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Arthur Ashkin: Father of the RETROSPECTIVE

Ren ´e-JeanEssiambrea,1

The father of the optical tweezers, Arthur Ashkin, graduating from Columbia in 1947, he enrolled at passed away peacefully at his home in Rumson, NJ, on as a Ph.D. student in nuclear , September 21, 2020, at the age of 98, two years after like his brother. By that time, Julius had already partici- being awarded the 2018 Physics . pated in the , where he worked closely with Feynman. Arthur liked to recall, half- Family and Childhood jokingly, that he lived all his life in the shadow of his Arthur Ashkin was born in , NY, on Septem- brother, the smart one of the family. At Cornell, Arthur ber 2, 1922, the son of humble eastern European met his wife, Aline, an undergraduate majoring in Jewish immigrants. Arthur’s father, Isador, was an or- chemistry, with whom he would happily spend the rest phan from , then part of Russia, while his of his life. mother, Anna, was from the province of Galicia, then After obtaining his Ph.D. in 1952, Arthur accepted part of the Austro-Hungarian Empire. a job offer from , the research arm of AT&T, Isador immigrated to the United States in 1910, where he joined the research department shortly before his nineteenth birthday. Upon his arrival of the Murray Hill Laboratory.* The first project he was at Ellis Island, his surname, Ashkenasy, was Ameri- assigned was on suppressing noise in a microwave canized to Ashkin. Having trained as a dental techni- amplifier, a goal later found to be impossible. “I al- cian in the orphanage where he was raised, Isador most got fired after a year of not making much prog- established his own dental laboratory on the Lower ress on this project,” Arthur recalled. After pulling East Side of Manhattan, specializing in dental pros- through this initial stressful period, Arthur became theses. Anna, who worked briefly as a secretary in Red freer to choose his own projects. For the next decade, Hook, Brooklyn, became a homemaker, raising Arthur he would pursue research on such topics as electron− and his siblings in Flatbush, Brooklyn. electron scattering and various aspects of the traveling Arthur had three siblings, a younger sister, Ruth, an wave tube. elder brother, Julius, and the first-born, Gertrude, who died at a young age. Ruth studied Greek and Latin Early Days of at The City College of and became an The second half of the 1950s was marked by a race to esteemed teacher in the New York City elementary build the first “optical maser,” a name commonly used school system. Julius became a . He was an for the laser at the time. The demonstration came in exceptionally gifted student who graduated early and 1960 when Theodore Maiman built a ruby laser (1). It became a close collaborator with many leading physi- took a few more years before Arthur started to ex- cists of the time, including , Edward periment with lasers, and he sometimes remarked, “I Teller, and . was late entering the laser field.” From the early 1960s to the mid-1970s, Arthur and his colleagues performed Studies and Joining Bell Labs a series of landmark laser experiments that would re- Arthur followed in his brother’s footsteps, enrolling as verberate for decades in optical research and paved the an undergraduate in physics at . way for the development of novel optical devices. World War II interrupted his studies. After being Several of the early laser experiments that Arthur drafted into the Signal Corps, he was then assigned to and his colleagues performed explored the optical the Columbia Radiation Laboratory as a technician, properties of ferroelectric crystals. He reported the building high-power magnetrons as part of the war first observation of continuous wave (cw) second- effort. He remembered those years as highly forma- harmonic generation (2) and, with his close colleague tive, helping him develop his experimental skills. After Gary Boyd, an early demonstration of cw parametric

aCrawford Hill Laboratory, Nokia Bell Labs, Holmdel, NJ 07733 Author contributions: R.-J.E. wrote the paper. Published under the PNAS license. 1Email: [email protected]. Published January 29, 2021. *The Murray Hill Laboratory is located at 600 Mountain Avenue in New Providence, NJ.

PNAS 2021 Vol. 118 No. 7 e2026827118 https://doi.org/10.1073/pnas.2026827118 | 1of4 Downloaded by guest on September 23, 2021 experimental investigations led to the observation of stimulated Raman (5, 6) and Brillouin scattering (7), four- wave mixing (8), and self-phase modulation (9). Arthur mentioned that these nonlinear optics experiments were inspired by his experiences early in his career working with high-power microwave amplifiers. “Iwas expecting to see new frequencies generated if we in- jected enough power in an ,” he said, when explaining his intuition on the observation of four-wave mixing in a few-mode fiber.

