In their work on the timing of ultrafast subatomic motions, physicist Matthias Kling and his colleagues not only generate incredible amounts of data, they present manage to present these data in intriguing and colorful ways. Indeed, many of the resulting graphics could be said to transform physical phenomena into something akin to art."
These visualizations are firmly based on experimental observations and measurements of physical processes occurring at the quantum mechanical level," says Kling, who is Professor of Ultrafast Nanophotonics at Ludwig-Maximilians-Universität (LMU) in Munich. Together with his team of laser physicists, Kling studies how electrons in atoms, molecules and nanoscale structures behave when they interact with light.
To this end, the researchers routinely produce pulses of intense laser light that last for a few femtoseconds (1 fs corresponds to a millionth of a billionth of a second) and often three orders of magnitude less (attoseconds). Samples of gas molecules or atoms are excited by trains of these ultrashort light flashes, and the resulting interactions are recorded with the help of a special camera, which reveals details of how the particles react. This technically demanding approach enables the physicists to acquire a better understanding of how the quantum world works.
"Imaging of processes at the quantum level usually requires the use of a camera with megapixel resolution. We take pictures at a rate of approximately 1000 per second," says Kling - and the experiments can last for anything from many hours to several days. The amount of data accumulated is enormous, and researchers in the field of attosecond physics are already running into difficulties owing to the limited storage capacity available on conventional hard disks.
Matthias Kling picks up the picture that lies on his desk, which shows a pattern reminiscent of the concentric wave forms that appear on the surface when a stone is dropped into a pond. "This comes from a study of a so-called buckyball or, more correctly, a fullerene molecule. Fullerenes take the form of a hollow, symmetrical cage made up of 60 interlinked carbon atoms. They look just like soccer balls, only 300 million times smaller," he explains.
In the study, these molecules were exposed to femtosecond pulses of laser light. The optical field of the laser first excites the quantum state of the fullerene to a higher energy level. "This then leads to the emission of electrons with highly characteristic properties, which are detected in the experiment. The energy distributions captured by our camera enable us infer many aspects of the quantum mechanical processes that underlie this interaction between light and matter." The colours reflect the relative numbers of electrons observed in different energy bands. Red and yellow represent large numbers of particles, shades of blue indicate the low end of the scale.
Another quantum portrait derived from experiments performed by Kling's group captures a specific light-matter interaction that lasts for just over 900 attoseconds. The star-shaped structure depicts one phase of the interaction between the optical field of a 40-fs laser pulse and atoms of an inert gas, which is mapped by the ultraviolet and infrared radiation that reveals the energies of the electromagnetic forces and particles involved.
"What one sees here is the recoil imparted to the ions formed upon the ejection of electrons from the gas molecules by the laser pulse," says Wilhelm Frisch, a student member of Matthias Kling's research group. This image thus shows the state of the system immediately after the incoming laser pulse has converted molecules of the electrically neutral gas target into positively charged ions.
The changing electromagnetic field associated with the laser pulse (indicated by the varying combinations of its two spectral components from left to right) then controls the behavior of the released electrons. "The polarity of the electric field associated with the light pulse changes its sign approximately one million billion times every second, and this causes the negatively charged electrons to behave in a whiplash-like manner. They wobble up and down," as Frisch explains.
Because this behavior occurs repeatedly during the 40-femtosecond duration of the pulse, the dynamics of the process can be resolved with attosecond precision - thus demonstrating that ultrashort laser pulses make it possible to control the motions of electrons with attosecond accuracy.
"As we learn to extract more information from interactions between light and matter, and penetrate deeper into the microcosmos, the better we understand the details of the elementary processes that take place at this scale," says Matthias Kling. "Our pictorial representations of experimental data enable us to test existing theories - and we sometimes find that there are none that allow us to directly reproduce the data in computer simulations. These visualizations thus contribute to the further development of theoretical physics, which in turn allows us to refine our description of the quantum world."
And as a bonus, many of the resulting graphics are aesthetically appealing - indeed beautiful enough to make one want to frame them and hang them up!
Text: Thorsten Naeser