Talk with Henrik Lemke, Group leader, PSI

What has been your main research activity and your most interesting scientific findings over the last few years ?

Being fascinated by the often astounding mechanisms giving materials a function in nature or technology, I had the chance to gain a number of new insights by building up and participating in multiple Free Electron Laser Experiments. These new large scale Light sources produce ultrashort and bright X-ray pulses which allow to observe the ultrashort-lived electronic and structural transitions outside equilibrium, which fundamentally determine the change of properties and thereby the function of materials. It became possible to observe electronic state changes after an optical material excitation, while following at the same time how the atomic structure rearranges by the changing potential landscape with respect to the equilibrium state. During my first years as an instrument scientist at the world-wide first hard X-ray FEL in the USA , we had observations of such processes in molecular systems in gas or liquid solution, approaching complex biological macromolecules, as well as in crystalline materials, especially also in correlated electron systems close to phase transitions at which electronic and magnetic properties drastically change. During the last years, my team has built up the Bernina insturment at SwissFEL which follows up on the experience gained at mainly LCLS by bringing more methods from X-ray science which are sensitive to atomic, magnetic and electronic structure to the ultrafast time scale. We combine this sensitivity with a sophisticated laser setup which allows to excite materials from lowest vibrational energy levels on few THz level up to electronic excitations in the UV range. Using this new ability we have obtained already a number of new and interesting findings in my team, like distinguishing different dynamics between magnetic and orbital structure, inducing phase transitions by induced ultrafast strain, as well as following the excitation of a multibody quantum state.

What are your upcoming main research goals ?

Working close to the instrumentation and with many visiting research groups doing their experiments at the station, we are trying to further push the boundaries of what parameters can be explored, while also streamlining to an extent that makes it easier for groups newer to the FEL science to take advantage of the experimental opportunities here. We are concentrating a lot of work to bring technically more challenging ways to selectively control state transitions by different types of light pulses and we are very curious to see to what extent different ways of excitation, for instance a broad excitation of vibrational states or a preferential excitation of magnetic or electronic vibrational modes, can change transitions, in their energetic path, speed and efficiency. In this context it is important to to push on highly technical developments that increase both the temporal resolution and signal to noise ratio in FEL experiments. Improvements here have already in the past opened several doors to new results that were otherwise invisible. We are hoping that we can push the time-resolution in pump/probe experiments closer to 10 fs, in order to study the often determining early electronic state transitions, or to follow the phase on vibrational excitation in the few 10s of THz.

How do you see your professional life in 10 years?

I hope to still work in the field of FEL science in ten or more years, as this would likely mean that our present work had further motivated the engagement of an increasing scientific community who would have solved a lot of questions, maybe even some greater problems with these experimental tools.

What do you like to do when you aren’t working on research?

I have a family with very young kids, who are taking most of my time beside research. Well, I guess every parent knows that growing up kids is a great deal of experimental science in itself, which may be tiring but has so many electrifying moments. If still feasible, I engage in some pre-parent hobbies like sailing, playing music, and realizing different types of tinkering or construction ideas. Where the latter lost quite a bit of importance, since working a good fraction on developing experimental equipment.

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