Prof. Stephan W. Koch from the Philipps University Marburg, Germany, died in Marburg on September 12 at the age of 69 after a short illness. He had recently retired as a Professor of Physics at the University of Marburg, a position he occupied since 1993 after spending the period 1986–1993 as a Professor of Optical Sciences and Physics at the University of Arizona. He continued to maintain a many decades long collaborative link to the Wyant College of Optical Sciences (Univ. Arizona) as an Adjunct Professor until his untimely death.

Stephan was a highly respected, pioneering theoretical physicist whose groundbreaking research on many-body theory shaped the scope of optical, quantum-optical, and electronic properties of solids. He was a prolific trendsetter, as evidenced by his extensive body of work that led to transformational insights and experimental advancements in semiconductor quantum optics, coherent ultrafast phenomena, lightwave electronics, the interface of optics and quantum transport, and laser technology. His name will always be associated with the semiconductor Bloch equations, so elegantly derived in his classic textbook coauthored with Hartmut Haug, Quantum Theory of the Optical and Electronic Properties of Semiconductors. During his time in Marburg, his contributions broadened to include the development of a fundamental approach to semiconductor quantum optics within the framework of a consistent many-body theory, that allows treatment of the effect of extremely strong optical and terahertz fields. The Marburg school of consistent many-body theory carries on Stephan’s scientific legacy. His last studies were on very strong excitonic correlations in two-dimensional transition metal dichalcogenide systems.

In the area of optoelectronics, Stephan Koch recognized early the importance of many-body Coulomb effects in semiconductor optics and particularly in semiconductor lasers. He played an important role in developing a predictive microscopic laser theory, starting at the level of electrons and holes interacting via the Coulomb interaction, combined with many-particle effects due to electron-phonon and electron-photon coupling. The approach continues to impact semiconductor laser advances because of its rigorous foundation. One notable example is the systematic improvement that solved the longstanding laser gain linewidth problem. The microscopic theory improved the predictive capability tremendously by removing the dephasing rate as an input parameter in laser gain calculations.  At the same time, the theory produced gain spectra that are in excellent agreement with experimentally measured data. More recent examples include the use of the complex optical susceptibility based on the semiconductor Bloch equations in modeling mode-locked lasers for frequency comb or ultrashort pulse generation, and heterogeneous-integrated lasers producing sub-Hz linewidths.

In addition to research, Stephan was involved in self-governance and in scientific organizations. Those activities included Dean of the Department of Physics and speaker of the Collaborative Research Center 383. He served as Divisional Associate Editor of Physical Review Letters and Topical Editor of the European Physical Journal, as well as a member of the Board of Trustees of the Physik Journal. He was a Fellow of OSA (now Optica). For his groundbreaking research and service to the scientific community, he received numerous awards, including the Leibniz Prize of the German Research Foundation and the Max Planck Research Award.

With Stephan Koch’s passing, we lost a very special colleague, friend, teacher, and mentor, whose intense dedication has made lasting impact on semiconductor optics. He was a very dedicated teacher, who made key contributions to numerous summer schools, and who not only passed his enthusiasm to students but also mentored numerous scientists, now spread worldwide in academia and industry.

Stephan Koch had many friends in the IEEE Photonics Society, who will miss his good humor and critical thinking. He had constantly challenged us to understand semiconductor lasers at the fundamental level of electron-electron, electron-phonon and electron-photon interactions.

Contributors:

Rolf Binder, University of Arizona, Tucson, Arizona
Weng Chow, Sandia National Laboratories, Albuquerque, New Mexico
Mackillo Kira, University of Michigan, Ann Arbor, Michigan
Andreas Knorr, Technical University of Berlin, Berlin, Germany
Torsten Meier, University of Paderborn, Paderborn, Germany
Jerome Moloney, University of Arizona, Tucson, Arizona