In the Blink of an EyeAlexander
Researchers want to freeze-frame the workings of atoms
with laser pulses just billionths of a billionth of a second long.
But first they must prove they really can produce a blast that
shortTo photograph something that happens
very quickly, you need a camera with lightning-fast shutter speed.
But to study how molecules behave and interact, no shutter is fast
enough. Instead, for a couple of decades, researchers have used
flashes of laser light little longer than a femtosecond; that's just
a millionth of a billionth of a second. Now they want to go even
quicker. Over the past few years, scientists have passed the
femtosecond frontier and are measuring their pulses in attoseconds:
billionths of a billionth of a second.
Such flashes should allow researchers to see inside an atom by
freeze-framing the motion of an electron around the nucleus. They
haven't got there yet--researchers are still learning how to produce
the laser pulses cleanly and to measure their length--but they are
looking forward to their first snapshots of the atom's interior. "I
hope we will discover something we haven't even dreamed of," says
Ursula Keller of the Swiss Federal Institute of Technology in
|The light fantastic.
Superfast laser pulses could open a new window on the
workings of matter.
CREDIT: FERENC KRAUSZ/MAX PLANCK
A host of phenomena in molecules, atoms, and even solid matter
takes place at attosecond time scales. "The dynamics of the
electrons [during ionization] is much faster than a femtosecond. At
one point they have to decide on which ion they go and sit, and this
happens very fast," says Keller. After a decade of work, researchers
are only just getting a glimpse of such processes. "At the moment
there are very few systems that are able to make these
measurements," says Ian Walmsley of the University of Oxford.
It's impossible to make an attosecond pulse with visible light
because its wavelength lasts more than a femtosecond, and so the
pulse would be less than a wavelength long. But in the early 1990s,
several researchers suggested a way to make short pulses using
shorter wavelengths, in the extreme ultraviolet (XUV) ranges. The
technique, known as high-order harmonics generation, involves
hitting atoms of a rare gas with a powerful femtosecond pulse from
an infrared laser. As the electric field component of the infrared
pulse oscillates back and forth, it rips electrons off the atoms,
and then it smashes them back into the nucleus. As the electrons
return to the ground state, they emit a burst of radiation that is a
combination of higher harmonics of the applied infrared frequency.
The result is a sharp attosecond-long XUV pulse.
Anne L'Huillier, now at Lund University in Sweden, pioneered this
technique during the 1990s while working at the French Atomic Energy
Commission's Saclay research center at Gif-sur-Yvette. But at first,
researchers were only able to make strings of attosecond pulses
about 1.3 femtoseconds apart. To get a snapshot of events inside the
atom, they needed clean, isolated attosecond pulses. Part of the
problem was that the infrared pulses used to make the attosecond
flashes were themselves untidy and chaotic. The shape of the
pulses--how the amplitude of the radiation rose to a peak then
subsided again--bore no relation to the electromagnetic waveform
that oscillated within it.
Researchers needed infrared pulses in which the maximum of the
electromagnetic wave coincides with the maximum of the pulse
envelope. Only that peak electromagnetic wave has the intensity to
generate an XUV burst. In 1999, Keller's group proposed a way to
make such a wave using a feedback mechanism that detects the state
of the electromagnetic wave and tweaks the laser that produces it.
But it was Ferenc Krausz of the Max Planck Institute for Quantum
Optics (MPQ) in Garching, Germany, who turned theory into reality.
In 2003, while he was at the Technical University of Vienna, his
group reported neat single XUV pulses. "The Vienna-MPQ group is now
clearly the leading group in this area. They have a system that
works, and it works well," says Walmsley.
Although the pulses were undoubtedly short, Krausz and his team
still had to prove that they were less than a femtosecond long.
Earlier this year Krausz employed a technique known as a "streak
camera" to measure the pulse length. He and his colleagues directed
an XUV flash at a target of neon atoms. The pulse tears electrons
from these atoms, and then the electric field of a second, infrared
light pulse sweeps them sideways into an electron detector. From the
energy distribution of these electrons, the researchers could
determine the duration of the x-ray pulse--a speedy 250 attoseconds.
To demonstrate what attosecond pulses can do, Krausz and his team
used them to make a waveform of light visible (Science, 27
August, p. 1267).
In a technique they've dubbed the "light oscilloscope," the team
ejected electrons from some atoms by blasting them with an
attosecond XUV pulse and then hit those electrons with a femtosecond
infrared pulse. During the small time window of 250 attoseconds, the
electric field associated with the infrared light wave accelerates
these electrons, which are then captured by a detector. From their
arrival times and energies, the team could deduce the shape of the
infrared light wave.
Several groups in Europe, North America, and Japan are now
gearing up to do similar research, says Walmsley. "The tools and
techniques of attosecond metrology are now really ready," says
Krausz. One such team is led by theoretical physicist Thomas Brabec
of the University of Ottawa. "We are working on potential
applications in atoms and clusters ... because [clusters] are the
transition between atoms and condensed matter," he says. And Ahmed
Zewail of the California Institute of Technology in Pasadena, who
received the 1999 Nobel Prize in chemistry for his pioneering work
in femtochemistry, is now also looking through this new window at
matter in a state never seen before: "If you can catch any system in
a very short time, then you are far from the equilibrium state of
these systems, and in refining more and more the time resolution,
you will find some interesting phenomena," says Zewail.
Alexander Hellemans is a writer in Naples,