surface plasmon polaritons travel
towards the tip of the funnel, where the conical shape of the funnel
results in a concentration of their fields. “The field on the
inside of the funnel can become a few hundred times stronger than the
field of the incident infrared light. This enhanced field results in
the generation of EUV light in the Xe gas.”, explains Prof. Mark
Stockman from GSU.
The nano funnel has yet another function. Its small opening at the exit
acts as “doorman” for light wavelengths. Not every opening
is passable for light. If the opening is smaller than half of a
wavelength, the other side remains dark. The 100 nanometer large
opening of the funnel did not allow the infrared light at 800 nm to
pass. The generated EUV pulses with wavelengths down to 20 nanometers
passed, however, without problems. “The funnel acts as an
efficient wavelength filter: at the small opening only EUV light comes
out.”, explains Prof. Seung-Woo Kim from KAIST, where the
experiments were conducted.
“Due to their short wavelength and potentially short pulse
duration reaching into the attosecond domain, extreme ultraviolet light
pulses are an important tool for the exploration of electron dynamics
in atoms, molecules and solids”, explains Seung-Woo Kim.
Electrons are extremely fast, moving on attosecond timescales (an
attosecond is a billionth of a billionth of a second). In order to
capture a moving electron, light flashes are needed, which are shorter
than the timescale of the motion. Attosecond light flashes have become
a familiar tool in the exploration of electron motion. With the
conventional techniques, they can only be repeated a few thousand times
per second. This can change with the nano funnel. “We assume that
the few femtosecond light flashes consist of trains of attosecond
pulses”, argues Matthias Kling, group leader at MPQ. “With
such pulse trains, we should be able to conduct experiments with
attosecond time resolution at very high repetition rate.”
The repetition rate is important for e.g. the application of EUV pulses
in electron spectroscopy on surfaces. Electrons repel each other by
Coulomb forces. Therefore, it may be necessary to restrict the
experimental conditions such that only a single electron is generated
per laser shot. With low repetition rates, long data acquisition times
would be required in order to achieve sufficient experimental
resolution. “In order to conduct experiments with high spatial
and temporal resolution within a sufficiently short time, a high
repetition rate EUV source is needed”, explains Kling. The novel
combination of laser technology and nanotechnology can help in the
future to record movies of ultrafast electron motion on surfaces with
so far unreached temporal and spatial resolution in the
attosecond-nanometer domain. Thorsten Naeser
Publication:
In-Yong Park, Seungchul Kim, Joonhee Choi, Dong-Hyub Lee, Young-Jin Kim, Matthias F. Kling, Mark I. Stockman & Seung-Woo Kim
Plasmonic generation of ultrashort extreme-ultraviolet light pulses
Nature Photonics, 16 October 2011, Doi: 10.1038/NPHOTON.2011.258Further information on attosecond physics at
http://www.attoworld.de
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