Discovery of Optical Trapping from Radiation Pressure Arthur Ashkin is considered by many to be the father of laser trapping of particles using radiation pressure. In 1970, at the age of 47, Arthur published the first observation that radiation pressure from lasers can “trap” transparent dielectric spheres (10). It was the dawn of laser optical trapping. In the same paper, Arthur discussed how optical trapping could also be applied to atoms and molecules. It is interesting to note that the manuscript was almost not submitted to Physical Review Letters. At the time, Bell Labs had a mandatory internal review to clear a manuscript be- fore it could be submitted for external publication. The review came back saying that the manuscript had no new physics, and, even though there was nothing Arthur Ashkin, in his backyard, looking through a wrong with it, it was not worthy of Physical Review magnifying glass. Image credit: Daniel Ashkin ’ (photographer). Letters. Arthur s boss at the time, Rudolf Kompfner, in a rare moment where he lost his cool, simply said, “Hell, just send it in!” The manuscript was easily ac- amplification (3). In 1966, Arthur discovered the phe- cepted for publication with congratulations and is now nomenon of optical damage due to photorefractive a milestone paper of Physical Review Letters (11). index modulation in LiNbO and other ferroelectrics (4). 3 Arthur published a second paper in Physical Re- It is around that time that the group moved from Murray † view Letters that same year, where he analyzed atomic Hill to the Holmdel Laboratory. beam deflection by a laser radiation pressure (12) In the second half of the 1960s, Arthur hired the based on the effect he had envisioned in his first pa- first three members of his group: John Bjorkholm, per. From the time of publication of these two seminal Roger Stolen, and Erich Ippen. He was viewed by papers in 1970 (10, 12) to the mid-1980s, Arthur “ ” them as not only the boss but also a teacher and a published a series of papers on optical trapping and mentor. When reminiscing with others about these days, Arthur sprightly declared, “Can you believe my luck to have hired these three guys.” Prior to 1972, optical fibers suffered from very high transmission losses, on the order of 1 decibel per meter, mainly due to impurities contaminating the glass. Interestingly, the glass material, in the form of optical fibers, microscopic spheres, or powerful len- ses, would be central to his scientific achievements throughout his career. Despite the drawback of high loss, early optical fibers enabled strong spatial con- finement of light that was maintained over distances well beyond what could be achieved in bulk materials. Therefore, optical fibers became a great “laboratory” for observing nonlinear effects. In a series of break- through experiments, Erich Ippen, Roger Stolen, and John Bjorkholm, with the help of Arthur, laid out the foundation of nonlinear optics in fibers. These From right to left: Arthur Ashkin, , and John Bjorkholm in 1986, around the time of the first † The Holmdel Laboratory was located at 101 Crawfords Corner demonstration of atom trapping. Reused with Road in Holmdel, NJ. permission of Nokia Corporation and AT&T Archives.

