Arthur Ashkin (1922–2020)

Obituary Arthur Ashkin (1922–2020)

Physicist who won Nobel for optical tweezers that trap atoms and proteins.

rthur Ashkin was the father of optical by using an ‘optical molasses’ created by three trapping. Using focused laser beams, counter-propagating sets of laser beams. The he manipulated particles ranging in basic idea had been proposed a decade earlier size from atoms to cells and their com- by Theodor Hänsch and Arthur Schawlow. ponents. In 2018, aged 96, he shared We showed that optical molasses slowed Athe Nobel Prize in Physics for “optical tweezers atoms enough for optical forces to trap them. and their application to biological systems”. Art, Bjorkholm and I tried, but failed for sev- He also made pioneering discoveries about the eral months, to demonstrate a large-volume forces that light exerts. Ashkin — ‘Art’ to most of trap proposed by Art. We rejected the single us— died on 21 September, aged 98. focused beam because the tiny trapping vol- Today, optical tweezers are indispensable ume would contain, at most, a few atoms in a to biology. They can measure movement cloud of millions. But we eventually realized with a precision equal to the diameter of an that thousands of atoms near the trap would atom, or the force generated by using the rapidly diffuse into the focal region and remain chemical energy of a single molecule of the cold and trapped. energy-carrying molecule ATP. Holding atoms cooled to just above absolute Ashkin was born in Brooklyn, New York, to zero in a single beam was analogous to trap- immigrant parents from what is now part of ping micrometre-sized polystyrene spheres in Ukraine. He and his older brother Julius taught water at room temperature. Once he realized themselves physics and calculus, and cobbled this, Art quickly showed that 0.025–10-μm

NOKIA BELL LABS together financial contributions from their diameter polystyrene particles in water could extended family to go to Columbia University in onto latex spheres micrometres in diameter be trapped in the one beam. A few months New York. When the United States entered the immersed in water. The water cooled the later, we trapped atoms. Ironically, all the Second World War in his second year, Ashkin’s spheres, eliminating the thermal radiometric ingredients for the optical-tweezers trap were mentor Sidney Millman convinced the army force, but the particles were still drawn into established by Art in 1970. to let him stay at Columbia to work on radar the high-intensity beam. Ashkin showed that In 1987, Art came into my lab with eyes spar- technology. the particle acts as a tiny lens that alters the kling and declared, “Steve, you’re not going For his PhD at Cornell University in Ithaca, momentum of the light. This rate of change to believe this, but I discovered LIFE!” He had New York, Ashkin measured the angular of momentum creates an equal but opposite found that bacteria growing in his experimen- dependence of electron–electron and elec- force on the particle that attracts it to higher tal set-up could be optically manoeuvred. tron–positron scattering accurately enough light intensity. When the light was turned off the bacteria to verify the full quantum electrodynamic swam away, but by steering the focal spot of (relativistic) theory. Then, invited by Millman, “His infectious passion for the laser they could be recaptured. By 1990, he went to Bell Laboratories in Holmdel, New several researchers were using optical twee- Jersey, where he stayed until his retirement in exploring optical forces zers to trap particles, atoms, single cells, 1992. At first, he worked on microwave phys- changed the course of many organelles, and individual biomolecules by ics. After Theodore Maiman demonstrated scientific careers.” attaching them to polystyrene spheres. the laser in 1960, Ashkin worked in this new The elegance and creativity of Ashkin’s ideas field and, with Gary Boyd, made important were extraordinary. He remained indefatiga- advances in non-linear optical devices. Ashkin also investigated the optical forces bly enthusiastic about science, working in his Since childhood, Ashkin had been fascinated on atoms. He demonstrated the atomic ver- home basement into his nineties. He happily by the forces due to light. In the mid-1960s, he sion of lensing — the ‘dipole force’ — with John taught many biophysicists how to duplicate the heard Eric Rawson at the University of Toronto Bjorkholm and Richard Freeman. In 1978, he optical-tweezers trap. His infectious passion for in Canada describe how tiny dust particles were proposed that a combination of the scattering exploring optical forces changed the course of trapped in a light beam inside a helium–neon and dipole forces could trap atoms. He high- many scientific careers, mine included. laser cavity. Later, Rawson showed the effect lighted an atom trap made of two opposing is due to the radiometric force. This is the laser beams, but also proposed “the concep- Steven Chu is the William R. Keenan Jr force that causes the black and white vanes in tually simplest trap” of one focused laser beam. professor of physics and professor of a Crookes radiometer, encased in a partially By 1980, the experimental work on atom molecular and cellular physiology at Stanford evacuated glass bulb, to rotate, as the result of trapping had stopped, but Art remained University, California. He was a fellow the thermal interactions with the residual gas steadfast. He piqued my interest after I department head with Ashkin at Bell Labs from molecules, but in the opposite direction to the moved to Bell Labs at Holmdel in 1983. In 1985, 1983 to 1987. In 1997, he received a share of scattering force due to photons recoiling from my research team, with Art and Bjorkholm, the Nobel Prize in Physics for laser cooling and the vanes. showed that light could cool atoms to less than atom trapping. Intrigued, Ashkin focused a laser beam a thousandth of a degree above absolute zero e-mail: [email protected]