Wolfgang Kaiser, one of the pioneers of the laser and the physics of ultrafast phenomena, died on 20 October 2023 at the age of 98. He was a full professor at the Technical University of Munich.
Wolfgang was born in Nuremberg on 17 July 1925. He studied physics, and in 1952 he received his doctorate on light absorption of silver halides under Rudolf Hilsch in Erlangen. In the same year, he went to the US as a postdoc at Purdue University. At Purdue, Wolfgang worked with H. Y. Fan on germanium and indium antimonide, using spectroscopy in the visible and IR to characterize impurity states and doping of those semiconductors.
In 1954 he moved to the US Army Signal Corps Engineering Laboratories in Fort Monmouth, New Jersey, as a research scientist. At this time, the Signal Corps was not regarded as a top research institution. However, it gave access to very good technical resources and allowed Wolfgang to continue his scientific career independently. At the Signal Corps, he studied impurities in silicon based on their vibrational fingerprint.
In the mid 1950s, technical applications of semiconductors focused on germanium, since no silicon crystals were available with adequate properties. Available material always became heavily n-type conducting when brought to high temperature. Wolfgang identified oxygen as the impurity causing that effect, and it became evident that it originated from the crystal growth in quartz crucibles. The problem was solved by preparation of crystals with the floating-zone technique, providing much lower oxygen concentration. Those achievements opened the door for the development of today’s semiconductor technology based on silicon.
For Wolfgang, the results generated a job offer from Bell Laboratories in Murray Hill, New Jersey, one of the leading scientific laboratories on semiconductor research at the time. From 1957 on, he could enjoy the stimulating atmosphere of the place, where he continued his investigations in solid-state physics. He solved the problem of different types of diamond by recognizing that the materials properties were heavily impacted by a substantial amount of nitrogen impurities. He also found hot electrons and carrier multiplication—the avalanche effect—in silicon.
At the beginning of the 1960s, the advent of the laser revolutionized physics. A few weeks after the press conference in July 1960 where T. H. Maiman had presented the first ruby laser, a group at Bell (R. J. Collins, D. F. Nelson, A. L. Schawlow, W. Bond, C. G. B. Garrett, and Kaiser) implemented a similar setup and demonstrated the basic emission properties defining an operating laser today. Later on, Wolfgang focused on the new light source and its applications. In 1961, together with Garrett, he found an unexpected fluorescence emission in the blue, when a europium-doped CaF2 crystal was illuminated in the red (694 nm) by intense light from a ruby laser. This first demonstration of two-photon absorption opened completely new avenues, from high-resolution spectroscopy to new types of microscopic imaging in biology and medicine.
In 1964 Wolfgang was appointed professor of experimental physics at the then Technische Hochschule of Munich. He was one of the founding members of the newly established department of physics, and he initiated an internationally renowned research group in laser physics and spectroscopy. He pioneered the development of picosecond lasers and the use of optical parametric processes to generate tunable ultrashort pulses. New spectroscopic methods were introduced. For example, the coherent Raman effect was discovered and further developed into time-resolved coherent Raman spectroscopy. He used such methodological achievements for the investigation of ultrafast phenomena on the pico- and later femtosecond time scale. Those include the phase and energy relaxation of molecular and lattice vibrations in condensed matter, the characterization of chemical elementary reactions, the elucidation of the primary processes of bacterial photosynthesis of bacteriorhodopsin and reaction centers, as well as the fastest electronic phenomena in semiconductor systems.
Wolfgang retired in 1993, but his influence on laser physics remained. Looking back, he was instrumental in bringing this field to Germany and establishing it, with over 240 publications. Until the end, he was highly interested in the latest developments in physics, optics, and spectroscopy.
The exchange of ideas with his colleagues and their support were particularly important to Wolfgang. There was a very open and constructive working and discussion atmosphere in his group. He took a lot of time for his diploma and doctoral students, encouraging them to develop their own ideas and to try out new things. That approach often led to unexpected findings, which were then discussed on long walks in the countryside. Many of his students went on to become heads of chair professorships or research institutes throughout Germany.
Wolfgang was extremely active in his private life into old age. His pronounced love of the fine arts deserves special mention. He was a regular visitor to Munich’s museums, and he liked to take people from his academic circle with him to introduce them to various aspects of art. He was particularly interested in the history and art of the Impressionists and Der Blaue Reiter. One of his lifelong passions was the mountains. Into his 80s, he was still able to climb from the Eibsee to the summit of the Zugspitze in one day, and he often spoke enthusiastically about his experiences. He regularly hiked in the foothills of the Bavarian Alps.
Wolfgang received several honorary doctorates and was elected member of the US National Academy of Sciences, the Bavarian Academy of Sciences, and the Academia Europaea. He was awarded the Max Born Prize of the German Physical Society (DPG) and the British Institute of Physics, the Lippincott Prize of the Optical Society of America, and the Stern–Gerlach Medal of the DPG, and he was decorated with the Maximilian Order for Science and the Arts of the State of Bavaria.
In Wolfgang we have lost our teacher and friend, whose memory we will always cherish with gratitude.
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