Shining a Light: Today’s Science reports on the winners of the 2018 Nobel Prize for Physics

October is always an extremely busy month for the editors of Infobase’s Today’s Science educational resource and database which serves to present all of the latest real-world news on discoveries and advances in the worlds of chemistry, biology, environmental science, space, physics, and technology. A key contributor to the month’s workload is the announcement of the various Nobel Prizes within each of the scientific disciplines. 2018 was no different with a range of exciting articles in place across the Today’s Science platform.

To mark the 2018 Nobel Prize we thought it fitting to include a deeply informative piece written by Catherine Nisbett Becker and taken from the many current Today’s Science pieces. The article focusses on the scientific development of the laser as recognised by the Nobel committee in its 2018 physics prize. We are a long way on from CD players and Luke Skywalker.

Shining a Light: Laser Tools Capture Nobel

by Catherine Nisbett Becker | October 2018

For under $10, you can buy a laser. You can use it to aid a presentation, tease a cat, or just point at something across a room. There are probably lasers in your home right now, in a laser printer, or a CD or DVD player. But the ubiquity of lasers can blind us to the fact that they are relatively new, complicated, and often difficult to control. All of this makes the accomplishments of the winners of the 2018 Nobel Prize in Physics all the more impressive.

The prize, worth just over $1 million, was awarded to three researchers. Half the prize money went to Arthur Ashkin, a retired researcher long associated with Bell Laboratories in Holmdel, New Jersey, for his development of “optical tweezers” capable of manipulating objects as small as an atom. Ashkin, 96, is the oldest person ever to win a NobelPrize in any category. The rest of the prize money was split equally between Gérard Mourou of the École Polytechnique in France and Donna Strickland of the University of Waterloo in Canada. Strickland, 59, is only the third woman ever to win a physics Nobel. She and Mourou, 74, were honored for developing a technique for strengthening laser pulses called “chirped pulse amplification (CPA).” The winners were selected by the Royal Swedish Academy of Sciences, and their names were announced on October 2.

 

 

Laser Light

The term “laser” is actually an acronym; it stands for “light amplification by stimulated emission of radiation.” Lasers were conceptualized simultaneously in the late 1950s by Charles Hard Townes and Arthur Leonard Schawlow at Bell Labs (both Townes and Schawlow subsequently received Nobel prizes), and by Gordon Gould, who was then a graduate student at Columbia University in New York City. Theodore Maiman built the very first laser in 1960 at Hughes Research Laboratories.

A laser is a light source with a “coherent” beam. All electromagnetic radiation can simultaneously be viewed as particle and wave; its waveformcan be characterized by a wavelength and an amplitude. When light is coherent, all the waves emitted by the light source are pointing in the same direction and have the same wavelength. For visible light, the wavelength corresponds to a particular color (red light has the longest wavelengths, while violet has the shortest). Because all the waves propagate in the same direction, the beam can be maintained over a long distance, unlike a flashlight that emits light in a cone, or a light bulb that throws light in all directions. Even at a distance, laser light can be focused on a small spot. And some lasers can emit short pulses—lasting no longer than a femtosecond, a millionth of a billionth of a second.

Almost immediately, physicists, engineers, and the general public were enormously excited by the potential applications of laser technology. Science fiction creators quickly imagined tractor beams and laser guns. And researchers quickly went to work.

Moving On Up

Because light is both wave and particle, it can exert pressure—something postulated as early as 1619, when Johannes Kepler tried to explain why comet tails always pointed away from the Sun. But radiation pressure is pretty weak. Consider that when you sunbathe, you feel the Sun’s warmth but don’t feel like you’re being pressed into the ground. The advent of lasers made it easier to study radiation pressure, and Arthur Ashkin was doing just that in the 1960s when he developed optical tweezers.

Optical tweezers are a way to use lasers to manipulate small objects, much like hand-held tweezers are used in everyday life. Optical tweezers take advantage of radiation pressure, which for very small objects can be a significant force. In 1970, Ashkin showed that narrow beams of laser light could push small charged particles in air and water in the direction of the beam. Moreover, Ashkin found that there was an additional force: a “gradient force” that drew the particles into the center of the beam. Next, Ashkin pointed two lasers in opposite directions, trapping a particle between the beams.

