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NanoNarratives: Dr. Lutz Schrottke, Expert in Terahertz Quantum Cascade lasers

Dr. Lutz Schrottke has spent more than 25 years at PDI, where his work on Quantum Cascade Lasers (QCL) laid the groundwork for one of the institute's Core Research Areas (CReAs). In this interview, Dr. Schrottke looks back on his academic path, his contributions to the development of THz QCLs, and their applications in fields like astronomy and plasma diagnostics. As he steps into retirement, he discusses the challenges and opportunities ahead for QCL research, and his plans to continue contributing to the field as a guest scientist at PDI.

Can you describe your academic journey and what inspired you to pursue a career in experimental physics?
From a young age, I was captivated by the natural sciences and mathematics. The beauty of equations and the precision of scientific inquiry fascinated me. Alongside that, I had a strong interest in technology. I attended the "Erweiterte Oberschule Heinrich Hertz," which had a strong focus on mathematics and physics, and it became clear early on that I wanted to study physics. My career wasn’t meticulously planned, but I always seized the opportunities that came my way. My early work at the "Zentralinstitut für Elektronenphysik" involved studying the optical properties of thin-film electroluminescent devices. In many ways, this work foreshadowed my later research on THz Quantum Cascade Lasers (QCLs).

What motivated you to focus on Quantum Cascade Lasers (QCL) and THz quantum-cascade lasers specifically?
After initially exploring optical spectroscopy and near-field scanning optical microscopy on semiconductor heterostructures, I began contributing to PDI’s efforts on QCLs, which are inherently complex. The collaborative nature of this research, particularly at PDI, played a significant role in my focus. Discussions about the potential and challenges of QCLs within the institute led to the development of a dedicated team. Initially, we worked on mid-infrared QCLs, but soon shifted to THz QCLs, as the GaAs-based materials PDI focused on were more suitable for THz applications than for mid-infrared ones.

Reflecting on your extensive career, what accomplishments are you most proud of from your time at PDI?
From the outset, my primary contribution was adopting theoretical models to design QCLs and developing their active regions. Our success was due to the comprehensive capabilities available at PDI—precise simulations, high-quality heterostructure growth via molecular beam epitaxy, photolithography for fabricating laser bars, and rigorous optical and electrical analysis. This synergy of expertise allowed us to push boundaries in QCL research.

What have been the most significant advancements in QCL in recent years, and how has your work contributed to these developments?
One of the highlights has been our collaboration with the German Aerospace Center (DLR) in Berlin. Together, we developed a 4.75 THz QCL, which was integral to the local oscillator in a heterodyne spectrometer for detecting atomic oxygen. This was used in the German Receiver for Astronomy at Terahertz Frequencies (GREAT) aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA), marking the first use of a THz QCL in an astronomical instrument. Another major achievement is our THz absorption spectrometer for measuring atomic oxygen density in plasmas, developed with the Leibniz Institute for Plasma Science and Technology (INP).

How do you envision the future of QCL research and its impact on technology and society?
There are some exciting challenges ahead, particularly in high-resolution spectroscopy for astronomy and plasma diagnostics. For atmospheric research, the European Space Agency is preparing for the Earth Explorer 12 mission in 2036. One of the mission’s candidates will require a 4.75 THz QCL as a local oscillator for observing atomic oxygen in Earth’s mesosphere and thermosphere, at altitudes of 50–150 km. Developing a satellite-proof laser for this mission will be key. In plasma diagnostics, the focus will be on detecting oxygen and fluorine, both essential in etching processes for the microelectronics industry. The challenge here will be creating QCLs that can operate under industry conditions.

In your opinion, what are the biggest challenges currently facing the field of quantum electronics and semiconductor research?
A key challenge in THz QCL research is the balance between innovation and practical applications. While we are making great strides in developing real-world applications, there is still a lot of fundamental research needed. One major obstacle is the need to increase operating temperatures for both continuous-wave and pulsed operation, which would reduce the cooling requirements and make these devices more practical for widespread use.

As you retire, what are your plans for the future? Do you intend to continue contributing to the scientific community in some capacity?
I remain deeply fascinated by the transition of THz QCLs from basic research to real-world applications, and I plan to continue observing the progress of the QCL group. As a guest scientist, I hope to contribute to novel designs for both QCLs and quantum-cascade detectors, as well as further refining the models we use for design purposes.