From the 10 year-old boy playing with a Crookes radiometer in the sunlight and pondering how light turns its vanes, to the nonagenarian busily building light-bending contraptions that harness the Sun’s rays in his home basement laboratory, Arthur ‘Art’ Ashkin, who peacefully passed away a year ago on 21 September 2021 at the ripe old age of 98, had a passion for invention and discovery like no other.
Ashkin´s insights and innovations had expanded and enriched diverse scientific fields – and will continue to do so long into the future. “His impact on science, on biology, on physics is enormous,” said Steven Chu (Nobel Prize in Physics 1997) at the Optical Society of America (OSA) 2021 symposium in Ashkin’s honour.
Most famously, his invention of optical trapping, more specifically optical tweezers, led to Ashkin’s 2018 Nobel Prize in Physics. It was in 1970 at Bell Labs, New Jersey, that he published his first and seminal paper on optical trapping using radiation pressure. The experimental setup used two laser beams pointed at micrometre-sized polystyrene spheres floating on water. These spheres were trapped due to forces arising from the momentum of the light itself, known as radiation pressure. It worked because radiation pressure that the laser light exerted on the object varied between the centre and sides, pressing the object towards the heart of the beam.
In 1971, Ashkin applied the same principle to small glass spheres in air, making them levitate. And then, over a decade later in 1986, Ashkin, Chu and coworkers managed to stop and trap atoms in a single laser beam. Optical tweezers were born.
Providing the ability to hold and move microscopic (and smaller) objects, it soon became clear optical tweezers were a game-changing innovation for the science of the small. Perhaps the most fruitful application of optical tweezers was discovered by Ashkin less than a year after their invention.
One day, he rushed into Chu’s lab with his eyes sparkling to declare: “I’ve discovered life!” Ashkin had used optical tweezers to trap bacteria in a laser beam. The breakthrough was revolutionary for the biological sciences. Now, scientists routinely trap and apply forces on living cells, viruses and biological molecules remotely without inflicting damage.
The ‘father of optical tweezers’ is perhaps less well-known as the ‘father of the photorefractive effect’ as well. But the latter discovery could prove just as important in the future. In the mid-1960s, Ashkin was experimenting with a lithium niobate crystal, aiming to switch one colour of intense laser light to another. However, when he directed a laser beam through the crystal, after a few minutes it began to distort and scatter the beam around the laboratory.
“He discovered this optical damage effect and it was a nuisance,” says friend and collaborator Gary Bjorklund. “But it was his credo that ‘when life gives you lemons, make lemonade’, so he wrote a paper about it.” In doing so, Ashkin exposed that the nuisance effect was caused by the laser changing the optical properties of the crystal – he had discovered the photorefractive effect. Photorefractive materials can capture sharp detail in intricate patterns of light and essentially store the image as a hologram from milliseconds to days, depending on the material. The image can then be wiped instantly by bathing the material in a uniform beam of light; perfect for data storage applications. Efforts are ongoing to bring holographic data storage using photorefractive crystals to practical use.
Collecting the Sun’s Rays
Upon his retirement from Bell Labs, Ashkin’s enthusiasm for science and invention remained undimmed. “Even in his late 80s, he was still just the same person, in terms of his enthusiasm about what he was doing, and his energy,” says Bjorklund.
At the 2021 OSA memorial, his wife Aline recalled how, even in his latter years, he could never switch off the scientific curiosity that had fuelled him throughout his life: “I often found him seated in his armchair or in his office with his head down and his eyes closed,” she said. “After hearing my suggestion that he lie down and take a nap, invariably he would raise his head, open his eyes and declare that he was thinking.”
Apart from spending some of this thinking time in his autumn years on a 941-page tome about optical trapping, most of it was taken up by a project in his basement – devising and improving a new device to capture and funnel light more efficiently, and thereby improve solar power.
“One of the hallmarks of Art’s entire career was the utter simplicity and elegance of his approach to difficult problems,” explains New York University’s David Grier, who spoke with Ashkin about his solar project only a few years ago. “And that was true of his solar collector too.”
Grier explains that there are many solar collectors already in existence, but they either have round apertures, making them inefficient, or they contain intricately curved surfaces, making them expensive to build. Ashkin’s solution was to design an arrangement of planar rectangular mirrors in a cone-like structure to collect as much light as possible. “His thing was really optimised for cost and manufacturability,” says Grier. Currently, some of Ashkin’s former colleagues are completing a paper he had started on the solar collector in the hopes of getting it published.
Beyond his scientific legacy, Ashkin has left an indelible imprint on those he inspired with his passion for science along the way. When Bjorklund joined Bell Labs in 1974, for instance, Ashkin left an immediate impression on the young researcher. “One of the things that really struck me was just his genuine enthusiasm for the research he was doing,” Bjorklund says. “He was also very, very clever about doing experiments.”
Nokia Bell Labs optical researcher René-Jean Essiambre – who befriended Ashkin in his latter years and gave his Nobel Lecture due to Ashkin’s poor health at the time – also gained a lot from spending time with the doyen. “He would suggest things to me that looked a little bit far-fetched at first. But after he explained things, I would realise his ideas were remarkably innovative,” Essiambre said in an interview for the Bell Labs blog. “His imagination was supported by tremendous intuition and sound science. I learned that I should let my imagination go, and not restrain myself.”
But perhaps Erich Ippen – MIT expert in nonlinear optics who worked under Ashkin at Bell Labs – put it best in his OSA memorial talk. Recalling Ashkin’s favourite phrase ‘when life gives you lemons, make lemonade’, Ippen paid a simple tribute to a man who enriched many lives: “He made lemonade out of us.”
To learn more about Arthur Ashkin’s fascinating life from the man himself, watch his 2018 Lindau Nobel Laureate portrait.