Matthias Kling and his team are exploring the world of electrons. But their scientific endeavours are also turning up some amazingly artistic results. These quantum optics physicists are coming up with colourful, abstract images that reveal insights into the behaviour of these elementary particles in atoms, molecules and nanostructures. Kling leads the Attosecond Imaging Junior Research Group, under Professor Ferenc Krausz at the Max Planck Institute for Quantum Optics. Here he explains how these pictures from the quantum cosmos are created and what conclusions the scientists are drawing from them.
The interaction of light and matter plays a big part in what happens inside atoms and molecules. That was something that fascinated me from a very early age. As a child I used to borrow the magnifying glass my grandfather used for reading and focused sunlight through it onto paper to make it burn. Still today I just love to carry out experiments. But these days I use lasers to explore what really happens, what the physical processes are, when light and matter interact.
It is the dynamics of the very lightweight, fast electrons that plays a key role in the interaction of light and matter. This dynamics takes place on incredibly fast time scales, sometimes even down to attoseconds. We are talking here of periods measured in billionths of a billionth of a second. It is perhaps easier to understand just how fast this is if I say that an attosecond is to a second what a second is to the age of the universe. In attosecond physics we research into ultrafast particle motion of this kind, using flashes of light that also only last for a few attoseconds.
I first came into contact with attosecond physics as a postdoc for Marc Vrakking at the Institute for Atomic and Molecular Physics in Amsterdam. Today we are still using the camera technology used then to measure the momenta of charged particles – it´s called velocity map imaging (VMI). In 2005 I had the chance of working with people from the LAP team, bringing a VMI from Amsterdam to Garching, on an experiment in which for the first time electrons were steered in a hydrogen molecule using the waveform of the electrical field of laser light, enabling us to control a chemical reaction. Phase-stable laser pulses were used for this. We were able to create these thanks to the frequency-comb technique for which in the same year Professor Theodor Hänsch received the Nobel Prize for Physics. In our Junior Research Group we are building on these initial results and are now looking at how to use such laser light to steer and observe electrons on attosecond time scales in complex systems.
I am interested in the complex physics of multi-electron systems, in which the interaction of matter and light is determined by collective phenomena. Nanostructures are ideal for investigating collective electron excitation (plasmons). Using attosecond techniques, plasmons can be directly observed. Their dynamics can be as fast as 100 attoseconds. My project proposal to the Deutsche Forschungsgesellschaft was based on this idea, and in 2007 the Emmy Noether Junior Research Group on Attosecond Imaging was formed at the MPQ. As well as researching into plasmons in nanosystems, we are also looking at electron dynamics in atoms and molecules on attosecond time scales.
The young spirit and high level of commitment in the junior research group is tremendously enjoyable. We have more ideas than we can work on and everyone in the group contributes their knowledge to the implementation of a project. I am very fortunate to have such talented and enthusiastic colleagues.
Yes, in the broadest sense we are 'photographing' the electrons. The basic principle of exposure is the same: in order to observe very fast processes in nature, you generally need cameras with exposure times that are shorter than the time scale of these processes. For a good photograph of a bullet flying through the air, you would need shutter speeds of a few microseconds, but if you want to observe the motion of atomic nuclei in molecules, you would have to have exposure times measured in femtoseconds. With electrons, attosecond exposure times are needed to obtain sharp pictures.
In our Attosecond Imaging Junior Research Group, we use special cameras to 'film' electrons. We usually take our images in a pump-probe experiment. First a flash of laser light, such as an attosecond pulse in extreme ultraviolet light, excites the atom or the molecule. The electron dynamics that takes place upon this excitation is probed with a second laser pulse. We vary the time interval between these two pulses, taking a camera image each time. When the images are then put together, a film of electron dynamics is produced, similar to how a motion picture conventionally is made.
The most frequent comment is how beautiful the images look. Generally these images present a two-dimensional cut from the 3D world of electrons. They often have a noticeable symmetry which is aesthetically very pleasing. In strong laser fields the systems often interact with more than one photon. The electrons ejected by the light display characteristic energy jumps. These correspond in each case to the absorption of an additional photon and thus show the quantum nature of the laser-matter interaction. This gives rise in the pictures to strong lines with characteristic spacings. The colours, too, are striking – the differences between the colours indicate the number of electrons. We can therefore draw conclusions about the emission process from the patterns and colour scale.
In atoms, for example, the absorption of a photon in extreme ultraviolet light causes an electron to be emitted from the inner shell. The resulting hole is filled up again and stability is reinstated. These reorganisation processes can involve the emission of more electrons, the emission of photons or repositioning inside the atom. Time scales here can be down into the attosecond range. Our images break down these processes in time and teach us a great deal about what goes on inside atoms and about the interaction of atoms with laser light.
Attosecond technology is still very young and we need to work on expanding the insights we have gained into the microcosm. The electron dynamics in multi-electron systems in particular is little understood. In our next experiments we want to understand the dynamics of plasmons in nanosystems at their shortest time scale. At the same time we want to achieve a very high spatial resolution and significantly improve on the imaging capability we have had so far. Another goal is a better understanding of the strongly coupled electron-nuclei dynamics in molecules in strong laser fields and temporal imaging of the electron motion in these systems. We have already carried out a first experiment in this area, and are still evaluating the results. We find that with this process of evaluating our measurements, we are always exploring new territory, opening doors where previously we were only permitted to look through the keyhole and this makes the research in this new field extremely exciting.
Interview and Photo: Thorsten Naeser