An orphan’s son in Brooklyn
Arthur Ashkin was born in Brooklyn, New York, on September 2, 1922, into a family of Ukrainian Jewish background. His father, Isadore, born in 1891 in the Russian Empire, had grown up in an orphanage with his older brother, Harry, never told how they came to be there. Harry escaped conscription into the Tsar’s army by leaving for America on Isadore’s passport, intending to mail it back; it never arrived. Stateless, Isadore paid his savings to be smuggled on a sealed train to Holland and then travelled steerage to New York, where an immigration agent shortened his name to Ashkin. Trained in the orphanage as a dental technician, he soon ran his own dental laboratory on Delancey Street in Manhattan.
Isadore married Anna Fishman, who had arrived from what is now Poland as a two-year-old. They raised three children in Brooklyn: Julius, a brilliant student who earned a physics PhD from Columbia at 22 and later worked on the atomic bomb at Los Alamos; Arthur; and Ruth, who studied Greek and Latin and devoted her career to teaching underprivileged children in a Brooklyn school. An older sister, Gertrude, had died young.
A radiometer in a pharmacy window
Ashkin was, in his own words, a product of Brooklyn’s public schools during the Great Depression. At the age of ten he became fascinated by a Crookes radiometer, the little glass bulb with vanes that spin in sunlight, which he saw in a pharmacist’s window. He bought one and worked out that the vanes were driven by heated air molecules, not by the pressure of light itself. A few years later he noticed that, after warming it outside and bringing it in, it spun the other way, as if pushed by light. The question of whether light could really push matter stayed with him for life.
He graduated from James Madison High School in 1940 and entered Columbia College to study physics. During the Second World War he served as a technician at Columbia’s Radiation Laboratory, building magnetrons for military radar, where he was mentored by Sidney Millman. After the war he finished his degree in 1947, then went to Cornell on the GI Bill, earning a PhD in nuclear physics in 1952 among physicists such as Hans Bethe and Richard Feynman.
Forty years at Bell Labs
Millman remembered him and hired him at Bell Telephone Laboratories in New Jersey, then one of the most productive research centres in the world. Ashkin first worked on microwave electron tubes for communications. He avoided nuclear physics partly because he did not want to compete with his brother, and resented being called “Ashkin’s brother Ashkin.” When the laser was invented in 1960 he switched to laser research. He made important contributions to nonlinear optics, to the behaviour of light in optical fibres and to parametric amplifiers, and he co-discovered the photorefractive effect, in which light changes a crystal’s refractive index, later used in optical data storage and signal processing.
Over a career of forty years at Bell Labs he published many papers and held 47 patents. He retired in 1992 but kept working in a laboratory at his home in Rumson, New Jersey.
Pushing with light
From the early 1960s Ashkin wanted to know whether the tiny momentum of photons could be used to move objects. Shining a laser at microscopic transparent beads in water, he found that they were pushed along the beam and, surprisingly, also pulled toward its centre, where the light was most intense. The reason is that light bends as it passes through a bead, and the changing intensity across the beam creates a force that draws the bead to the brightest region. In 1970 he published the first paper on optical trapping, using two opposing beams; Bell Labs’ theorists had objected to it, but his boss Rudolf Kompfner told him to send it in, and by his account it became the most referenced paper of that year. It also suggested that atoms and molecules might one day be trapped in the same way.
In 1971 he levitated glass spheres with laser light. Then, working with his colleague Joseph Dziedzic and others, he developed the single-beam gradient trap: one laser beam sharply focused through a microscope lens, which holds a particle in three dimensions at the focus. The paper describing it appeared in 1986. The device became known as optical tweezers.
His Nobel autobiography records that the 1970 paper went on to gather more than 5,000 citations and was later selected for a collection of the most important atomic physics papers of the twentieth century in the Physical Review. The levitated spheres of 1971 were themselves put to use, for example in an optical version of Millikan’s classic oil-drop experiment, and to show that particles close to the size of atoms could be held, Ashkin and Dziedzic trapped plastic spheres only a few billionths of a metre across.
