2018 Nobel Prize in Physics

Ashkin, Mourou, and Strickland share 2018 Nobel Prize in Physics
Left to right: Arthur Ashkin, Gérard Mourou, and Donna Strickland. Credits: Bell Labs, Alexis Cheziere/CNRS Photothèque, and University of Waterloo
Arthur Ashkin, Gérard Mourou, and Donna Strickland are to be awarded the 2018 Nobel Prize in Physics “for groundbreaking inventions in the field of laser physics,” the Royal Swedish Academy of Sciences announced on Tuesday. Ashkin, formerly of Bell Labs in New Jersey, will receive half the prize of 9 million Swedish krona (roughly $1 million); Mourou, of École Polytechnique in France and the University of Michigan, and Strickland, at the University of Waterloo in Canada, will split the other half.

The Royal Swedish Academy is honoring Ashkin for his invention of optical tweezers to trap and manipulate particles and living cells. In the 1970s and 1980s, he discovered that the radiation pressure in laser beams could be used not only to push small objects but also to confine and manipulate them. Although the initial targets of manipulation were latex beads, Ashkin soon expanded the technique to atoms, viruses, DNA, and other biological specimens.

Mourou and Strickland together developed chirped pulse amplification (CPA), in which a laser pulse is stretched, amplified, and then compressed to increase its power. The ultrafast, high-intensity tabletop lasers that ensued have spurred advances in data storage, materials manufacturing, and the study of femtosecond- and even attosecond-duration phenomena. Citing its mission to recognize inventions that benefit humankind, the academy also highlighted how Mourou and Strickland’s work made possible production of surgical stents and the use of lasers to correct vision.

When she receives her medal in December, Strickland will become the third woman to receive the physics prize out of 209 laureates, and the first since Maria Goeppert Mayer in 1963.

Trapped by light

At 96 years old, Ashkin is the oldest person to receive a Nobel Prize. When he was half that age, in 1970, he was at Bell Labs studying the use of light’s radiation pressure to propel objects. The main challenge in observing the optical force was to avoid heating the sample with the light, which causes a thermal gradient force that is usually orders of magnitude larger than the force due to radiation pressure. Ashkin tackled that problem by shining laser light on a transparent, nonabsorbing system: micron-sized latex spheres immersed in water.

Ashkin’s seminal 1970 paper in Physical Review Letters reports the beads’ acceleration in the direction of the laser beam, as one would expect from the collective nudge of a bundle of photons. But it also describes a second, less intuitive force that is directed toward the axis of the pulse. The force emerges from the refraction of the light as it passes through the curved interface between the low refractive index of the water and the higher-index bead, and it’s strongest at the core of the beam where the light intensity is highest (see Physics Today, November 2010, page 13).

Ashkin continued working on such optical manipulation, and in 1986 he set up an experiment with Bell Labs colleague Steven Chu using lenses to focus light on the beads. Ashkin and Chu expected the beads to move toward the high-intensity center of the beam and jet forward. Instead, the spheres stopped in their tracks. The momentum transfer from the scattered light leaving the sphere had imparted a backward force to counteract the beam’s forward push. “That’s what Nobel Prizes are made of,” says New York University optical physicist David Grier. “An obvious truth hiding in plain sight.”

Building on Ashkin’s work, Chu and others directed their research toward trapping and cooling atoms with lasers. Along with William Phillips and Claude Cohen-Tannoudji, Chu received the 1997 Nobel Prize in Physics for that work (see Physics Today, December 1997, page 17); some people in the field, Ashkin included, felt he should have been recognized by the Nobel committee.

In this optical trapping and imaging apparatus, a laser travels up through an objective, reaching the sample from below. The sample is imaged using light from above, which travels down through the objective and is then relayed to a camera. Credit: David Grier
Unlike some of his colleagues, Ashkin was attracted to the biological potential of his optical tweezers. In the late 1980s, he demonstrated the optical trapping of viruses and living cells, such as bacteria, using a lower-energy IR laser to avoid searing the specimens. Since then, researchers have used optical tweezers to stretch strands of DNA, prod red blood cells, and tie molecular knots. In a widely cited 1993 paper in Nature, researchers optically trapped the protein kinesin, which transports molecular cargo inside eukaryotic cells, and measured the force it applied to a bead to which it was affixed.

And the research continues. Grier and his colleagues recently introduced holographic optical trapping, in which a computer shapes a single laser beam into tens or hundreds of optical traps, each capable of manipulating objects in 3D. They’ve also developed a laser setup—a rudimentary tractor beam—that pulls objects in the opposite direction of beam propagation.

Ultrafast, ultrapowerful pulses

Whereas Ashkin exploited the properties of lasers to manipulate objects, Mourou and Strickland worked on manipulating laser pulses. Within five years after the development of the laser in 1960, researchers had found multiple ways to shorten the duration of pulses, and thus intensify the power, by six orders of magnitude. But by concentrating solely on pulse length, the high intensity (power per unit area) of the generated pulses was becoming impractical to work with. Amplifiers and other optical components were suffering damage, and pulses were propagating erratically due to extreme intensity gradients in the beam. As a result, laser intensity and power barely improved between the mid 1960s and the mid 1980s.

Inspired by a technique developed for microwaves, Mourou, then at the University of Rochester in New York, and his graduate student Strickland set out to amplify a stretched-out—and thus less intense—pulse and then recompress it. In their pivotal experiment, Mourou and Strickland sent a nanojoule pulse through an optical fiber. Due to positive group velocity dispersion within the fiber, the red component of the light propagated faster than the blue. The stretched, lower-energy-density pulse was then amplified and passed through a pair of parallel diffraction gratings, which allowed the blue component to catch up to the red. The reassembled 2 ps pulse had three orders of magnitude more power than the original pulse (see the article by Mourou, Christopher Barty, and Michael Perry, Physics Today, January 1998, page 22). The 1985 paper on the CPA technique, published in Optics Communications, was Strickland’s first publication. According to Google Scholar, it has been cited 4677 times.

The original CPA technique had some flaws—for one, the shape of the reassembled pulse didn’t perfectly match that of the original. Once again, Mourou and Strickland drew inspiration from others’ work on longer-wavelength light. Along with colleagues at Rochester, they built a pulse compressor based on a design for the telecommunications industry. Everything came together when the Rochester researchers and a visiting scientist named Patrick Maine produced a 1 TW pulse—1 J embedded in 1 ps—on a tabletop. The “Maine event,” as the researchers called it, triggered a radical change in the use of short-pulse lasers. With no fear of frying their apparatus, researchers could ditch liquid-dye amplifiers in favor of titanium:sapphire and other solid-state media with superior performance.

Most importantly for research scientists, the CPA technique spurred the proliferation of the tabletop terawatt, or T3, laser. Laser technology that was once viable only for major research institutions could soon be done in university physics labs for hundreds of thousands, rather than many millions, of dollars. Scientists use such lasers, which can now pack petawatts of power, to probe atom ionization, electron–nuclear coupling, and other subfemtosecond processes, among other research. (See Physics Today, January 2018, page 18, and June 2018, page 20.) “I always felt that ultrafast lasers have not received the attention they deserved,” says Ursula Keller, who directs ultrafast-laser-physics research at ETH Zürich. “When you look at the impact, it’s enormous.”

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