Posted: August 13, 2007 |
Snapshots of electrons |
(Nanowerk News) No flash of light can be shorter than the
time it takes the wave carrying the flash to
perform a full oscillation. A team headed
by Prof. Ferenc Krausz, Director, Max Planck Institute of Quantum Optics in Munich, Germany, has now succeeded
in generating – for the first time – flashes
of intense laser light that deliver more than
half of their energy within a single wellcontrolled
wave cycle. Atoms exposed to
this extreme light pulse emit an attosecond
X-ray pulse (1 attosecond = one billionth of
a billionth of a second) whose wave
components, if oscillating more slowly,
would represent nearly all colours of
visible light, all the way from blue through
green and yellow to red. The resultant
“white” pulse has an expected duration of
about 100 attoseconds and is composed of
more than a million X-ray photons.
Therefore, it is brief enough, and powerful
enough to capture the motion of electrons
moving on molecular orbitals. Real-time
observation of the electrons that bind
atoms together will provide invaluable
insight into the microscopic origin of the
formation and deformation of molecules.
The results were reported in the July issue
of New Journal of Physics and
featured on the cover of Science (August
10, 2007).
|
Light is a wave in which the oscillating electromagnetic field changes its direction and strength with
awesome rapidity. In the case of visible light, these changes occur several 100 trillion times per
second. Consequently, it takes a visible light wave only several thousand attoseconds to perform
an oscillation. A team of researchers at the Ludwig Maximilians University Munich and the Max
Planck Institute of Quantum Optics led by Prof. Ferenc Krausz has now produced intense flashes
of visible laser light containing more than 50% of their energy within a single oscillation cycle. This
single, large amplitude, field oscillation is used to exert a well-controlled ultrastrong force on
charged particles, such as electrons, allowing unprecedented precise control of their motion in and
around atoms.
|
On the crest of this ultra-intense wave cycle, the force is strong enough to pull an electron away
from an atom with almost 100% probability. The freed electron is first pulled away from the atom
with a speed of several thousand km/s. Even at this high velocity, the electron can travel only
several nanometers before it is turned around and directed back toward its parent atom by the next
half wave cycle, which exerts its force in the opposite direction. As a result, only some two
thousand attoseconds after being freed, the electron is recombined with its parent atom, emitting
an X-ray pulse during the recombination.
|
In a conventional laser pulse, consisting of many wave cycles, this process of recombination and
X-ray emission occurs many times, once during each half wave cycle. In striking contrast, the
intense, hypershort laser pulse generated by the LMU-MPQ team ("Attosecond Control and Measurement: Lightwave Electronics") allows only one energetic
recombination to occur, resulting in the emission of a single isolated burst of soft-X-ray light
spanning a spectral range equivalent to the entire visible spectrum. The hypershort laser light
pulse is used to illuminate a large number of atoms (in a gas jet), inducing this energetic electronatom
recombination in a large number of atoms in unison. These individual atoms all emit an
ultrashort X-ray burst in the same way at the same time, collectively giving rise to a powerful X-ray
pulse delivered in a collimated, laser-like beam.
|
The ultra-strong, single wave cycle of the hypershort laser pulse, results in the emission of vastly
different frequencies (colours) within the X-ray pulse. By slicing out only the central portion of this
wide band of frequencies, 170 attosecond X-ray pulses were generated. This result suggests
that by using the entire band of available X-ray frequencies, which is more than twice as broad as
the spectrum used to produce the 170-as pulse, X-ray pulses substantially shorter than 100
attoseconds can be generated. A mirror capable of reflecting and focusing all these X-ray waves is
currently under development.
|
Once available, it is likely to lead to the production of the world’s
first light source producing powerful laser-like X-ray flashes of duration shorter than 100
attoseconds – the first source producing sub-100-as light. In the near future these X-ray pulses will
allow researchers to take “freeze-frame” snapshots of electrons moving in molecules, allowing
reconstruction of the motions that control information transfer on molecular scales as well as
structural changes of both small and large (bio-) molecules. These snapshots may reveal the
ultimate speed and size limits in electronics, the mechanisms of biological information transfer, and
the microscopic origins of the function and malfunction of biological macromolecules.
|