Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Observing the birth of a spectral line

11.11.2016

Ultrashort intense laser pulses cut into a fundamental quantum phenomenon

For the first time, physicists managed to observe in real time how an atomic spectral line emerges within the incredibly short time span of a few femtoseconds, verifying a theoretical prediction. This has been possible by applying a very fast temporal switch:


Absorption in a helium as it depends on the photon energy of the exciting extreme-ultraviolet flash of light and the time delay to the ionizing near-infrared laser pulse acting as a cut-off gate.

graphics: MPIK

An intense laser pulse cuts off the natural decay shortly after excitation by a preceding laser pulse. The build-up of the asymmetric Fano line shape of two quantum-mechanically interfering electrons in the Helium atom is measured by varying the time delay between the two laser pulses.

In a classical picture, the electrons in an atom are allowed to revolve only on certain orbits around its nucleus – or in terms of quantum mechanics occupy certain orbitals or energy levels. Light may lift (excite) an electron into a higher orbit, if its energy (color) matches the energy difference of the orbitals.

That is why the atom only absorbs light of certain colors, called its absorption spectrum. In most cases, the single spectral lines are symmetrically shaped, but under some conditions asymmetric line shapes appear, which are termed Fano profiles.

The decay of doubly excited helium stands for such a case: One of the two excited electrons falls back to the lowest-energy (ground) state after a collision with the other electron, thereby kicking the latter out of the atom. As the free electron is no longer limited to discrete energy levels, physicists are speaking about the coupling of a discrete state to a continuum, a general phenomenon at work in many different processes in nature, and fundamentally at the interface of quantum (discrete energies) and classical (continuous energies) mechanics.

Theoretical calculations predicted that the corresponding Fano profile does not emerge instantaneously but takes some time to develop: In helium, the unfolding of the line shape takes a few femtoseconds – some millionths of a billionth of a second.

Recently, experimental physicists from the MPI for Nuclear Physics (MPIK, Heidelberg), together with theoretical physicists at the Vienna University of Technology and the Kansas State University succeeded to take a movie of an emerging Fano spectral line. To gain access to the short time scales, they used two ultrashort laser-controlled flashes of light. The first one in the extreme ultraviolet excites both electrons of the helium atom. Some femtoseconds later, the second, intense, near-infrared laser flash triggers ionization ahead of time, i.e., it cuts off the natural autoionization decay process.

Alexander Blättermann, postdoc in the group of Thomas Pfeifer at the MPIK, explains: “You may think of the excited helium atom as an oscillating dipole that produces the optical absorption line. The subsequent strong infrared pulse acts as a temporal switch stopping this oscillation before the line has fully built up.” By varying the time delay between the two laser pulses – this has been done with a precision of less than a femtosecond – the researchers tracked the evolution of the line shape in real time.

“The experimental results nicely show how the Fano profile builds up with increasing time delay”, says Andreas Kaldun who has recently moved from the MPIK to SLAC in Stanford. At very short time delays, the spectral line is completely smeared out to a broad and flat band. With increasing time delay, the dipole is granted more and more time to oscillate, thus the line narrows and steepens step by step and finally approaches the original Fano profile, in very good agreement with the theory prediction.

“Our results thus not only verify the prediction, but at the same time demonstrate the power of the applied time-gating method for the exploration of the origin and evolution of many different fundamental quantum processes that could thus far only be studied by interpreting their static absorption spectra”, concludes Thomas Pfeifer.

The study of such atomic processes by different experimental methods has always fueled the evolution of physics (e.g. the discovery of quantum mechanics in the past) and remains an active topic of international contemporary research. In the same issue of Science magazine, a completely independent team of scientists from France and Spain have used the complementary method of time-resolved photoelectron spectroscopy to obtain a view on the Fano resonance "from the outside" of the atom.

This was achieved by reconstructing the time-dependent outgoing quantum-mechanical electron wavepacket (DOI: 10.1126/science.aah5188). Together with the time-resolved view "from the inside" by the time-gated dipole approach as described above (DOI: 10.1126/science.aah6972), atomic physics continues to transform our perception of the building blocks of nature. On the long run, this understanding eventually leads to technological discoveries. Physics revolutions of the past brought the laser and x-ray sources, the future could bring laser-controlled chemistry and molecule-sized ultrafast computers and devices.

