| Radiologists
and biologists have been dreaming - ever since the discovery
of lasers - of a compact laboratory source emitting
X-rays in one direction in a laser-like beam. Such a
source would permit X-ray images to be recorded with
far higher resolution at vastly reduced dose levels,
allowing early-stage cancer diagnosis at dramatically
reduced risk. Microscopes furnished with this source
would make nanometer-sized biomolecules perceivable
in their natural surrounding (in vivo). It may take
many years before this dream comes true, but the experiment
reported by an Austrian-German collaboration led by
Ferenc Krausz indicates a promising way of realizing
the dream some day. Researchers at Vienna University
of Technology, the University of Würzburg, the
University of Munich and Max Planck Institute of Quantum
Optics demonstrated the first source of laser-like X-rays
at a wavelength of 1 nanometer with a compact laboratory
apparatus [Nature 433, 596 (2005)] in an experiment
in Vienna, funded by the Austrian Science Fund.
The
colour of light is determined by the length of one
cycle of the electromagnetic wave (referred to by
physicists as the wavelength) that makes up light.
Red light has a wavelength of around 700 nanometers,
whereas our eye perceives radiation as violet light
if its wavelength is only 400 nanometers. Light of
even shorter wavelength is invisible (ultraviolet
light), and with the wave cycle shortened to less
than 1 nanometer, the X-ray regime is entered.
The
Austrian-German team focused a sequence of intense
ultrashort flashes of red light at a gas of helium
atoms to convert 700-nm laser light into a 1-nm wave
of X-ray light emitted by the excited atoms (Figure).
The intense laser field makes the negatively charged
electron cloud perform giant oscillations around the
positively charged atomic core, thereby turning the
atoms into antennas. Because of the giant amplitude
of their oscillations, these radiate waves not only
at the wavelength of the driving laser (700-nm) but
also at shorter wavelengths. Since the antennas are
in phase over in time, they also keep time when emitting
their waves. Although these tiny "atomic"
waves are extraordinarily faint, because they all
oscillate in time they add to build up an X-ray wave
of significant intensity delivered in a highly-directed
beam parallel to the incident laser.
The
phenomenon described above is not new. It has become
a standard technique for routinely producing laser-like
extreme ultraviolet radiation at wavelengths down
to the 10 nanometer regime. Pushing the frontier of
this technology to ever shorter wavelengths has proven
ever more difficult because it requires atoms to be
exposed to laser light of ever greater intensity,
which tends to disintegrate the atoms. What makes
the situation even worse, the free electrons ripped
off the atoms by the strong laser field impede the
buildup of an intense wave from the faint "atomic"
waves.
The
Vienna-Würzburg-Munich team have overcome these
problems by irradiating the atoms with the world’s
shortest high-intensity laser pulses, lasting merely
5 millionths of a billionth of a second (= 5 femtoseconds).
These pulses hit the atoms so abruptly that they have
no time to disintegrate before emitting the X-ray
burst. Thanks to this extremely short interaction
time, the researchers have not only managed to break
the nanometer barrier but also created a source of
X-ray bursts that may, for the first time, be briefer
than 0.1 femtosecond (= 100 attoseconds). The X-ray
beam delivered by the new source is - at present -
too weak for any practical applications, but the research
team are confident that technical improvements will
boost the X-ray power by several orders of magnitude.
Once this feat is achieved, this novel research tool
will open up new prospects in several areas of physics,
biology and materials science.
Original work:
J.
Seres, E. Seres, A.J. Verhoef, G. Tempea, C. Streli,
P. Wobrauschek, V. Yakovlev, A. Scrinzi, C. Spielmann,
F. Krausz
Source of coherent kiloelectronvolt X-rays
Nature 433, 596; 10. Januar 2005
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ISSN 0170-4656
Contact:
Prof.
Ferenc Krausz
Max-Planck-Institute of Quantum Optics, Garching,
Germany
Tel.: +49 89 32905-612
Fax: +49 89 32905-314
E-mail: ferenc.krausz@mpq.mpg.de
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