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A Laser Clock with a Quantum Hand
What exactly happens when a molecule breaks apart? A phenomenon like this can be investigated with short laser pulses. But there is a problem: one has to deal with very different time scales simultaneously. The electrons move so fast that they have to be studied on an attosecond time scale. The heavier particles in the molecule, on the other hand, hardly move at all within an attosecond. If the distance between them increases and the molecule finally breaks apart, these dynamics can only be observed on a much larger time scale.
The solution was to link different timers. The fast timer is the rotation of the light field. Like the clock hand, the direction of the electric field of the elliptically polarized laser light – its polarization vector – rotates, but at an extremely fast speed - once every 2.5 femtoseconds. This continuous change of direction can be used to investigate how the fast motion of the electrons is related to the slow motion of the protons. The slower proton movement can be analyzed by looking at the energy of the protons after the molecule fracture.
original publication (Dec. 2019): Physical Review Letters
additional info: Viewpoint in Physics
Press release of TU Vienna
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Compression of multi-mJ CEP-stable 3.2µm pulses in a hollow-core waveguide
We demonstrate an extension of high-power pulse compression deeper into the challenging IR spectral range around 3.2 μm wavelength, where the effects of material absorption, dispersion, and free electron disturbance on nonlinear propagation become increasingly limiting parameters. 5 mJ, 80 fs pulses from a KTA parametric amplifier were spectrally broadened in a large-core hollow fiber with argon as the nonlinear medium and compressed down to two-cycle duration.
original publication (Nov. 2016): Optica
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Filamentation in Air for Atmospheric Research
When an extremely intense laser pulse is sent through the air, it can focus itself, creating a narrow filament of light. By shooting such filaments into the sky and analysing back-scattered light, it would be possible to trace pollutants in the atmosphere. To achieve this, lasers with mid-infrared wavelengths are required. However, reaching the critical power to produce such a filament with mid-infrared laser beams is very difficult. At these wavelengths, laser filaments have only been produced in high pressure gas tubes. Together with collaborators from Moscow State University we succeeded in building a new kind of mid-infrared laser which is so intense that it ignites laser filaments in the air at normal atmospheric pressure.
original publication (Feb. 2015): Scientific Reports
Press release of TU Vienna
Selected media response:
Der Standard, Austria
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Creating Bright X-Ray Pulses with intense mid-infrared laser pulses
The experiments combined a novel driver system based on Optical Parametric Chirped Pulse Amplification (OPCPA) with an x-ray target chamber from Max Born Institute, Berlin. Pulses of 80 fs duration and up to 18 mJ energy at a center wavelength of 3900 nm (3.9 µm) were focused down onto a 15 µm thick copper tape. This scheme allows for generating an unprecedented number of 109 hard x-ray photons at a 0.154 nm wavelength per driving pulse. A comparison with previous experiments performed with 800 nm driver pulses shows that the enhancement of the x-ray flux in the new scheme scales with the square of the wavelength ratio, i.e., (3900 nm/800 nm)2, roughly a factor of 25. The results pave the way for a new generation of table-top hard x-ray sources, providing up to 1010 x-ray photons per pulse at elevated, e.g., kilohertz repetition rates.
original publication (May 2014): Nature Photonics
Press release of TU Vienna
Selected media response:
Der Standard, Austria
Welt der Physik
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Optimization of Quantum Trajectories Driven by Strong-Field Waveforms
The electric field of a focused ultra-short laser pulse can pull on the outer electrons in an atom as strongly as the nucleus to which they are bound. During the few-femtoseconds (10-15 s) long duration of a single lightwave oscillation cycle, an electron can thus be detached and steered along trajectories around its parent atom, which are governed by the precise “shape” (time evolution) of the lightwave cycles. The capability to sculpt the lightwave cycle thus yields control on the quantum-level of single atoms and on a sub-femtosecond time-scale. Here we demonstrate a major advance in this field: we optimize the lightwave-steered electron trajectories with more finesse than ever before to strongly enhance the generation of laser-like extreme-ultraviolet (XUV) pulses emitted when the accelerated electron recollides with its parent atom.