Original publication:
Observing the ultrafast build-up of a Fano resonance in the time domain, A. Kaldun, A. Blättermann, V. Stooß, S. Donsa, H. Wei, R. Pazourek, S. Nagele, C. Ott, C. D. Lin, J. Burgdörfer, T. Pfeifer, Science, 11 November 2016, DOI: 10.1126/science.aah6972

Contact:
Prof. Dr. Thomas Pfeifer, MPI für Kernphysik
phone: +496221 516380
e-mail: thomas.pfeifer@mpi-hd.mpg.de

Prof. Dr. Joachim Burgdörfer, Technische Universität Wien
phone:+ 43 1 58801 136 10
e-mail: joachim.burgdoerfer@tuwien.ac.at

Prof. Dr. Chii-Dong Lin, Kansas State University
phone: +1-785-532-1617
e-mail: cdlin@phys.ksu.edu

Weitere Informationen:

http://www.mpi-hd.mpg.de/mpi/en/pfeifer/pfeifer-division-portal/ Division Pfeifer at MPIK

Dr. Bernold Feuerstein | Max-Planck-Institut für Kernphysik

Further reports about: Electrons Kernphysik MPIK energy levels femtoseconds laser pulses physics quantum mechanics

More articles from Physics and Astronomy:

nachricht Quantum optics allows us to abandon expensive lasers in spectroscopy
22.11.2017 | Lomonosov Moscow State University

nachricht Nano-watch has steady hands
22.11.2017 | University of Vienna

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Nanoparticles help with malaria diagnosis – new rapid test in development

The WHO reports an estimated 429,000 malaria deaths each year. The disease mostly affects tropical and subtropical regions and in particular the African continent. The Fraunhofer Institute for Silicate Research ISC teamed up with the Fraunhofer Institute for Molecular Biology and Applied Ecology IME and the Institute of Tropical Medicine at the University of Tübingen for a new test method to detect malaria parasites in blood. The idea of the research project “NanoFRET” is to develop a highly sensitive and reliable rapid diagnostic test so that patient treatment can begin as early as possible.

Malaria is caused by parasites transmitted by mosquito bite. The most dangerous form of malaria is malaria tropica. Left untreated, it is fatal in most cases....

Im Focus: A “cosmic snake” reveals the structure of remote galaxies

The formation of stars in distant galaxies is still largely unexplored. For the first time, astron-omers at the University of Geneva have now been able to closely observe a star system six billion light-years away. In doing so, they are confirming earlier simulations made by the University of Zurich. One special effect is made possible by the multiple reflections of images that run through the cosmos like a snake.

Today, astronomers have a pretty accurate idea of how stars were formed in the recent cosmic past. But do these laws also apply to older galaxies? For around a...

Im Focus: Visual intelligence is not the same as IQ

Just because someone is smart and well-motivated doesn't mean he or she can learn the visual skills needed to excel at tasks like matching fingerprints, interpreting medical X-rays, keeping track of aircraft on radar displays or forensic face matching.

That is the implication of a new study which shows for the first time that there is a broad range of differences in people's visual ability and that these...

Im Focus: Novel Nano-CT device creates high-resolution 3D-X-rays of tiny velvet worm legs

Computer Tomography (CT) is a standard procedure in hospitals, but so far, the technology has not been suitable for imaging extremely small objects. In PNAS, a team from the Technical University of Munich (TUM) describes a Nano-CT device that creates three-dimensional x-ray images at resolutions up to 100 nanometers. The first test application: Together with colleagues from the University of Kassel and Helmholtz-Zentrum Geesthacht the researchers analyzed the locomotory system of a velvet worm.

During a CT analysis, the object under investigation is x-rayed and a detector measures the respective amount of radiation absorbed from various angles....

Im Focus: Researchers Develop Data Bus for Quantum Computer

The quantum world is fragile; error correction codes are needed to protect the information stored in a quantum object from the deteriorating effects of noise. Quantum physicists in Innsbruck have developed a protocol to pass quantum information between differently encoded building blocks of a future quantum computer, such as processors and memories. Scientists may use this protocol in the future to build a data bus for quantum computers. The researchers have published their work in the journal Nature Communications.

Future quantum computers will be able to solve problems where conventional computers fail today. We are still far away from any large-scale implementation,...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Ecology Across Borders: International conference brings together 1,500 ecologists

15.11.2017 | Event News

Road into laboratory: Users discuss biaxial fatigue-testing for car and truck wheel

15.11.2017 | Event News

#Berlin5GWeek: The right network for Industry 4.0

30.10.2017 | Event News

 
Latest News

Corporate coworking as a driver of innovation

22.11.2017 | Business and Finance

PPPL scientists deliver new high-resolution diagnostic to national laser facility

22.11.2017 | Physics and Astronomy

Quantum optics allows us to abandon expensive lasers in spectroscopy

22.11.2017 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>