2of4 | PNAS Essiambre https://doi.org/10.1073/pnas.2026827118 Arthur Ashkin: Father of the optical tweezers Downloaded by guest on September 23, 2021 its applications (13). He pioneered optical levitation (14), performed an optical version of the Millikan ex- periment, and performed high-precision Mie scatter- ing measurements. His efforts toward optically trapping atoms also progressed. Together with John Bjorkholm, he demonstrated strong transverse con- finement and defocusing of atomic beams by fre- quency tuning an overlapping copropagating laser beam (15, 16). There were also important theoretical advances that Arthur contributed to during that period. His good friend James Gordon developed a quantum model to understand the stability of radiation traps. It was based largely on numerous discussions with Ar- thur and insights he provided (17). At the time of de- termining authorship, Arthur’s reaction was, “Jim, I should not be a co-author of this paper. I don’t un- derstand this stuff,” to which Jim answered, “You clearly explained the problem to me. That’s more than enough.” A few years later, Arthur and Jim published another theoretical paper, led by Arthur this time, Arthur Ashkin in his homemade laboratory in his basement in November 2018. where they demonstrated that the resonant radiation pressure, or scattering force, alone could not stably unusual circumstances: applying optical trapping and trap objects having a scalar polarizability tensor. They manipulation techniques to biology. He and Joseph named their finding the optical Earnshaw theorem Dziedzic, his long-time technical assistant turned friend, (18), in analogy to the Earnshaw theorem in electro- obtained tobacco mosaic virus (TMV) samples from Bell statics. To get around the optical Earnshaw theorem, Labs colleagues at the Murray Hill Laboratory. Arthur one could consider cases where polarizability de- andJosephwereableto“solidly” trap the rod-like–shaped pends on the state of polarization of light, like for atoms with a degenerate ground state. For such virus at either end. While these studies were per- atoms, a stable trap involving only the scattering force formed, the TMV sample solutions were kept open to “ ” becomes possible. This is what happens in a magneto- the air. One morning, bugs were discovered, trap- optical trap. But there was another very important ped by an optical tweezers inadvertently left on result in that remarkably fruitful paper by Arthur and overnight. Arthur and Joseph quickly identified the Jim (18). It concluded that trapping atoms with the bugs as motile bacteria that were trapped alive. This dipole force, the gradient force held dear by Arthur, serendipitous event sparked a series of experiments should be possible. Such atom trapping was demon- on trapping a variety of living organisms (22, 24, 25). strated experimentally in 1986 (19, 20). The first reports of these single-laser-beam trap- ping experiments created an immense interest for The Optical Tweezers employing optical tweezers as a new tool in the field In 1985, at the age of 63, Arthur observed that a single of biology (26). Optical trapping and optical manipu- tightly focused laser beam can trap a dielectric sphere lation of biological systems are now well-established in all three spatial dimensions simultaneously using fields having their own dedicated conferences. An radiation pressure. It was the realization of an optical example of biological optical tweezers technology trap he had himself envisioned in 1977 (15). He and his that Arthur was particularly fond of is the “handle colleagues reported the discovery in a landmark paper technique” that consists of attaching a dielectric (21), which became the most cited article of all time in sphere, or bead, to molecules that are too small to be Optics Letters. They demonstrated, experimentally, directly manipulated by optical tweezers. Using such a that a single laser beam can trap dielectric spheres technique, one can monitor, very precisely, the posi- from 10 micrometers down to 25 nanometers in di- tion of the bead and thereby the molecule attached to ameter, a remarkable range of eight orders of mag- it. It enables, for instance, measuring with accuracy the nitude in volume of the trapped spheres. A year later, process of transcription of a single DNA template by a he gave the “single-beam gradient force optical trap” single molecule of RNA polymerase (27). a more appealing name, “optical tweezers” (22). The discovery of the optical tweezers is at the core of his Retirement from Bell Labs 2018 Physics Nobel Prize, whose prize motivation is After his retirement from Bell Labs in 1992, Arthur “for the optical tweezers and their application to bi- remained very active. He built his own laboratory in his ological systems” (23). basement, starting from a few pieces of equipment donated by Bell Labs. He wrote many highly influential Biological Applications papers and a book on optical trapping. He also reg- In early 1987, Arthur decided to try an idea he laid out ularly visited Bell Labs, where each visit was an event. as far back as 1970 but thought may only work under He delivered informative and entertaining speeches at

Essiambre PNAS | 3of4 Arthur Ashkin: Father of the optical tweezers https://doi.org/10.1073/pnas.2026827118 Downloaded by guest on September 23, 2021 ‡ the annual picnic at the Crawford Hill Laboratory. Ar- ence (2003), the (2004), his being named thur was an articulate orator, delivering both technical an Honorary Member of (2009), the content and jokes with a great sense of timing. In the (2018), his induction into the last 15 years of his life, he became passionate about National Inventors Hall of Fame (2013), and the Edison renewable energy and worked toward creating an ef- Patent Award (2019). ficient and inexpensive way to capture solar power. He was a Fellow of IEEE, OSA, SPIE, APS, and American Association for the Advancement of Science. Awards and Recognitions Arthur received many awards and honors from his Epilogue peers. Among them are his election to the National Those who have had the chance to know and work Academy of Engineering (1984), the Institute of Elec- with Arthur experienced his contagious passion for trical and Electronics Engineers (IEEE) Photonics Soci- science. He reveled in doing “small science,” that is, ’ ety s Quantum Electronics Award (1987), the Charles working in small groups, where people get to know Hard Townes Award (1988), the Rank Prize in Opto- each other well and enjoy a great deal of freedom. Electronics (1993), his election to the National Acad- Arthur Ashkin was an exceptionally creative person ’ ’ emy of Sciences (1996), The Optical Society s(OSAs) who was greatly admired and liked by his colleagues. Frederick Ives Medal/Jarus W. Quinn Endowment ’ ’ (1998), the American Physical Society s(APSs) Joseph Acknowledgments F. Keithley Award for Advances in Measurement Sci- Many people contributed to this obituary. I would like to thank Arthur’s wife, Aline, his sons, Michael and Daniel, and his daughter, Judith Herscu. I also thank Roger Stolen, John Bjorkholm, Erich ‡ The Crawford Hill Laboratory is located at 791 Holmdel Road in Ippen, Stephen Harris, Alain Aspect, and Jean-Pierre Huignard for Holmdel, NJ. providing feedback and historical perspective.