Selected for a "Viewpoint in Physics - spotlighting exceptional research"
by the American Physical Society
original publication (May 2014): Physical Review X
Press release of TU Vienna
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Steering chemical reactions with ultrashort laser pulses
In molecules, the constituent atoms are bound together by their electrons, whose motions can be very fast. Rearranging the atoms, or splitting them apart, is ultimately determined by the electrons. A fundamentally important and fascinating question is then to what extent, and by which mechanisms, electronic processes can influence the slower molecular restructuring and splitting processes, or even determine their outcome. The answer to this question may also guide the design of novel methods for controlling chemical reactions. We could show that selective fragmentation of acetylene and ethylene molecules can be achieved by controlled distortion of the electron cloud using very short laser pulses. Selectivity can be achieved either by the intensity and duration of the pulse, or by the relative alignment of the molecule with respect to laser field oscillations.
selected for a "Viewpoint in Physics - spotlighting exceptional research"
by the American Physical Society
original publications (April 2014):
Physical Review Letters
Physical Review X
Press release of TU Vienna
Selected media response:
Der Standard, Austria
ORF (Austrian Broadcasting)
Welt der Physik
Chemie.de
Photonics.com
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Laserlight controls the fragmentation of polyatomic molecules
Splitting a molecule is an example of an elemental chemical reaction. We have for the first time been able to show that it is possible to influence, i.e. to initiate it or to suppress, the splitting of a specific molecular bond in various polyatomic molecules with up to 10 atoms in a controlled manner using the carrier-envelope phase of laser pulses with a duration of only a few femtoseconds. Our result shows that the movement of the atomic nuclei on the time-scale of many femtoseconds can be influenced indirectly by controlling the much quicker motion of the electrons on the attosecond time-scale.
selected for a "Viewpoint in Physics - spotlighting exceptional research"
by the American Physical Society
published in Physical Review Letters in Dec 2012
Press release of TU Vienna
Online-article in Der Standard
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Kiloelectron-Volt X-rays from a mid-infrared laser source
A breakthrough in laser science was achieved in our labs: a new method of producing bright light pulses at X-ray energies was developed. The radiation covers a broad energy spectrum and can therefore be used for a wide range of applications, from materials science to medicine. Up until now, similar kinds of radiation could only be produced in particle accelerators (synchrotrons), but now a laser laboratory can also achieve this.
published in Science in June 2012
Press release of TU Vienna
Online-article in Der Standard online
Online-article from ORF (Austrian Broadcasting)
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Watching an electron being born
A strong laser beam can remove an electron from an atom – a process which takes place almost instantly. We could study this process with a time resolution of less than ten attoseconds. Our team succeeded in watching an atom being ionized and a free electron being “born”. These measurements yield valuable information about the electrons in the atom, which up until now hasn't been experimentally accessible, such as the time evolution of the electron’s quantum phase – the beat to which the quantum waves oscillate.
published in Physical Review Letters in May 2012
Press release of TU Vienna
Online-article in Der Standard online
Online-article from ORF (Austrian Broadcasting)
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Discovery of a new ionization mechanism in polyatomic molecules
We discovered a novel highly efficient ionization mechanism in hydrocarbon molecules of various sizes that drives the molecules into unexpectedly high charge states at very moderate laser intensities. Subsequently it leads to the complete molecular decomposition via a two-stage Coulomb explosion that involves the concerted emission of all protons with remarkably high kinetic energies.
published in Physical Review Letters in April 2011
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Observation of attosecond ionization in glasses
We observed an optical signature of attosecond ionization modulation with a focused few-cycle laser pulse in a transparent solid.The method holds promise for developing an attosecond metrology for bulk solids, in which, unlike in the established attosecond metrology of gases and surfaces, direct detection of charged particles is unfeasible.
published in Physical Review Letters in April 2011
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An optical method to observe attosecond tunnel ionization
In a collaboration with Moscow State University we could experimentally demonstrate that attosecond electron tunneling dynamics can be observed as an optical signal. The results of this work pave the way toward an all-optical metrology of subcycle ionization dynamics in systems in which free electrons cannot be directly measured.
published in Physical Review Letters in April 2010
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