obituary In memory of Arthur Ashkin Radiation pressure exerted by light was a lifelong passion for Arthur Ashkin. He foresaw that light pressure could do useful work and invented the optical tweezers that can trap microscopic objects, from small ‘living things’ down to individual atoms.

orn on 2 September 1922, the son of as stimulated Brillouin and Raman immigrants from Eastern Europe, scattering in fibres and four-wave mixing, Arthur Ashkin (pictured) was always sometimes referred to as four-photon B 5 proud of his humble heritage. Despite mixing at the time, were first demonstrated not having been raised in a scholarly by Art’s group. environment, he possessed an almost In 1966, Art attended a presentation by limitless passion for learning that lasted Rawson, Hara and May at the International until his passing on 21 September 2020. Quantum Electronics Conference (IQEC) At ten years old, Ashkin was fascinated in Arizona, where they showed a video on by the Crookes radiometer. He would ‘runners and bouncers’, small particles that bring it to sunlight to observe the vanes moved around seemingly at random inside turning. He learned a few years later that a laser cavity6,7. The motion was quickly this behaviour was due to the thermal identified as being of thermal origin, but forces from the air remaining in the glass this demonstration reignited Art’s childhood bulb. He also found out that, with a higher desire to demonstrate that radiation pressure level of vacuum, the vanes would turn in could do useful work if thermal effects could the opposite direction due to radiation be suppressed. pressure. It was the first time Ashkin became It was in 1969 that Art observed for the aware that radiation pressure could do first time that radiation pressure can trap useful work. His desire to understand the small dielectric spheres. He published his Crookes radiometer ultimately led him to findings in a milestone paper in Physical demonstrate the optical tweezers at the age Review Letters, but not without difficulties. of 63. It is this invention that won him the Arthur Ashkin in his garden around 1975. The challenge did not come from the journal 2018 Nobel Prize in Physics. Courtesy of Daniel Ashkin. but from a mandatory internal review at Bell Art, as he was known to friends Labs. That review stated that the manuscript and colleagues, was raised in Flatbush, was not worthy of Physical Review Letters. a neighbourhood in New York City’s Despite this obstacle, the manuscript, Brooklyn borough. His father, Isador, the Columbia Radiation Laboratory under submitted in its original form, was easily was an orphan from Odessa, then part of the supervision of Sidney Millman. The accepted and is considered by most as Russia. His mother, Anna, emigrated in high-power magnetron was intended to be signalling the birth of optical trapping early childhood from Galicia, then part of used in radar. by radiation pressure8. Art also discussed the Austro-Hungarian Empire. Art’s last Art started at Bell Labs in 1952, trapping of atoms using radiation pressure name, Ashkin, a shortened version of the immediately after obtaining his PhD from in that paper. original Ashkenasy, was the creation of an Cornell University in nuclear physics. He The next year, Art demonstrated optical immigration officer when Isador arrived joined the microwave research department levitation where radiation pressure is used at Ellis Island in 1910 as a young man. Art at the Murray Hill Laboratory, where he was to compensate gravity9. Recognizing that grew up with his brother, Julius, and sister, focused on improving the travelling-wave optical levitation can be used as a sensitive Ruth. An older sister, Gertrude, died at a tube amplifier. His first experiments in detection mechanism, Art used this tool to young age. Julius was two years Art’s senior. optics started around 1962, a few years optically measure the charge of the electron Julius became a notable physicist at a very after the laser was demonstrated. A series and to perform very precise measurements young age and played an important role of discoveries and inventions followed. He of Mie scattering and of the dimensions of in the Manhattan Project. The ‘Ashkin’ in is credited with the discovery of the optical objects10. the Ashkin–Teller model refers to Julius. damage created by the photorefractive effect Optical trapping of atoms employing In the early days of the brothers’ careers, in lithium niobate and lithium tantalate3. He radiation pressure remained an overarching leading physicists who worked with Julius also was the first to observe second-harmonic goal for Art. Together with John Bjorkholm sometimes referred to Art as ‘Ashkin’s generation from a continuous-wave signal at and his long-time technical assistant and brother, Ashkin’. optical frequencies4. friend, Joseph Dziedzic, he performed a The United States’ involvement in World From the mid- to late-1960s, Arthur series of experiments towards that goal11. War II interrupted Art’s undergraduate hired three young researchers, John They were joined by Steven Chu in the early studies in physics at Columbia University. Bjorkholm, Roger Stolen and Erich Ippen. 1980s. After another series of breakthrough He was drafted into the Signal Corps, a In the decade that followed, under Art’s experimental realizations, stable atom division of the US Army. Starting in 1942, supervision and guidance, they performed trapping was achieved. This achievement he worked as a technician on the rising sun experiments that laid the foundation of was part of the scientific discoveries on atom magnetron1,2 as part of the war effort at nonlinear optics in fibres. Such phenomena trapping and cooling that led to the 1997

