Interview with Marcus Seidel

Since November 2019, our former colleague Marcus Seidel has been working at the Deutsches Elektronen Synchrotron (DESY) in Hamburg. Currently, a major upgrade for the free electron laser FLASH is in progress. In this context Marcus is also involved in the development of laser systems. In this interview he talks about his work and what the future of Free Electron Lasers could look like.


Why did you move so far north from Munich?

I was actually not afraid to go “so far north” since I had already been studying in Rostock for several years. But I would not say that going north was really on purpose. After spending two years abroad my family and I had the desire to move back to Germany. We checked several sites and at the end Hamburg suited us best. I believe that Hamburg is a great place to continue my scientific career. Although I would claim that Munich is still the “science capital” of Germany, I feel like science has been really boosted here over the past years. We have got many new buildings of DESY, Max-Planck and the university on our campus which is even supposed to grow to a “Science City” over the next decades.


How does life in Hamburg differ from life in Munich?

Well, I have changed my fast food preference from “Leberkäsesemmel” to “Fischbrötchen“. A deeper answer to this question is actually quite hard at present. I have spent most of the time here under all these corona restrictions, and thus cannot tell up to now how life in Hamburg is in normal times. Nevertheless, under the given circumstances, a great advantage of the city has turned out during the past summer: The beaches of the Baltic and the North Sea are only a good hour drive away from our home. Hence, even without traveling we had some summer holiday feeling this year.


Your specialty is Free Electron Lasers. Can you give us an overview of your work at DESY?

Although I am working at FLASH, I am not too much into the free electron laser itself. Close to 90 % of the experiments carried out at the FLASH facility are excite-probe experiments where conventional lasers excite the sample under test to a specific state. The free electron laser is then probing the state dynamics. With “conventional” I mean that these lasers are actually based on the process of stimulated emission and are very compact compared to the large scale free electron laser which is based on wiggling of electron bunches. In short, I am mainly developing ultrafast lasers for these excite-probe experiments, and thus I am staying close to my research field at MPQ. We have just completed the development of a new laser system for user experiments. Last week we handed it over to our operations team. In two weeks first external users will work with it. It is the first laser at FLASH which is based on pulse compression instead of parametric amplification. We try to pick up current trends of the scientific community, do our own research on it and adapt these trends to the very specific laser specs we need. Since this new laser system is ready now, I can spend at present more time on an own independent research project and on student supervision.


How does your work differ from that at MPQ?

The main difference is that DESY is a user facility. Therefore, our first priority is safety and the second is to serve our users. Our own research results are certainly important for providing our users cutting-edge tools but they are ranked lower in our priority list. For instance, the lasers we develop do not have to provide record-breaking parameters but must be extremely reliable. Our users plan their free electron laser experiment many months in advance and have only a very limited beam time of one week to make it work. They cannot afford to have one of our lasers not running properly. For that reason all researchers of our laser group got on-call duty twice a year, i.e. they may have to get up in the middle of the night to fix laser issues at FLASH. Fortunately, that has not yet happened to me. During the past three years our lasers were always more than 95 % of the requested time available.


Where do you see Free Electron Lasers technologically in 10 years?

At present, FLASH is the only free electron laser worldwide that provides more than 1000 pulses per second. That will change in the near future as new facilities in the US and China will open for user operation. Hence, I can claim that the FEL developments are somehow driven by the same motivation that I had when I did my PhD at MPQ: increasing repetition rate increases data rate and grants access to photon-hungry applications, for example in solid-state physics. Another important trend, which is also at the heart of the currently starting FLASH 2020+ upgrade, is seeding of free electron lasers. The FEL radiation is at present initiated by a spontaneous emission process which leads to uncertainties in the arrival time and the spectral width of the pulses. “Seeding” means that this spontaneous process is suppressed by the injection of photons with precise timing and wavelength control. I would say that the attosecond physics community and the FEL community will approach each other in the upcoming years. FELs are doing excellent in pulse energy and wavelength tunability but only processes in the 10s of femtosecond range can be tracked. Attosecond pulses on the other hand could benefit a lot by becoming more energetic but the temporal resolution they provide is fantastic. Seeding of FLASH will be an important milestone towards reaching the attosecond frontier also with free electron lasers.


And what will we be able to explore with FELs?

Just recently, I was very impressed again by the powerfulness and the versatility of high brightness light sources. Our colleagues from the synchrotron Petra III investigated how different drugs interact with a corona dummy virus. The brightness of free electron lasers is even higher than that of synchrotrons. The applications are manifold. We divide them into four categories. First, in experiments with atoms and molecules a better understanding of high-field ionization dynamics will be developed, charge-transfer processes will get a closer look and chemical reactions in space will be mimicked and examined. Time and space resolved studies of chemical reaction pathways are the second main field of applications. Here, the scientific interest reaches from fundamental aspects like the understanding of dynamics at conical intersections to applied research, for instance on carrier dynamics in photovoltaics. Third, the condensed matter community typically gets a significant amount of beam time at our FLASH facility. Photoelectron spectroscopy is a heavily used tool in solid state physics and high repetition rate FELs provide outstanding parameters for such experiments. In particular, we are looking forward to gaining better understanding of quantum effects that trigger highly application relevant phenomena like high temperature superconductivity, colossal magnetoresistance or metal-insulator phase transitions. Last but not least, the extraordinarily high brightness and the short wavelengths of FEL sources result in unique imaging capabilities. Whereas our European XFEL colleagues can resolve structures down to the atomic level, the wavelengths we provide at FLASH are well-suited for resolving nanometer-size objects. Especially large biomolecules will become of interest after our upgrade. This is the vision that we have about upcoming experiments. However, there are two calls per year for experiment proposals at FLASH. So, we are looking forward to exciting suggestions that our external users come up with.