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Ever smaller, ever faster
Femtochemistry comes of age. Molecules are getting photographed via femtosecond light pulses. A lot of
questions remained unanswered.When scientists started to „take pictures“ of the movements of atoms in molecules during chemical reactions at the end of the 1980s, this had little to do with classical photography. The principle, however, remained the same: A short exposure time is used to produce sharp pictures. The structure of molecules changes within femotoseconds, which means within millionths of a billionth second. This is why an exposure time which also lasts femtoseconds was necessary. And scientists found it in laser technology thanks to advanced femtosecond laser flashes. Despite groundbreaking successes and the Nobel Prize for the Egyptian scientist Ahmed Zewail (in 1999), questions remained unanswered. What happens inside the electron shell prior to changes and what are the consequences of these motions in structure?
At the beginning of the new millennium it was only known from theory that these processes are a hundred to a thousand times faster than atomic movements. They happen within attoseconds (one thousandth of a femtosecond), being beyond the reach of femtochemistry. -
A new dimension of time
The first sub-femtosecond light pulses were measured at the Vienna University of Technology. Attophysics was born.
A new dimension of time is being explored. At the Vienna University of Technology, scientists – for the first time – succeed in producing and measuring light flashes which last less than one femtosecond. In doing so, the scientists focus laser pulses lasting seven femtoseconds on neon atoms. Their electrons absorb the light’s energy and subsequently emit it in the form of X-ray light flashes. The flashes emitted at the shortest wave lengths (in this experiment, around 13 nanometres) are isolated. They last only 650 attoseconds. Attosecond physics was „born“
(Nature, November 29, 2001).people involved:
M. Hentschel, R. Kienberger, C. Spielmann,
G. A. Reider, N. Milosevic, T. Brabec, P. Corkum, U. Heinzmann, M. Drescher and F. Krausz.
Nature, November 29, 2001
Setup of the AS-beamline, which produced the
first 650 attosecond X-ray lightflashes.Reinhard Kienberger and Michael Hentschel
(from left) work on the AS-Beamline in the laboratory of the Vienna University of Technology. -
Diving deep into the atom
For the first time real-time insights into electron processes deep inside the atom are obtained.
For more than a decade it has already been possible to explore the movements of atoms and molecules during chemical reactions by means of femtosecond laser pulses. However, the considerably lighter electrons are much quicker: Deep inside atoms, a vacant quantum state, i.e. a „hole“, may be filled by an electron at the „margin“ of an atom within a few thousand to a few hundred attoseconds. By means of the 650 attosecond X-ray flashes it is now possible to observe such a transition between two quantum states nearby the atomic nucleus directly. The attosecond flash knocks out an electron from an inner shell of a Krypton atom. Electrons at the „margin“ of the atom try to fill up the „hole“ as quickly as possible. During that process, energy is released through the emission of a second electron, an Auger electron, as it is called. The emission time of the Auger electron is determinated by the interaction with the laser pulse, lasting seven femtoseconds. This means that – for the first time – real-time insights into electron processes deep inside the atom are obtained
(Nature, October 24, 2002).people involved:
M. Drescher, M. Hentschel, R. Kienberger,
M. Uiberacker, V. Yakovlev, A. Scrinzi,
T. Westerwalbesloh, U. Kleineberg, U. Heinzmann and F. Krausz.
Nature, October 24, 2002
Markus Drescher and Ferenc Krausz (from left)
in front of the vacuum chamber of the first Attosecond beamline.Setup of the AS-beamline which produced the
first isolated attosecond pulses. -
Control over light waves
Laser pulses with a reproducible wave form are generated. They contain only a few wave oscillations.
Up until the end of the 20th century there had been no tool to measure or control the high-speed oscillations of light waves. The breakthrough was made with the development of a frequency comb at the Max-Planck-Institut für Quantenoptik (MPQ) shortly before the millennium. The tool allows for the precise measurement of the frequency of laser light and thus for the energy distances of quantum states. The new technology opens the door to control the temporal evolution of the electric (and the magnetic) field of laser pulses. The Vienna University of Technology uses this technology to generate laser pulses with a reproducible wave form which contains only a few wave oscillations. The controlled oscillations of the electric field mean controlled forces on electrons. This allows for the first time control of electron movements in atoms and atomic systems within attoseconds. The consequences are far reaching. First experiments confirm that it is possible to reproducibly generate attosecond flashes (Nature, February 6, 2003) and to use light as an attosecond stopwatch.
people involved:
A. Baltuška, Th. Udem, M. Uiberacker,
M. Hentschel, E. Goulielmakis, Ch. Gohle,
R. Holzwarth, V. Yakovlev, A. Scrinzi,
Th. W. Hänsch and F. Krausz.
