Arthur Ashkin: Father of the optical tweezers

The father of the optical tweezers, Arthur Ashkin, passed away peacefully at his home in Rumson, NJ, on September 21, 2020, at the age of 98, two years after being awarded the 2018 Physics Nobel Prize.

Arthur Ashkin, in his backyard, looking through a magnifying glass. Image credit: Daniel Ashkin (photographer).

From right to left: Arthur Ashkin, Steven Chu, and John Bjorkholm in 1986, around the time of the first demonstration of atom trapping. Reused with permission of Nokia Corporation and AT&T Archives.

Arthur Ashkin in his homemade laboratory in his basement in November 2018.

Family and Childhood

Arthur Ashkin was born in Brooklyn, NY, on September 2, 1922, the son of humble eastern European Jewish immigrants. Arthur’s father, Isador, was an orphan from Odessa, then part of Russia, while his mother, Anna, was from the province of Galicia, then part of the Austro-Hungarian Empire.

Isador immigrated to the United States in 1910, shortly before his nineteenth birthday. Upon his arrival at Ellis Island, his surname, Ashkenasy, was Americanized to Ashkin. Having trained as a dental technician in the orphanage where he was raised, Isador established his own dental laboratory on the Lower East Side of Manhattan, specializing in dental prostheses. Anna, who worked briefly as a secretary in Red Hook, Brooklyn, became a homemaker, raising Arthur and his siblings in Flatbush, Brooklyn.

Arthur had three siblings, a younger sister, Ruth, an elder brother, Julius, and the first-born, Gertrude, who died at a young age. Ruth studied Greek and Latin at The City College of New York and became an esteemed teacher in the New York City elementary school system. Julius became a physicist. He was an exceptionally gifted student who graduated early and became a close collaborator with many leading physicists of the time, including Richard Feynman, Edward Teller, and Hans Bethe.

Studies and Joining Bell Labs

Arthur followed in his brother’s footsteps, enrolling as an undergraduate in physics at Columbia University. World War II interrupted his studies. After being drafted into the Signal Corps, he was then assigned to the Columbia Radiation Laboratory as a technician, building high-power magnetrons as part of the war effort. He remembered those years as highly formative, helping him develop his experimental skills. After graduating from Columbia in 1947, he enrolled at Cornell University as a Ph.D. student in nuclear physics, like his brother. By that time, Julius had already participated in the Manhattan Project, where he worked closely with Feynman. Arthur liked to recall, half-jokingly, that he lived all his life in the shadow of his brother, the smart one of the family. At Cornell, Arthur met his wife, Aline, an undergraduate majoring in chemistry, with whom he would happily spend the rest of his life.

After obtaining his Ph.D. in 1952, Arthur accepted a job offer from Bell Labs, the research arm of AT&T, where he joined the microwave research department of the Murray Hill Laboratory.* The first project he was assigned was on suppressing noise in a microwave amplifier, a goal later found to be impossible. “I almost got fired after a year of not making much progress on this project,” Arthur recalled. After pulling through this initial stressful period, Arthur became freer to choose his own projects. For the next decade, he would pursue research on such topics as electron−electron scattering and various aspects of the traveling wave tube.

Early Days of Nonlinear Optics

The second half of the 1950s was marked by a race to build the first “optical maser,” a name commonly used for the laser at the time. The demonstration came in 1960 when Theodore Maiman built a ruby laser (1). It took a few more years before Arthur started to experiment with lasers, and he sometimes remarked, “I was late entering the laser field.” From the early 1960s to the mid-1970s, Arthur and his colleagues performed a series of landmark laser experiments that would reverberate for decades in optical research and paved the way for the development of novel optical devices.

Several of the early laser experiments that Arthur and his colleagues performed explored the optical properties of ferroelectric crystals. He reported the first observation of continuous wave (cw) second-harmonic generation (2) and, with his close colleague Gary Boyd, an early demonstration of cw parametric amplification (3). In 1966, Arthur discovered the phenomenon of optical damage due to photorefractive index modulation in LiNbO₃ and other ferroelectrics (4). It is around that time that the group moved from Murray Hill to the Holmdel Laboratory.^†

In the second half of the 1960s, Arthur hired the first three members of his group: John Bjorkholm, Roger Stolen, and Erich Ippen. He was viewed by them as not only the “boss” but also a teacher and a 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 lenses, would be central to his scientific achievements throughout his career. Despite the drawback of high loss, early optical fibers enabled strong spatial confinement 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 breakthrough experiments, Erich Ippen, Roger Stolen, and John Bjorkholm, with the help of Arthur, laid out the foundation of nonlinear optics in fibers. These 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. “I was expecting to see new frequencies generated if we injected enough power in an optical fiber,” 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 before 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 wrong with it, it was not worthy of Physical Review 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 accepted for publication with congratulations and is now a milestone paper of Physical Review Letters (11).

