ARTICLE-IN-A-BOX Nicolaas Bloembergen (1920–2017)∗ Physicist, Father of Nonlinear Optics The father of nonlinear optics (NLO), Prof. Nicolaas Bloembergen, affectionately nicknamed ‘Nico’ by his students and associates, was an exceptional physicist with equal expertise in experimental and theoretical physics. The 1981 Nobel Prize in Physics was awarded to him jointly with Arthur Schawlow for the development of laser spectroscopy and to Kai Siegbahn for developing high-resolution electron spectroscopy. At that time, he was the Gerhard Gade University Professor at Harvard University. Bloembergen was born in Dordrecht, the Netherlands, on March 11, 1920, as the second child amongst six in an affluent family. His father, a chemical engineer and an executive encouraged his passion for science at a young age. He joined the Utrecht University of the Netherlands to study physics. He managed to receive his Master’s degree just before the Nazis closed the university. After spending the rest of the wartime in hiding, Bloembergen left the war-ridden Netherlands in 1945 to pursue graduate studies at Harvard University with Professor Purcell. Just five weeks before the arrival of Bloembergen, Purcell had co-discovered nuclear magnetic resonance (NMR) spectroscopy, based on the concept that atomic nuclei can absorb electro- magnetic radiation in the radio-frequency region in the presence of a magnetic field. During his graduate studies, Bloembergen developed the first NMR machine and outlined the physics of nuclear magnetic relaxation, which essentially dealt with how NMR could be used to sense the motions of water molecules, for example, through the radio-frequency response of its pro- tons. His first striking discovery in physics was the concept of ‘motional narrowing’, which resulted in one of the seminal papers on nuclear magnetic resonance, popularly known as the BPP paper published in 1948 [1]; one of the most highly cited articles in Physical Review. Mo- tional narrowing is the counterintuitive concept that spectral lines become narrower and more resolved as the nuclear spins are disturbed more frequently. This concept is used to explain the line shapes for all aspects of spectroscopy across all research fields and all frequency bands. The same idea led to the development of magnetic resonance imaging (MRI) as the sharp spec- tral lines first observed by Bloembergen for protons in water were later used for medical MRI, enabling physicians to view soft tissues inside the body in a non-invasive manner. Bloembergen went back to the Netherlands and submitted his thesis, Nuclear Magnetic Relax- ation, at the University of Leiden in 1947, as he had completed all the preliminary examinations ∗Vol.25, No.12, DOI: https://doi.org/10.1007/s12045-020-1086-8 RESONANCE | December 2020 1653 ARTICLE-IN-A-BOX in the Netherlands. Given the widespread utility of this work, Springer published his thesis as a book in 1948 [2]. He also received his PhD degree from Leiden in the same year. After that, Bloembergen continued as a postdoctoral fellow at Leiden for about a year with Prof. Gorter. While at Leiden, he met Huberta Deliana Brink (known as Deli), and the two were married in 1950. Bloembergen returned to Harvard as a Junior Fellow of the Society of Fellows in 1949. Though, as a Harvard Junior Fellow, he studied microwave spectroscopy and nuclear physics at the Harvard cyclotron, he chose to work on smaller-scale spectroscopy experiments even- tually. He was inducted as an Associate Professor at Harvard in Applied Physics in 1951. In 1956, Bloembergen created the crystal maser, which was an extension of his previous work in microwave spectroscopy and was more powerful than the standard gaseous version. In 1957, he became the Gordon McKay Professor of Applied Physics, and in 1958, he became a nat- uralized United States citizen. His discovery of the three-level solid-state microwave maser, the predecessor to the solid-state optical laser, naturally followed from his experience with the radio frequency relaxation processes of the BPP paper. This discovery earned Bloembergen the prestigious Oliver E. Buckley Condensed Matter Prize of the American Physical Society in 1958. He was also honoured with the Stuart Ballantine Medal by the Franklin Institute in 1961. Some people thought that his research impact on the development of maser should have entitled him to be a part of the 1964 Nobel Prize. This was because Bloembergen had found a practical approach to generate population inversion. Population inversion, which is a prereq- uisite for lasers, is an unusual situation, where more members of a physical system exist at a higher energy state as compared to the lower one. Thus, his practical scheme of three-level pumping enabled the development and widespread adoption of the laser. The three-stage crys- tal maser was dramatically more powerful than earlier gaseous masers, and that has become the most widely used microwave amplifier. In the early 1960s, as soon as Theodore Maiman revealed the discovery of