1 T. H. Maiman, Stimulated optical radiation in ruby. Nature 187,493–494 (1960). 2 A. Ashkin, G. D. Boyd, J. M. Dziedzic, Observation of continuous optical harmonic generation with gas masers. Phys. Rev. Lett. 11, 14–17 (1963). 3 G. D. Boyd, A. Ashkin, Theory of parametric oscillator threshold with single-mode optical masers and observation of amplification in

LiNbO3. Phys. Rev. 146, 187–199 (1966). 4 A. Ashkin et al., Optically-induced refractive index inhomogeneities in LiNbO3 and LiTaO3. Appl. Phys. Lett. 9,72–74 (1966). 5 E. P. Ippen, Low-power quasi-CW Raman oscillator. Appl. Phys. Lett. 16, 303–305 (1970). 6 R. H. Stolen, E. P. Ippen, A. R. Tynes, Raman oscillation in glass optical waveguide. Appl. Phys. Lett. 20,62–64 (1972). 7 E. P. Ippen, R. H. Stolen, Stimulated Brillouin scattering in optical fibers. Appl. Phys. Lett. 21,539–541 (1972). 8 R. H. Stolen, J. E. Bjorkholm, A. Ashkin, Phase-matched three-wave mixing in silica fiber optical waveguides. Appl. Phys. Lett. 24, 308–310 (1974). 9 R. H. Stolen, C. Lin, Self-phase modulation in silica optical fiber. Phys. Rev. A 17, 1448–1453 (1978). 10 A. Ashkin, Acceleration and trapping of particles by radiation pressure. Phys. Rev. Lett. 24, 156–159 (1970). 11 Physical Review Letters, Letters from the past—A PRL retrospective. https://journals.aps.org/prl/50years/milestones. Accessed 4 December 2020. 12 A. Ashkin, Atomic-beam deflection by resonance-radiation pressure. Phys. Rev. Lett. 25, 1321–1324 (1970). 13 A. Ashkin, Applications of laser radiation pressure. Science 210, 1081–1088 (1980). 14 A. Ashkin, J. M. Dziedzic, Acceleration levitation by radiation pressure. Appl. Phys. Lett. 19, 283–285 (1971). 15 A. Ashkin, Trapping of atoms by resonance radiation pressure. Phys. Rev. Lett. 40, 729–732 (1978). 16 J. E. Bjorkholm, R. R. Freeman, A. Ashkin, D. B. Pearson, Observation of focusing of neutral atoms by the dipole forces of resonance- radiation pressure. Phys. Rev. Lett. 41, 1361–1364 (1978). 17 J. P. Gordon, A. Ashkin, Motion of atoms in a radiation trap. Phys. Rev. A 21, 1606–1617 (1980). 18 A. Ashkin, J. P. Gordon, Stability of radiation-pressure particle traps: An optical Earnshaw theorem. Opt. Lett. 8,511 –513 (1983). 19 S. Chu, J. E. Bjorkholm, A. Ashkin, A. Cable, Experimental observation of optically trapped atoms. Phys. Rev. Lett. 57, 314–317 (1986). 20 A. Ashkin, Optical trapping and manipulation of neutral particles using lasers. Proc. Natl. Acad. Sci. U.S.A. 94, 4853–4860 (1997). 21 A. Ashkin, J. M. Dziedzic, J. E. Bjorkholm, S. Chu, Observation of a single-beam gradient force optical trap for dielectric particles. Opt. Lett. 11, 288–290 (1986). 22 A. Ashkin, J. M. Dziedzic, Optical trapping and manipulation of viruses and bacteria. Science 235, 1517–1520 (1987). 23 Nobel Foundation, The Nobel Prize. https://www.nobelprize.org/prizes/physics/2018/ashkin/facts/. Accessed 6 December 2020. 24 A. Ashkin, J. M. Dziedzic, Internal cell manipulation using infrared laser traps. Proc. Natl. Acad. Sci. U.S.A. 86, 7914–7918 (1989). 25 A. Ashkin, K. Schütze, J. M. Dziedzic, U. Euteneuer, M. Schliwa, Force generation of organelle transport measured in vivo by an infrared laser trap. Nature 348, 346–348 (1990). 26 A. Ashkin, Optical Trapping and Manipulation of Neutral Particles Using Lasers: A Reprint Volume with Commentaries (World Scientific, 2006). 27 K. C. Neuman, S. M. Block, Optical trapping. Rev. Sci. Instrum. 75, 2787–2809 (2004).

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