Arthur published a second paper in Physical Review Letters that same year, where he analyzed atomic beam deflection by a laser radiation pressure (12) based on the effect he had envisioned in his first paper. From the time of publication of these two seminal papers in 1970 (10, 12) to the mid-1980s, Arthur published a series of papers on optical trapping and its applications (13). He pioneered optical levitation (14), performed an optical version of the Millikan experiment, and performed high-precision Mie scattering measurements. His efforts toward optically trapping atoms also progressed. Together with John Bjorkholm, he demonstrated strong transverse confinement and defocusing of atomic beams by frequency 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 Arthur and insights he provided (17). At the time of determining authorship, Arthur’s reaction was, “Jim, I should not be a co-author of this paper. I don’t understand 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, where they demonstrated that the resonant radiation pressure, or scattering force, alone could not stably trap objects having a scalar polarizability tensor. They named their finding the optical Earnshaw theorem (18), in analogy to the Earnshaw theorem in electrostatics. To get around the optical Earnshaw theorem, one could consider cases where polarizability depends on the state of polarization of light, like for atoms with a degenerate ground state. For such atoms, a stable trap involving only the scattering force becomes possible. This is what happens in a magneto-optical trap. But there was another very important result in that remarkably fruitful paper by Arthur and Jim (18). It concluded that trapping atoms with the dipole force, the gradient force held dear by Arthur, should be possible. Such atom trapping was demonstrated experimentally in 1986 (19, 20).

The Optical Tweezers

In 1985, at the age of 63, Arthur observed that a single tightly focused laser beam can trap a dielectric sphere in all three spatial dimensions simultaneously using radiation pressure. It was the realization of an optical trap he had himself envisioned in 1977 (15). He and his colleagues reported the discovery in a landmark paper (21), which became the most cited article of all time in Optics Letters. They demonstrated, experimentally, that a single laser beam can trap dielectric spheres from 10 micrometers down to 25 nanometers in diameter, a remarkable range of eight orders of magnitude in volume of the trapped spheres. A year later, he gave the “single-beam gradient force optical trap” a more appealing name, “optical tweezers” (22). The discovery of the optical tweezers is at the core of his 2018 Physics Nobel Prize, whose prize motivation is “for the optical tweezers and their application to biological systems” (23).

Biological Applications

In early 1987, Arthur decided to try an idea he laid out as far back as 1970 but thought may only work under unusual circumstances: applying optical trapping and manipulation techniques to biology. He and Joseph Dziedzic, his long-time technical assistant turned friend, obtained tobacco mosaic virus (TMV) samples from Bell Labs colleagues at the Murray Hill Laboratory. Arthur and Joseph were able to “solidly” trap the rod-like–shaped virus at either end. While these studies were performed, the TMV sample solutions were kept open to the air. One morning, “bugs” were discovered, trapped by an optical tweezers inadvertently left on overnight. Arthur and Joseph quickly identified the bugs as motile bacteria that were trapped alive. This serendipitous event sparked a series of experiments on trapping a variety of living organisms (22, 24, 25).

The first reports of these single-laser-beam trapping experiments created an immense interest for employing optical tweezers as a new tool in the field of biology (26). Optical trapping and optical manipulation of biological systems are now well-established fields having their own dedicated conferences. An example of biological optical tweezers technology that Arthur was particularly fond of is the “handle technique” that consists of attaching a dielectric sphere, or bead, to molecules that are too small to be directly manipulated by optical tweezers. Using such a technique, one can monitor, very precisely, the position of the bead and thereby the molecule attached to it. It enables, for instance, measuring with accuracy the process of transcription of a single DNA template by a single molecule of RNA polymerase (27).

Retirement from Bell Labs

After his retirement from Bell Labs in 1992, Arthur remained very active. He built his own laboratory in his basement, starting from a few pieces of equipment donated by Bell Labs. He wrote many highly influential papers and a book on optical trapping. He also regularly visited Bell Labs, where each visit was an event. He delivered informative and entertaining speeches at the annual picnic at the Crawford Hill Laboratory.^‡ Arthur was an articulate orator, delivering both technical content and jokes with a great sense of timing. In the last 15 years of his life, he became passionate about renewable energy and worked toward creating an efficient and inexpensive way to capture solar power.

Awards and Recognitions

Arthur received many awards and honors from his peers. Among them are his election to the National Academy of Engineering (1984), the Institute of Electrical and Electronics Engineers (IEEE) Photonics Society’s Quantum Electronics Award (1987), the Charles Hard Townes Award (1988), the Rank Prize in Opto-Electronics (1993), his election to the National Academy of Sciences (1996), The Optical Society’s (OSA’s) Frederick Ives Medal/Jarus W. Quinn Endowment (1998), the American Physical Society’s (APS’s) Joseph F. Keithley Award for Advances in Measurement Science (2003), the Harvey Prize (2004), his being named an Honorary Member of the Optical Society (2009), the Nobel Prize in Physics (2018), his induction into the National Inventors Hall of Fame (2013), and the Edison Patent Award (2019).

He was a Fellow of IEEE, OSA, SPIE, APS, and American Association for the Advancement of Science.

Epilogue

Those who have had the chance to know and work with Arthur experienced his contagious passion for science. He reveled in doing “small science,” that is, working in small groups, where people get to know each other well and enjoy a great deal of freedom.

Arthur Ashkin was an exceptionally creative person who was greatly admired and liked by his colleagues.