The smallest of all worlds

He deals with things that remain hidden: Christoph Lienau has established the "Ultrafast Nano-Optics" research area at the university. In the international research community, Oldenburg is now known as the "city of short impulses".

Oldenburg is not just the urban residential city in north-west Germany, the city of the "EWE Baskets" or the "City of Science", which the Stifterverband für die Deutsche Wissenschaft (Donors' Association for German Science) named it in 2009. Oldenburg is, at least for a small community of highly specialised physicists, "the city of short impulses". At the beginning of the year, physicist Christoph Lienau was a guest at a scientific conference in Xiamen, a city of four million people in south-east China. When he tried to explain where he was from, a young scientist interrupted him: "Oldenburg, that's the city where they can generate short light pulses."

Over the past eight years, Lienau has worked hard with the "Ultrafast Nano-Optics" working group at the Institute of Physics to ensure that Oldenburg achieves this reputation in the scientific community. Together, they have developed ground-breaking nano-optical processes that allow the optical properties of nanostructures to be better understood and utilised.

Although ultrafast nano-optics is still in its infancy, "it is one of the most exciting and promising areas of physics research," says Lienau. Scientific progress is rapid: nanoresearch and nanotechnology have become increasingly important for many branches of industry in recent years. The spectrum of applications ranges from pan or window coatings and innovative ultrasonic sensors to nanoclusters as computer memories, super-fast semiconductors and nano-tuning for solar cells. Nanotechnology could even play a decisive role in the next generation of computers. According to experts, the future belongs to optical computers based on nanostructured light switches.

Ultra-short laser pulses

And Lienau and his team are providing insights into this smallest of all worlds. Using ultrashort laser pulses, they can visualise things and processes that are so tiny that they cannot be seen with conventional optical microscopes. The laser flashes used by the research team are "ultrashort", lasting just a few femtoseconds - a femtosecond is a trillionth of a second. Thanks to these impulses, it is possible, for example, to make statements about the function of material structures that are only about one ten-thousandth the size of a human hair.

In conversation, Lienau admits that his field of research is something for physics geeks, for experimental physicists with a great enthusiasm for state-of-the-art measurement technology. "We certainly don't do this work to get rich or to contribute directly to increasing the profits of companies. We are first and foremost interested in gaining knowledge," he explains. Lienau and his team conduct basic research in order to create new knowledge. And the physicist adds with a slightly ironic undertone: "We want to recognise people - let's call them nerds - who have just as abstract and, if possible, creative ideas as we do." Ideas that might even seem abstruse and absurd at first, because they question traditional knowledge. And this recognition from such nerds is a great reward.

And Lienau has received this recognition: he has been able to publish his research findings in a large number of internationally renowned scientific journals. Journals such as "Science" and "Nature Photonics". His ultrashort laser pulses are now so much in demand that he has to turn down invitations to scientific conferences all over the world - due to a lack of time.

Centre of Excellence for Ultrafast Optics

Lienau's research was born on the natural sciences campus of the University of Oldenburg. There, the physicist and his colleagues have set up a very special laboratory. A laboratory like few others in the world, a kind of Centre of Excellence for the spectroscopy of nanostructures. This is where the powerful ultrashort pulse laser systems are located, without which the working group's research success would have been impossible.

To the uninitiated, the facility looks like a model railway world turned upside down, as if a confused optician had scattered miniature mirrors senselessly across a work surface. But behind the chaos of light sources, mirrors and prisms lies a sophisticated, highly complex system.Lienau's staff have arranged the optical devices in the laboratory rooms in such a way that the respective laser system generates the desired ultrashort pulses to measure optical effects. The students and doctoral candidates work with commitment and energy on the experimental set-ups, which provide the material for their final theses and doctoral dissertations. Designing and setting up the experiments and precisely aligning the lasers, mirrors and prisms usually takes several weeks, while the actual measurements only take a few days.

Ultrashort pulse light sources, the respective laser systems that form the starting point for the experiments, are readily available from specialised scientific retailers, the doctoral candidates report. However, these no longer fulfil the group's requirements. The pulses that these laser systems generate are usually either too long or have the wrong light frequency. This is why they have to make their own "home-made stuff" - as the doctoral candidates affectionately call the arrangements they have designed.

The sophisticated technology of short laser pulses for observing and analysing fast dynamic processes is essentially based on the same considerations as photography. If you want to photograph a racing car at full speed, for example, the exposure time must be very short so that it is captured sharply on film.

And Lienau's research relies on ultra-short exposure times: When he began his research, the time resolution of the pulses generated was still in the range of a hundred femtoseconds. This was sufficient to visualise the movement of atomic nuclei in molecules and thus follow the course of chemical reactions. Ahmed Zewail from the renowned California Institute of Technology, where Lienau worked as a postdoctoral researcher, was awarded the Nobel Prize in Chemistry for this work in 1999. In the meantime, Lienau and his team in Oldenburg have succeeded in reducing the pulse duration to well below ten femtoseconds and thus visualising the movements of individual electrons. A development that also benefits research in the field of renewable energies. It makes it possible to elucidate energy conversion in nanostructures, which is highly relevant for applications.