Nature, February 6, 2003
Experiments confirm that it is possible to reproducibly generate attosecond flashes and to use light as a stopwatch. -
Time captured in a picture
The first light-field-controlled streak camera measures 250 attosecond X-ray flashes and visualizes light waves by means of attosecond flashes.
The use of attosecond flashes for the study of the microcosm requires the ability to measure them reliably. As early as in 1834 the physicist Charles Wheatstone was able to measure the duration of a light flash of an electric discharge by means of a rotating mirror. The mirror reflected the impinging light to different spatial positions. The length of the picture and the knowledge of the angular velocity of the mirror provided the duration of the flash. The streak camera was born. In order to increase the speed, the rotating mirror is replaced by the force the electric field light exerts on electrons. The electrons are knocked out by an attosecond flash, which is to be measured in the presence of the laser wave which had previously been used to generate it. Depending on the moment the removed electrons are captured by the quickly changing electric field they are accelerated differently. The resulting velocity distribution of the released electrons provides the streaked image of the attosecond pulse. Around 170 years after Wheatstone’s experiment the first light-field-controlled streak camera measures 250 attosecond X-ray flashes (Nature, February 26, 2004) and, for the first time, visualizes light waves by means of attosecond flashes (Science, August 27, 2004). Since then it has been the standard measuring instrument for attosecond science.
people involved:
R. Kienberger, E. Goulielmakis, M. Uiberacker, A. Baltuška, V. Yakovlev,
F. Bammer, A. Scrinzi, T. Westerwalbesloh, U. Kleineberg, U. Heinzmann,
M. Drescher, and F. Krausz.
Nature, February 26, 2004
E. Goulielmakis, M. Uiberacker, R. Kienberger, A. Baltuška, V. Yakovlev,
A. Scrinzi, T. Westerwalbesloh, U. Kleineberg, U. Heinzmann,
M. Drescher and F. Krausz.
Science, August 27, 2004
Eleftherios Goulielmakis, Andrius Baltuška, Reinhard Kienberger and Matthias Uiberacker (from left) in the Laserlab. The team made the first picture of a lightwave by using attosecond light flashes. -
On the scent of electrons
Electron tunnelling in atoms and Angstrom-scale electric charge transport in solids can now be scuritinized directly in the time domain.
With techniques for the reproducible generation and reliable measurement of attosecond soft-X-ray pulses and near-single-cycle field-controlled visible-infrared laser pulses in place, widespread applications to real-time observation and control of electronic processes in atomic, molecular and solid-state systems could begin. The dynamics of fundamental processes such as electron tunnelling in atoms (Nature, April 5, 2007) and Angstrom-scale electric charge transport in solids (Nature, October 25, 2007) can now be scuritinized directly in the time domain.
people involved:
M. Uiberacker, Th. Uphues, M. Schultze, A. J. Verhoef, V. Yakovlev,
M. F. Kling, J. Rauschenberger, N. M. Kabachnik, H. Schröder, M. Lezius,
K. L. Kompa, H. G. Müller, M. J. J. Vrakking, S. Hendel, U. Kleineberg,
U. Heinzmann, M. Drescher, and F. Krausz.
Nature, April 5, 2007
A. Cavalieri, N. Müller, Th. Uphues, V. Yakovlev, A. Baltuška, B. Horváth,
B. Schmidt, L. Blümel, R. Holzwarth, S. Hendel, M. Drescher,
U. Kleineberg, P. M. Echenique, R. Kienberger, F. Krausz and
U. Heinzmann.
Nature, October 25, 2007
Matthias Uiberacker (right) and Martin Schultze (left) working at the AS-1 at MPQ. 
Adrian Cavalieri and his team explored the electriccharge transport in solids at the Laboratory for Attosecond Physics. -
Breaking the 100 as-barrier
Lightflashes allow for real-time insights into the processes of the microcosm with a time resolution of a few tens of attoseconds.
