(1920- ) Dutch/American Theoretical and Experimental Physicist, Laser Spectroscopist
Nicolaas Bloembergen made his mark on 20th-century physics through his contributions to the technology of the laser, the revolutionary device that creates and amplifies a narrow, intense beam of coherent light, and its forerunner, the maser. His three-level crystal maser proved strikingly more powerful than earlier gaseous masers and became the most widely used microwave amplifier. Bloembergen went on to develop laser spectroscopy, which allows high-precision observations of atomic structure. On the basis of his laser spectroscopic studies, he established the theoretical foundation for the science of nonlinear optics, a new theoretical approach to the analysis of how high-intensity coherent electromagnetic radiation interacts with matter. Bloembergen also did seminal work in nuclear magnetic resonance.
Bloembergen was born on March 11, 1920, in Dordrecht, the Netherlands, the second of six children born to Auke Bloembergen, who was a chemical engineer, and Sophia Maria Quint, who had a degree that qualified her to teach French but instead devoted herself to family duties. Before Nicolaas reached school age, the family moved to Bilthoven, a suburb of Utrecht, where, he recalls, “We were brought up in the Protestant work ethic characteristic of the Dutch provinces.” Encouragement of intellectual attainments and a level of frugality beyond that required by family income were twin pillars of his childhood. However, his parents also urged him to participate in field hockey, water sports, and skating on the Dutch waterways, rather than sit constantly over his books. At the age of 12 he entered the municipal gymnasium in Utrecht, whose rigid curriculum emphasized the humanities; he did not discover his love of science until the last years of secondary school. He describes his choice of physics as “probably based on the fact that I found it the most difficult and most challenging subject.”
In 1938, he entered the University of Utrecht, where he was allowed to assist in the research of a graduate student, G. A. W. Rutgers, with whom he published his first paper, “On the Straggling of Alpha Particles in Solid Matter,” in 1940. That same year, however, the Nazis invaded the Netherlands, and Bloembergen’s mentor, Professor L. S. Ornstein, was forced to leave the university in 1941. He managed to obtain the Dutch equivalent of a master’s degree in science degree just before the university was closed completely in 1943. For the next two years, until the war’s conclusion in 1945, he went into hiding, “eating tulip bulbs” and reading about quantum mechanics “by the light of a storm lamp.”
At war’s end, he was accepted to do graduate work by Harvard University, and, with the help of his father and the Dutch government, he left behind the devastation of Europe. His arrival at Harvard coincided with the detection, by e. m. purcell, R. V. Pound, and Henry Torrey, of nuclear magnetic resonance (NMR) in condensed matter (an effect that results when waves of radio frequency are absorbed by the nuclei of matter in a strong external magnetic field). Bloembergen was accepted as a graduate assistant to develop the early NMR apparatus. At the time the field of NMR in solids, liquids, and gases was unexplored, and Bloembergen and his Harvard colleagues gathered a rich harvest of findings, which they published in 1948 in a landmark paper, “Relaxation Effects in Nuclear Magnetic Resonance,” in The Physical Review. Bloembergen did part of this work back in the Netherlands, at the Kammerlingh Onnes Laboratory in Leiden, in 1947 and 1948; there he discovered the nuclear spin relaxation mechanism by conduction electrons in metals and by paramagnetic impurities in ionic crystals, the phenomenon of spin diffusion, and the large shifts induced by internal magnetic fields in paramagnetic crystals. He was the first physicist to measure NMR relaxation times, that is, the decay time of the NMR process, accurately and discovered that NMR relaxation times could be measured in seconds or fractions of seconds. This result made NMR a practical research tool, with applications in chemistry, medicine, and physics, including a method for analyzing the structure of molecules and for producing the contrast needed to create images of tissues in the human body. Bloembergen wrote his doctoral thesis, “Nuclear Magnetic Relaxation,” using these same materials, and submitted it, in 1948, to the University of Leiden, where he had already filled all his other requirements for the Ph.D.
During a vacation trip with the Physics Club in Leiden, in the summer of 1948, he met Huberta Deliana Brink (known as Deli), a premed student, whom he married in Amsterdam in 1950. The young couple immigrated to the United States, where they would raise three children and become citizens in 1958. With Deli as “a source of light in my life,” he picked up his career at Harvard. While a junior fellow there he broadened his experimental background to include microwave spectroscopy and some nuclear physics at the Harvard cyclotron; however, he preferred “the smaller-scale experiments of spectroscopy, where an individual or a few researchers at most, can master all aspects of the problem.” Thus, in 1951, he returned to NMR research; his group made a number of significant discoveries that led Bloembergen to propose a three-level solid state maser in 1956.
He did not try to build a working laser, after charles hard townes and arthur leonard schawlow published their proposal for an optical maser, doubting that a small academic lab with no previous experience in optics could succeed. He did, however, recognize in 1961 that his laboratory could take advantage of some of the new research opportunities made possible by laser instrumentation. His group started a program in a field that became known as nonlinear optics, the study of the behavior of high-intensity coherent radiation in matter, and published their early results in a 1965 monograph. Nonlinear optical methods of spectroscopy are based on the mixing of two or more high-intensity coherent light waves in an optically active medium for which the principle of superposition, valid for ordinary electromagnetic radiation, breaks down. Bloembergen and his group demonstrated this type of nonlinear optical phenomenon shortly after the laser was introduced and comprehensively explored the theory describing it around the same time. Bloembergen’s method of nonlinear four-wave mixing, in which three coherent light waves act together in generating a fourth coherent light wave, made it possible to generate laser light far outside the visible range, in both the infrared and the ultraviolet spectra. This greatly extended the range of wavelengths accessible to laser spectroscopy studies. One example of this is a special form of four-wave mixing called coherent anti-Stokes Raman scattering (CARS), which has been applied in studies of widely differing kinds—from optimization of combustion processes in automobile engines to the study of the transport of chemical elements in biological tissues. For this seminal work Bloembergen shared the 1981 Nobel Prize in physics with Schawlow, the coinventor of the laser.
While performing this work Bloembergen enjoyed a rich academic life at Harvard, including innumerable travels abroad.
He served on a 1986 committee to study President Ronald Reagan’s “Star Wars” program and concluded that, in order for it to work, a decade of laser weapon research would be needed. The committee’s findings confirmed the scientific community’s “gut feeling” that it was not practical.
In addition to writing monographs on nuclear magnetic relation and nonlinear optics, he has published over 300 papers in scientific journals. In June 1990 he retired from the faculty at Harvard and became professor emeritus. He and his wife had a special feeling for Tucson, so after his retirement from Harvard, in 1991 he became an unpaid professor of physics at the University of Arizona, Tucson, where he continues to pursue research at the university’s optical institute. Also in 1991, he served as president of the American Physical Society.
Bloembergen’s theoretical and experimental work with masers and lasers led to a vast spectrum of practical applications, from surgical techniques to boring and cutting of metal to the development of fiber optics. In a 1998 talk, he foresaw an increasing laser use in scientific applications, noting that there is currently an “enormous push” for high-powered semiconductor lasers.