From atoms to living cells
Optical tweezers helped make it possible to trap atoms. Ashkin’s ideas and techniques fed directly into the work of his Bell Labs colleague Steven Chu on cooling and trapping atoms with laser light, which earned Chu a share of the 1997 Nobel Prize. Ashkin’s Nobel autobiography notes that many felt he should have been included in that award.
Trapping atoms was harder than trapping beads. Atoms are tiny and, at room temperature, move at hundreds of metres per second, so the light forces had to be stronger and the atoms had to be slowed first. According to the Nobel committee, everything fell into place in 1986, when optical tweezers were combined with laser-cooling methods for stopping atoms, allowing single atoms to be held in a beam of light.
Ashkin himself turned to biology, by accident. Trying to trap ever smaller particles, he used samples of tobacco mosaic virus and left one open overnight. The next day it was full of bacteria, which his laser caught as they swam past. His green laser killed them, so he switched to infrared light, in which the bacteria survived the trap and even reproduced. In 1987 he reported trapping living viruses and bacteria without damaging them, and he went on to show that the tweezers could reach inside cells and move their internal structures without breaking the cell membrane.
A tool used in laboratories everywhere
Optical tweezers gave biologists something they had never had: a way to hold and pull on a single molecule or cell without touching it, and to measure forces of a few trillionths of a newton. The first molecular motor mapped in detail with them was kinesin, the protein that walks along the cell’s internal scaffolding carrying cargo. Since then optical tweezers have been used to watch DNA being copied and unzipped, to measure how proteins fold, to study how viruses package their genes and how muscles contract, and to sort and examine individual sperm, blood and cancer cells.
Today laser tweezers are standard equipment in biophysics laboratories around the world, and commercial instruments are sold for research. Holographic versions can create thousands of traps at once, and the Nobel committee noted that they could be used to separate healthy blood cells from cells infected with malaria. The same principle underlies new methods of manipulating nanoparticles and testing quantum physics with levitated objects.
The oldest laureate
On October 2, 2018, Ashkin was awarded half of the Nobel Prize in Physics “for the optical tweezers and their application to biological systems,” sharing it with Gérard Mourou and Donna Strickland, who were honoured for a method of producing ultra-short, high-intensity laser pulses. At 96 he was the oldest person ever to receive a Nobel Prize, a record that stood until the following year. He did not travel to Stockholm. Earlier honours included the Charles Hard Townes Award, the Frederic Ives Medal, the Harvey Prize in 2004 and election to both the National Academy of Engineering and the National Academy of Sciences; in 2013 he was inducted into the National Inventors Hall of Fame.
Family and Jewish roots
Ashkin’s story is a classic Jewish immigrant story of the early twentieth century: an orphaned father escaping the Russian Empire, a mother brought from Eastern Europe as a toddler, a small business on the Lower East Side, and children who rose through New York’s free public schools and colleges into the professions. His parents’ three children became a physicist, a laser pioneer and a teacher of poor children.
He met his wife, Aline, at Cornell, when she came across him splitting shale with a prospector’s pick on the shore of Lake Cayuga, looking for fossils. They married in 1954 and lived for more than fifty years in Rumson, New Jersey. Aline taught high school chemistry for fifteen years. Their children are Michael, a professor of art at Cornell, who studied Hebrew and Arabic at university; Judith; and Daniel, a ceramics scientist. They had five grandchildren and two great-grandchildren.
Death and legacy
Arthur Ashkin died at his home in Rumson on September 21, 2020, at the age of 98.
He took a childhood question about whether light can push things and turned it into one of the most versatile instruments in modern science. Because of his optical tweezers, researchers can hold a single cell in a beam of light, stretch a strand of DNA, or watch a protein motor take one step at a time. Much of what we now know about the molecular machinery of life, knowledge that underpins research into diseases from cancer to infection, was measured with his laser fingers.
Sources & further reading
Arthur Ashkin, Biographical, NobelPrize.org · The Nobel Prize in Physics 2018, Popular information, NobelPrize.org · Nobel-winning physicist Arthur Ashkin, Ph.D. ’52, dies at 98, Cornell Chronicle · Arthur Ashkin, Wikipedia · Researched from the sources above and checked against them. Spotted an error? Tell us.
A life of tikkun olam. This is part of tikkun olam, the Jewish idea of repairing the world, and every one of us can add to it.