Nano-energy research

Prof. Dr Uwe Schneidewind, then President of the University of Oldenburg, gave Lienau the idea of working on renewable energies - as the physicist recalls with a smile from his beginnings in Oldenburg: "Professor Schneidewind called for 'research with a regional focus in order to help anchor the university in the region'. And I then replied: 'No, I certainly can't do that for the time being. Our strength lies in basic research. We have to use this strength to make a lasting contribution to the success of our university'." Lienau then became intensively involved in research into renewable energies at the University of Oldenburg. "When I saw what the physicist Jürgen Parisi and his team were developing for energy and semiconductor research, what Carsten Agert had set up with his NEXT ENERGY research institute, I realised: These are highly relevant fields of research to which we can make a contribution. But not by simply building on what already exists. We must succeed in utilising our expertise in quantum physics to elucidate the microscopic processes of energy conversion in nanostructures and thus open up a different perspective on energy research."

Basic research for the solar cell of the future

Together with the then Dean of School V - School of Mathematics and Science, Prof. Dr Martin Holthaus, the idea was born in 2011 to establish nanoenergy research as a new research focus at the university and also to anchor "ultrafast nano-optics" more firmly in Oldenburg. Since then, the research area has become an integral part of the university with more than 80 scientists from physics, chemistry and biology and a state graduate programme "Nano-Energy Research" for 15 doctoral candidates.

Whether conventional batteries, lithium-ion cells, innovative lithium-air batteries, organic solar cells or organic light-emitting diodes - all consist of nanostructures in which energy is converted from one form to another. The most striking example is solar cells, where light is converted into electricity. Lienau's team is working on visualising the processes that take place on the inside. "For example, we look inside the solar cell to see how electricity is generated on a molecular scale," explains the physicist. To do this, the scientists break down the solar cell into its smallest components - atoms and molecules - and track their movement on extremely short time scales. The centre of interest is the interplay between the incident light and the atoms and molecules within the solar cell. In this way, the physicists hope to understand the microscopic, quantum mechanical principles of current generation.

These processes are so complex that they have so far eluded direct scientific observation. However, in his latest publication in the scientific journal "Science", Lienau describes how he and a team of international scientists succeeded in filming the conversion of light into electricity in an organic solar cell. In this way, they were able to show for the first time how the light-induced electron transfer in such a cell takes place in detail and that the quantum mechanical wave character of the electrons plays a decisive role in this process.

Lienau is thus conducting fundamental research for future key technologies. He is convinced that his investigations and experiments can contribute to the development of more powerful solar cells and batteries in the medium term. "Some materials are better suited to energy conversion than others - we can use our nano-optical measurement methods to analyse why this is the case right down to the molecular level." And the scientist adds: "In order to increase the efficiency of batteries or solar cells, we need to understand the underlying operating principles as well as possible - if only to understand why nature often uses different conversion architectures in biological systems than we physicists and chemists currently do in artificial light conversion systems."

The Spirit Of Wechloy

Lienau's research would be inconceivable without a broad network of national and international scientists. The collaboration with Italian research teams from Milan and Modena, internationally recognised experts in ultrafast physics, is particularly intensive. But there are also many scientists at the University of Oldenburg with whom Lienau co-operates on an interdisciplinary basis.

In order to design and produce new, artificial light collection complexes, the physicist works very closely with Prof Dr Jens Christoffers, Professor of Organic Chemistry. Together with biology professors Karl W. Koch and Henrik Mouritsen, he is investigating the similarities and differences between biological and artificial energy conversion systems. And in physics, nano-energy research is supported in particular by the experimental physicists Prof. Dr Matthias Wollenhaupt, an expert in "customised", ultra-short light pulses, and Prof. Dr Niklas Nilius, an expert in scanning probe spectroscopy.

"We have a rapidly developing research culture and a special cohesion on the natural sciences campus in Wechloy," says Lienau. Colleagues from other universities have also noticed this. "At conferences and congresses, I keep hearing that Oldenburg has a reputation as a university with creative researchers who work closely together across disciplines. Colleagues envy us for the short distances, for the fact that biologists, physicists and chemists cooperate so well here that they have no fear of contact and their research is mutually beneficial," explains Lienau.

International young researchers

The fact that Lienau's research is known far beyond the borders of the city of short impulses is also shown by the fact that a good 20 national and international young scientists are conducting research in his working group. One example is Humboldt Fellow Dr Parinda Vasa, who came to Oldenburg from India as a postdoctoral researcher to investigate metallic semiconductor structures using nano-optical methods and complete her habilitation. Even before she completed her habilitation, she was offered professorships at the most renowned universities in India. She currently holds the professorship for ultrafast spectrography, plasmonics and nano-optics in the Department of Physics at the Indian Institute of Technology Bombay. "Word is getting around that we have a thriving research landscape here in Oldenburg," reports Lienau with pride. Our doctoral candidates and postdocs are now scattered all over the world.

The scientist is hoping for further important research impulses and even better international visibility, particularly from the "Nano-Energy Research" doctoral programme, which he is running together with the Emden/Leer University of Applied Sciences and which is funded by the state of Lower Saxony. The state doctoral programme combines scientific issues from energy research with basic research in physics and chemistry. Lienau is certain that it offers an excellent opportunity to attract even more talented and eager young scientists to Oldenburg.

And Lienau also has the youngest scientists in mind. Last year, chemistry and physics teacher Silvia Beckhaus set up a nanotechnology laboratory for schoolchildren at the Altes Gymnasium Oldenburg in collaboration with "Ultrafast Nano-Optics". Here, pupils can use an atomic force microscope to gain insights into the nanocosmos.