On October 20, 2023, Prof. Wolfgang Kaiser passed away at the age of 98. He was one of the pioneers in laser and ultrafast physics and served as a professor at the Technical University of Munich.

Wolfgang Kaiser was born on July 17, 1925, in Nuremberg, and he began studying physics at Ludwig-Maximilians-University in 1943, with interruptions due to his service in the Wehrmacht. In 1952, he earned his Ph.D. in Erlangen under Rudolf Hilsch on the topic of "A new light absorption of silver halides." Following this, he went to Purdue University in the USA as a post-doc to investigate semiconductors using infrared spectroscopy. From 1954 to 1964, he worked at the US Army Signal Corps Engineering Laboratories in Fort Monmouth, studying impurities, particularly oxygen in silicon, which laid the foundation for modern semiconductor technology. In 1960, at Bell Laboratories in Murray Hill, he successfully implemented the ruby laser, previously presented by Maiman in a press conference, with typical laser properties such as low divergence and spectral bandwidth. In 1961, along with C. G. B. Garrett, Kaiser discovered two-photon absorption, with broad applications from physics to medicine.

Wolfgang Kaiser was appointed as a professor of experimental physics at the Technical University of Munich in 1964. He belonged to the founding generation of the newly established Physics Department and established an internationally renowned research group in laser physics and spectroscopy. He pioneered the development of picosecond lasers and the use of optically parametric processes for generating tunable ultrashort pulses. New spectroscopic methods were introduced, including the discovery of coherent Raman effect, further developed into time-resolved coherent Raman spectroscopy. These methodological advancements were used to study ultrafast phenomena on the picosecond and later femtosecond time scales, such as phase and energy relaxation of molecular and lattice vibrations in condensed matter, characterization of chemical elementary reactions, elucidation of primary processes in bacterial photosynthesis, and fastest relaxations in semiconductor systems.

Wolfgang Kaiser retired in 1993, but his influence on laser physics in Germany persisted. In retrospect, he played a significant role in bringing laser physics to Germany and establishing it with over 240 publications. He remained highly interested in the latest developments in laser physics and spectroscopy until the end.

The exchange with his colleagues and their promotion were particularly important to Wolfgang Kaiser. In his research group, there was a very open and constructive working and discussion atmosphere. He dedicated much time to his students, encouraging them to develop their own ideas and try new things. This often led to unexpected insights, which were then discussed during long walks. Many of his students later became leaders of chairs or institutes throughout Germany.

In his personal life, Wolfgang Kaiser remained extremely active until old age. His pronounced love for the visual arts is noteworthy. He was a regular visitor to Munich museums and often took people from his scientific circle there to introduce them to various aspects. He was especially interested in the history and art of the Impressionists and the Blue Rider. A lifelong passion was his love for the mountains. Even at the age of over 80, he was able to climb from Lake Eibsee to the summit of the Zugspitze in one day and often enthusiastically shared his experiences. Until recently, he regularly hiked in the Bavarian foothills of the Alps.

Wolfgang Kaiser held multiple honorary doctorates and was a member of the Bavarian Academy of Sciences, the National Academy of Sciences of the United States, and the Academia Europaea. He was also awarded the Maximilian Order for Science and Art. He received the Max Born Prize from the German Physical Society (DPG) and the Institute of Physics in the UK, the Lippincott Prize from the Optical Society of America, and the Stern-Gerlach Medal from the DPG.

With Wolfgang Kaiser, we have lost our teacher and friend, whose memory we will always cherish with gratitude.