For the first time, scientists succeed in producing light flashes which last less than 100 attoseconds. This is possible due to the development of the femtosecond laser technology to its ultimate limits. It produces laser pulses in which the electric field performs hardly more than one single strong oscillation, lasting about 3 femtoseconds (Science, August 10, 2007). That means that the light wave contains only one high wave crest which is followed by a deep valley. The power the electric light field exerts on the electrons is most powerful at the top of the ridge and at the deepest spot of the valley. The former removes the electrons from the noble gas atoms, the latter – due to the opposite direction of the force – hurls the electrons a little later back to the vicinity of the atomic nucleus with a high amount of energy. When the electrons impinge upon the atom they evoke a few attosecond oscillations of the electric charge in the atoms. These tiny dipoles synchronously radiate a correspondingly short X-ray flash from the ensemble of atoms. This results in more than 100 million photons within 80 attoseconds (Science, June 20, 2008). The flashes allow for real-time insights into the processes of the microcosm with a time resolution of a few tens of attoseconds.
people involved:
E. Goulielmakis, M. Schultze, M. Hofstetter, V. Yakovlev, J. Gagnon,
M. Uiberacker, A. L. Aquila, E. M. Gullikson, D. T. Attwood, R. Kienberger,
F. Krausz and U. Kleineberg.
Science, June 20, 2008
Eleftherios Goulielmakis shows the Guiness-certificate for the shortest light flashes worldwide. 
The beamline AS1 is producing the shortest light-flashes worlwide.
The consist of more than 100 million photons within 80 attoseconds. -
Real time access to electron motion
Attosecond-tools provide realtime access to electron motion.
Whereas the former study provides fundamental insight into strong-field-atom interactions, the latter capability will be beneficial to advancing electronics towards its ultimate speed limits, to light-wave frequencies. Meanwhile, these tools also provide real-time access to valence electron motion inside atoms
(Nature, August 5, 2010) as well as molecules. Moreover, precision measurement of the timing of photoemission with an precision of less than 10 attoseconds now reveals the role of electron correlations in this fundamental process (Science, June 25, 2010).people involved:
M. Schultze, M. Fieß, N. Karpowicz, J. Gagnon, M. Korbman, M. Hofstetter, S. Neppl, A. Cavalieri, Y. Komninos, Th. Mercouris, C. A. Nicolaides,
R. Pazourek, S. Nagele, J. Feist, J. Burgdörfer, A. M. Azzeer, R. Ernstorfer,
R. Kienberger, U. Kleineberg, E. Goulielmakis, F. Krausz, and V. Yakovlev.
Science, June 25, 2010
E. Goulielmakis, Z. Loh, A. Wirth, R. Santra, N. Rohringer, V. Yakovlev,
S. Zherebtsov, T. Pfeifer, A. M. Azzeer, M. F. Kling, S. R. Leone
and F. Krausz.
Nature, August 5, 2010
Setup of the beamline AS-2 at MPQ. Measurements of the timing of photoemission with an precision of less than 10 attoseconds reveals the role of electron correlations in this fundamental process. 
Artificial view into an attosecond experimental chamber. -
Tailor-made light waves
Synthesized waveforms of lightpulses consist of less than a full oscilation.
In order to control electrons, the waveform of light must be tailored within one oscillation period. That requires „ultra white“ laser light, which involves not only all colours of the visible spectrum, but also infrared and ultraviolet waves, as well as a tool which combines these waves oscillating at different speeds. In 2011, LAP researchers developed a „light wave synthesizer“, as it is called, which allows the control of the oscillation of light with state-of-the-art precision. Using this technology, the scientists succeeded in creating completely new waveforms in the pulses. They may now consist of less than a full oscillation and hence last only around two femtoseconds. With that the physicists generated the shortest flashes of visible light observed to this day. It is now possible to control the temporal evolution of the force exerted by light on the attosecond scale. Thus, the technology promises, for the first time, the precise control of electron movements. Since all the energy of the electromagnetic field clusters in a tiny time window, the new tool allows for a stimulation of processes within one femtosecond and opens the door to the attosecond pump-probe experiments (Science, October 14, 2011).
people involved:
A. Wirth, M. Hassan, I. Grguras, J. Gagnon, A. Moulet, T. Luu, R. Santra,
Z. Alahmed, A. M. Azzeer, V. Yakovlev, V. Pervak, F. Krausz and
E. Goulielmakis.
Science, October 14, 2011
Tran Trung Luu, Adrian Wirth, Mohammed Hassan, Eleftherios Goulielmakis, Vladimir Pervak, Antoine Moulet and Vladislav Yakovlev (from left), developed the Light Wave Synthesizer. 
A light field synthesizer divides incident white light into three color channels and modifies it afterwards. The composition creates laser pulses with a complex, however fine adjustable waveform.
