COMMENT OBITUARY Nicolaas Bloembergen (1920–2017) and optics pioneer whose work led to magnetic resonance imaging.

icolaas Bloembergen had three green laser pointers. major achievements, any one of A more fundamental concept was non- which could have warranted his linear susceptibility, the optical response of NNobel Prize in . He was a pioneer in illuminated material. Normally, this response nuclear magnetic resonance (NMR) and would depend on the input frequen- in non-linear optics, as well as in , cies. Bloembergen realized that non-linear the work mentioned in the Nobel citation. susceptibility would also contribute to the IRA WYMAN/SYGMA/GETTY Self-effacing, he sometimes called himself amount of free energy in the material (free “a glorified engineer”. He was thrilled that energy depends on both the input and output his intellectual pursuits turned out to be use- frequencies). Thus, there is always one extra ful in wider society. I recall him excitedly frequency in non-linear susceptibility. telling a weekly group meeting in the early Bloembergen and his team also made 1970s, “NMR is now being used for medical clear that many seemingly disparate physi- imaging!” cal effects (including second-harmonic gen- Bloembergen died aged 97 in Tucson, eration and change in refractive index with , on 5 September. Born in 1920, electric field) all stem from the same physical in , the , he passed process. Similarly, there are vast numbers of through the crucible of the Second World third-order non-linear optical effects, which War. He studied physics at Utrecht Uni- seem different, yet have the same origin. versity in the Netherlands, but his educa- counter-intuitive observation that spectral Today, Internet communication to data tion was interrupted when Nazi authorities lines become sharper and narrower the centres, as well as under-sea optical com- closed the university. During the Dutch more frequently the nuclear spins are dis- munication cables beneath the Pacific and famine in 1944, he recalled, he ate boiled turbed. The concept is used to explain the Atlantic oceans, need to take full account of tulip bulbs. One in ten of his college frater- shape of lines in all facets of , in these non-linear effects to function. nity brothers perished, having been caught all research fields and across all frequency After retiring from Harvard in 1990, and killed as members of the underground, bands. The sharp spectral lines that Bloem- Bloembergen worked as a visiting scientist at shot as hostages or deported as Jews, or hav- bergen observed for protons in water were the in Tucson, where ing succumbed to illness and malnutrition. later used for medical magnetic resonance he accepted a professorship in 2001. In January 1946, he boarded a ship for the imaging, enabling physicians to view soft tis- He often said that he was just lucky to be , bound for graduate studies sue inside the body for the first time. at the right place, at the right time. But there with Edward Purcell at Working to enable the precursor of the were many other scientists at that time — in Cambridge, Massachusetts. Just five weeks laser — the — he found a practical and Bloembergen always seemed to be at the earlier, Purcell had co-discovered NMR — way to generate a population inversion, an right place, over and over again. the concept that atomic nuclei can absorb unusual situation in which more members His achievements arose from his great radio-frequency electromagnetic radiation of a physical system exist at a higher energy scientific and personal integrity. He never in the presence of a magnetic field. There level than at a lower one. Population inver- deceived himself about what was important followed 18 months of intense experimen- sion is a prerequisite for lasers, and Bloem- in physics, and he never went for the easy tal work, in which Bloembergen laid out the bergen’s scheme, three-level pumping, solution. He was both a theoretician and an physics of nuclear magnetic relaxation: how enabled the development and widespread experimentalist. And although he seemed to NMR could be used to sense the motions of, adoption of the laser. His colleague Lester have a laissez-faire attitude in his lab, that for example, water molecules, through the Hogan teased rivals at Bell Labs, telling them may be because he was always so busy. He radio-frequency response of its protons. that, once they realized how simple Bloem- was a role model, loved by his students. He This formed the basis of his PhD thesis, bergen’s idea was, “you will kick yourself in was always excited when they brought him which became such a useful guide to the new the pants”. new data, and he took every opportunity to technology that hundreds of unauthorized In 1961, Bloembergen and his research brag about their achievements. They were photocopies were soon in circulation. His group at Harvard examined how light of suf- his greatest pride. ■ thesis was ultimately reissued as a book, ficient intensity can change the properties of Nuclear Magnetic Relaxation (Springer, a material that it interacts with. For example, is the James & Katherine 1948) that sold well for decades. Bloember- refractive index becomes a function of light Lau engineering chair and professor in gen spent a year back in the Netherlands intensity — hence the phenomenon now the electrical-engineering and computer- at , before returning to called non-linear optics. His group pub- sciences department, and director of the Harvard, where he was made an associate lished three long papers in Physical Review NSF Center for Energy Efficient Electronics professor in 1951. in 1962–64, exploring the phenomenology Science (E3S), University of California, During his thesis work, Bloembergen’s and fundamentals of this concept. Among Berkeley, Berkeley, California 94720-1770, first striking physics discovery was the the more minor insights was quasi-phase USA. concept of motional narrowing. This is the matching, today used to create the green in e-mail: [email protected].

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