Meet our MPSP Fellows: Interview with Stefan Karsch
In today's interview, Prof. Dr. Stefan Karsch reveals the most surprising photonics application he has ever encountered in his research.
He is the group leader of the High Field Lasers & Applications Group (HiFLAG), and Professor for Physics at the Ludwig-Maximilians-University in Munich.
What first sparked your interest in photonics, and how did it lead you to the Max Planck School of Photonics (MPSP)?
I have always been fascinated by lasers, mainly due to the colorful laser beams I saw as a high school kid during the Munich laser exhibition. As my „Facharbeit“ for the high school exam, I built a holography setup with a HeNe laser and recorded holograms on film, which I still have in my office today. During my studies at the Technical University of Munich, laser physics was obscured a bit by the strong nuclear and particle physics groups, so I first embarked on an accelerator physics course and then worked in nuclear physics as my first PHD topic. But when the opportunity arose to work on particle acceleration with lasers at the Max Planck Institute of Quantum Optics, I jumped onto the train and never got off. As our group was involved with Max Planck School of Photonics (MPSP) through Ferenc Krausz from the start, I became part of it as well.
As a fellow, what has been your biggest challenge in the field of photonics, and how have you overcome it?
The biggest challenge for an experimental physicist, is and always has been, attracting and selecting the best young talent to her/his field. Here, MPSP has made a difference by helping to find and hire three of my brightest students in recent years. The other challenge is trying to stay competitive and at the forefront of one’s field. I am not sure that I have overcome these challenges yet. They are a bit perpetual in nature. All other scientific challenges, of which there are many, pale in comparison. But for those scientific challenges, I have always found that honest collaboration and synergistic work between equal partners can solve a lot of problems instead of relying on fierce competition.
Collaboration plays a major role in research. What has been your most rewarding collaborative experience, and how did it push the boundaries of your work?
We are collaborating with many institutes, but our longest-standing collaboration are with Helmholtz-Center Dresden-Rossendorf (HZDR), as part of the Helmholtz society, and Oxford University. While HZDR gives us access to cutting-edge experimental facilities and excellent experimental experience, our Oxford friends always find a solution to complex physics problems that my small group (and brain) struggles to solve. And meetings in Oxford invariably end with a visit to the local pubs, which is hard to beat for having interesting conversations.
What are some of the most surprising or unexpected applications of photonics that you’ve encountered in your research or studies?
As a senior researcher, the answer to this question has to be quite specific, since one becomes very specialized over time. Our quest for higher electron energies from a laser-wakefield accelerator (LWFA) led us to add a second plasma behind the first one, hoping that the laser pulse exiting the first plasma would drive another accelerator in the second one, thus boosting the energy. What we observed instead was that a 40 MeV electron could be completely stopped in 1.5 mm of rather dilute plasma. This is one-fourth of the stopping range, even in lead. We deduced that this strong stopping had to be mediated by the collective excitation of a plasma wave in the second jet. So, we used this idea to look into whether this secondary wave, driven by electrons, can be used for accelerating particles as well. This crazy idea has been extremely successful since then, spawning a major research collaboration between leading plasma acceleration groups and many high-profile publications. It may even provide a way of producing unprecedentedly bright electron beams in a very compact setup.
What advice would you give to students or early-career researchers aspiring to make an impact in the field of photonics, and how can they stay motivated through challenges?
Choose your field of work according to your personal preferences, and don’t be shy to change subjects if you feel that your chosen topic doesn’t interest you enough. If you love your work and it gives you full reward, motivation comes naturally.
How does your research at the Max Planck School of Photonics differ from your previous experiences in photonics, and what unique opportunities have you found here?
Not so much, to be honest. Before my time as a university professor, I worked at the Max Planck Institute of Quantum Optics, where I already had ample chance to experience the unique research atmosphere there. MPSP is quite unique in attracting and training excellent students from all over the world. While its ability to attract students rivals that of other Max Planck graduate programs, its training aspect is truly unique. But as a member of a non-teaching institute, these benefits are hard for me to take advantage of, unless an MPSP student applies to do their thesis project in my group.
You’ve been part of the Max Planck School of Photonics for some time now. What’s been the most memorable moment or breakthrough you’re particularly proud of during your time here?
The most memorable moments have been the PHD defense of my first MPSP research phase candidate. So far, there has been one, and another one is coming up soon.
Looking ahead, what excites you most about the future of photonics, and where do you hope your research will take you in the next few years?
I hope that as a university we can make an impact on groundbreaking research to improve and compactify laser-based particle accelerators that make the benefits of accelerators available for a broader range of institutions and users. If we can do that by developing new laser concepts and new plasma schemes, my work has been successful.