John Joannopoulos, the Francis Wright Davis Professor of Physics at MIT and director of the MIT Institute for Soldier Nanotechnologies (ISN), died Aug. 17 at the age of 78, MIT announced. Joannopoulos was a prolific researcher in the field of theoretical condensed-matter physics, and an early pioneer in the study and application of photonic crystals. Many of his discoveries, in the ways materials can be made to manipulate light, led to transformative and life-saving technologies, from chip-based optical waveguides, to wireless energy transfer to health-monitoring textiles, to precision light-based surgical tools. John Joannopoulos. Courtesy of MIT/Jose-Luis Olivares. In the 1990s, Joannopoulos shifted his focus from electronics to photonics, spurred by a paper by physicist Eli Yablonovitch at UCLA. Noticing parallels with his work in electronics, Joannopoulos and his research group applied their first-principles approach to predict the fundamental behavior of photons in different classes of materials. His group was among the first to pioneer the field of photonic crystals and the study of how materials can be manipulated at the nanoscale to control the behavior of light traveling through. He co-authored the first textbook on the subject in 1995. Three years later, he overturned a more-than-century-old assumption of how light should reflect. The assumption predicted that light shined onto a structure made of multiple refractive layers could reflect back, but only for a limited range of angles. Joannopoulos and his group showed that the opposite is true. If the structure’s layers followed a particular design criteria, the structure as a whole could reflect light coming from any and all angles. This structure, was called the “perfect mirror.” Joannopoulos attended the University of California, Berkeley where he received a bachelor's degree in 1968, and a Ph.D. in 1974, both in physics. That same year, he joined the faculty at MIT, where he would spend his 50-plus year career. In 2006, Joannopoulos became director of MIT’s Institute for Soldier Nanotechnologies. Joannopoulos helped to translate many basic science insights into practical applications. He was a cofounder of six spinoff companies based on his fundamental research, and helped to create dozens of other companies, which have advanced technologies as wide-ranging as laser surgery tools, to wireless electric power transmission, transparent display technologies, and optical computing. He was awarded 126 patents for his many discoveries, and has authored over 750 peer-reviewed papers. Joannopoulos was elected to the National Academy of Sciences and the American Academy of Arts and Sciences. He was also a fellow of both the American Physical Society and the American Association for the Advancement of Science. He was the recipient of many scientific awards and honors including the Max Born Award, and the Aneesur Rahman Prize in Computational Physics.