laser, Bloembergen extended his spectroscopy research into tunable lasers and developed a high-precision tech- nique to observe the atomic structure. His research on laser spectroscopy, in turn, led to his inception of NLO, wherein he created a new theoretical way to analyze the interaction of elec- tromagnetic radiation with matter. Bloembergen’s interest in NMR led to the development of masers. Exactly at the same time as the lasers were being developed in the early 1960s, Bloembergen used them to study the optical properties of matter at high light intensities with his research group. Peter Franken and co-workers at the University of Michigan observed second-harmonic generation soon after the construction of the first laser. This discovery led Bloembergen and his group to devise a theoretical model for the mixing of two or more light waves and to experiment with mixing multiple light waves. They examined how the light of sufficient intensity can change the properties of a material that it interacts with. For example, 1654 RESONANCE | December 2020 ARTICLE-IN-A-BOX the refractive index, which is otherwise a characteristic material property, becomes a func- tion of light intensity—hence the phenomenon is now called nonlinear optics. Bloembergen’s group published three long papers in Physical Review during 1962–64 [3–5], exploring the phenomenon and the fundamentals of this concept. Among his relatively minor insights was the concept of quasi-phase-matching, which is nowadays used to create the green colour in green laser pointers. A more fundamental concept was nonlinear susceptibility, the optical response of the illumi- nated material. Usually, this response would depend on the input light frequencies. Since free energy depends on both input and output frequencies, Bloembergen realized that nonlinear sus- ceptibility would additionally contribute to the free energy of the material. There is, therefore, always one extra frequency in nonlinear susceptibility. In his classic monograph, Nonlinear Optics, published in 1965 [6], Bloembergen outlined his group’s experimental and theoretical efforts to analyze the interaction of electromagnetic radiation with matter. Bloembergen wanted to essentially extend the general principles of Maxwell’s electromagnetic theory and quantum mechanics to include the higher-order interactions between light and matter, in terms of their nonlinear susceptibilities. Bloembergen and his team also made clear that many seemingly disparate physical effects (including second-harmonic generation and the change in refractive index with the change in an electric field) all stem from the same physical process. Similarly, there are vast numbers of third-order nonlinear optical effects, which seem different, yet have the same origin. The application and control of NLO effects have, in recent years, become an essential part of a variety of research and development, such as optical communication, biomedical optics, and chemical analysis. Bloembergen received many more awards including the Guggenheim Fellowship in 1957, be- sides the few mentioned above. The President of the United States, Gerald Ford, honoured him with the award of the National Medal of Science in 1975. In 1979, he received the Fred- eric Ives Medal of the Optical Society of America (OSA). He was elected an OSA Fellow in 1967 and was subsequently elected an OSA Honorary Member in 1984. Bloembergen was also bestowed the highest honour in Physics in the Netherlands by the Lorentz Medal of the Royal Dutch Academy of Science in 1978. He was very active in promoting physics during his career, also serving as the President of the American Physical Society in 1991. In 1974, he became the Rumford Professor of Physics, and in 1980, he became the Gerhard Gade University Professor. Additionally, Bloembergen served as a visiting professor in Paris in 1957, at the University of California, Berkeley from 1964 to 1965. He visited Leiden University in 1973 as the Lorentz Guest Professor. The Indian Academy of Sciences elected Bloembergen as Honorary Fellow in 1978, and, thereafter, he came as the Raman Chair Professor to Ban- galore in 1979 to deliver a series of lectures [7]. During this visit, he also visited the Indian RESONANCE | December 2020 1655 ARTICLE-IN-A-BOX Institute of Technology Kanpur. From 1996 to 1997, he became a visiting scientist in the Col- lege of Optical Sciences at the University of Arizona. In 1991, Prof. Bloembergen retired from the faculty of Harvard University, USA, as Gerhard Gade University Professor Emeritus. He remained active at Harvard until he and his wife moved to Tucson in 2001. He published over 300 papers. It was during his last period of stay at Harvard in 1993 that I became acquainted with him, personally, while I worked as a postdoctoral fellow in the Oxford Building, located right across the street. I used to join the group meetings of Prof. Eric Mazur, who, at one point, worked as a postdoctoral fellow of Bloembergen before starting his independent career at Harvard.