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Nobel Prize in Physics being awarded to became contaminated with bacteria that in which they can be made useful. The last Steven Chu, Claude Cohen-Tannoudji and were found inadvertently trapped inside 15 years of his life were devoted to studying William Phillips. the optical tweezers. To the surprise of Art ways to harness the energy of the Sun at The basic concept that led to the optical and Dziedzic, the bacteria remained alive low cost, with the hope of reducing global tweezers came to Art in 197712, and he in the trap for several minutes. If the power warming and improving the quality of life in demonstrated it with his colleagues in of the green laser used for trapping was developing countries. Arthur Ashkin was a 198513. The idea that an object can be increased, there would be a “wild display remarkable inventor and scientist who cared stably trapped in space within a single of light scattering” before it settled down deeply about people and who was beloved by laser beam may appear counterintuitive at to a steady state and the bacterium would his colleagues. ❐ first. Art had demonstrated in 1969 that a die by ‘opticution’, a term coined by Art to transparent sphere could be trapped in the mean death by light. A change of wavelength René-Jean Essiambre ✉ transverse direction of a laser beam due from the 514.5-nm visible green laser to a Murray Hill Laboratory, Bell Labs, Nokia, New to its transverse gradient of intensity. The 1.06-μm infrared yttrium/aluminium laser Providence, NJ, USA. sphere, however, experienced a net forward where bacteria absorb less light enabled ✉e-mail: [email protected] force due to light scattering off the sphere bacteria to be kept alive for hours. It even that pushed it in the same direction as the allowed cell reproduction within the Published online: 19 February 2021 laser. To provide a confinement in the axial optical trap to be observed. Following this https://doi.org/10.1038/s41566-021-00768-0 direction of the laser, Art realized that by serendipitous trapping of bacteria, Art and tightly focusing the beam with a strong Dziedzic went on to trap and study all kinds References lens, one creates a large axial intensity of living things, including chloroplasts, 1. Ashkin, A. Phys. Rev. 69, 701 (1946). 2. Millman, S. & Nordsieck, A. T. J. Appl. Phys. 19, 156–165 (1948). gradient over a short distance just beyond spermatozoa, organelles of the paramecium 3. Ashkin, A. et al. Appl. Phys. Lett. 9, 72–74 (1966). 14 the focal point. That gradient produces a and the giant amoeba Reticulomyxa . These 4. Ashkin, A., Boyd, G. D. & Dziedzic, J. M. Phys. Rev. Lett. 11, backward gradient force that compensates experiments led to the development of new 14–17 (1963). 5. Stolen, R. H. J. Light. Technol. 26, 1021–1031 (2008). the forward scattering force. The result is applications of optical tweezers in biology. 6. Rawson, E. G., Hara, A. H. & May, A. D. In Proc. International a three-dimensional optical trap created It became possible to study very precisely Quantum Electronics Conference 8C-3 (OSA, 1966). within a single laser beam. He called his the interactions of individual biological 7. Rawson, E. G. & May, A. D. Appl. Phys. Lett. 8, 93–95 (1966). 14 8. Ashkin, A. Phys. Rev. Lett. 24, 156–159 (1970). invention the optical tweezers and he was molecules, including DNA . 9. Ashkin, A. & Dziedzic, J. M. Appl. Phys. Lett. 19, 283–285 (1971). awarded the 2018 Nobel Prize in Physics “for The exceptional experimental skills of Art 10. Ashkin, A. Science 210, 1081–1088 (1980). the optical tweezers and their application to were well known at Bell Labs. He was often 11. Ashkin, A. Proc. Natl Acad. Sci. USA 94, 4853–4860 (1997). 12. Ashkin, A. Phys. Rev. Lett. 40, 729–732 (1978). biological systems”. consulted by colleagues on how to perform 13. Ashkin, A., Dziedzic, J. M., Bjorkholm, J. E. & Chu, S. Opt. Lett. Art had trapped spherical objects difficult experiments. He had a bag of tricks 11, 288–290 (1986). with his optical tweezers but it remained like no other. For instance, he invented a 14. Ashkin, A. Optical Trapping and Manipulation of Neutral Particles unclear if objects of irregular forms could free-space polarization-independent variable Using Lasers: A Reprint Volume with Commentaries (World Scientifc, 2006). be ‘captured’ as easily by the tweezers. Art optical attenuator based on diffraction on a 15. Ashkin, A. Nonpolarizing wire-grid difraction-type optical and Dziedzic decided to try trapping the metallic comb15. The attenuation of a laser attenuator. US patent 3,620,599 (1969). Tobacco Mosaic Virus (TMV) that has beam could be varied continuously from a rod-like shape, about 16 times longer transparency to in excess of 30 dB. It could Acknowledgements than its diameter. Fortunately, they were handle high continuous-wave powers and Many people contributed to this obituary. I would like to thank his wife Aline, his daughter, Judith Herscu, and able to grab the virus by either end and produce high-quality beams. sons, Michael and Daniel. Many thanks to R. Stolen, J. stabilize it. During that study, the TMV Art saw himself as an inventor first. He Bjorkholm, E. Ippen, G. Bjorklund and S. Harris who samples were left exposed to the air and enjoyed ‘playing with toys’ and finding ways kindly provided feedback on the manuscript.

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