Then he went a step further: he found a way to use only one beam to trap a small particle, called the “optical levitation trap.” He pointed one laser in the opposite direction of the force of gravity. Gravity pulled downward, while the laser beam pushed upward, and the particle was held at the point where those two forces canceled each other out. This was a milestone, but to be an impractical method.

But then, in 1986, Ashkin and his colleagues figured out how to use just one laser to trap a particle without relying on gravity. This method, originally called “the single-beam gradient force optical trap,” is now called optical tweezers. Ashkin sent his laser beam through a lens before letting it hit the particle. On one side of the lens, all the light waves were pointing in the same direction, but on the other side, the light had been refracted depending on its path through the lens. Parts of the beam that had provided the original gradient force now acted against the radiation pressure at the beam’s center, trapping the particle. Ashkin showed that he could trap, in water, charged particles as small as tens of nanometersacross and as large as tens of micrometers across.

Ashkin’s method was originally used to trap atoms and charged particles, but he soon began to experiment with using optical tweezers to manipulate small biological samples. By switching from a green laser to an infrared one, he found he could use the method on viruses and living cells, including parts of plant cells and amoebas. Thereafter, optical tweezers could be widely used not only by physicists and chemists but also by biologists.

Taking the Pulse

More than half a million people get Lasik eye surgery every year, where laser pulses lasting only a femtosecond create small incisions in the surface of the eye. But in the 1960s and 1970s, laser pulses were too weak to do that kind of work. If the pulses were amplified too much, they would damage both the amplifying material and the laser itself. The only way to create strong laser pulses was to create huge, expensive lasers, the kind of thing found in national research institutes, but not in ordinary laboratories or medical facilities. And anyway, the payoff was small—huge lasers had to cool off between pulses, which meant only a few shots per day were possible. In 1985, at the University of Rochester in upstate New York, Strickland, then a doctoral candidate, and Mourou, her Ph.D. supervisor at the time, were inspired by radar technology to develop the chirped pulse amplification (CPA) technique, the breakthrough the field needed.

 

CPA has three steps. First, a short laser pulse is stretched in time by orders of magnitude, so the peak power of the pulse is reduced by orders of magnitude. This step is a little like slowing down a sound recording. Originally, Strickland and Mourou ran the pulse through optical fiber to stretch it, but they quickly replaced the optical fiber with a pair of diffractiongratings, which helped increase the power of the pulse at the end. The second step is to amplify the stretched pulse. Because the power had been reduced in step one, the amplifying material isn’t damaged in step two. The third step is to speed the pulse back up again, now with a much stronger peak power.

 

Over time, improvements in the CPA technique have decreased the duration of pulses and increased their peak power. Now, laser pulses can be as short as a femtosecond and contain a petawatt (a quadrillion watts) of power. Physicists are now planning for laser pulses as short as an attosecond, a thousandth of a femtosecond. And in Prague, the capital of the Czech Republic, a facility is being developed that is intended to accommodate a 10-petawatt laser, which will make it easier for physicists to study extreme states of matter.

CPA applications have been incredibly wide-ranging, from high-energy particle physics to industry to clinical medicine. And CPA devices have shrunk from gigantic pieces of equipment used only in national laboratories to tabletop systems.

Better, Faster, More

Swedish inventor Alfred Nobel endowed the Nobel Prizes to recognize individuals whose achievements had most benefited humanity. Ashkin, Strickland and Mourou were recognized in 2018 because their contributions to laser technology, while continuing to be improved upon, have already significantly benefited both scientific research and everyday life.

Discussion Questions

What are possible applications of lasers that would use the techniques honored in this year’s prize?

The Chirped Pulse Amplification method discussed above in effect increases the power of a laser pulse by concentrating the energy in an extremely short period of time. A somewhat analogous effect can be seen in devices that achieve high pressure by narrowing the footprint on which pressure is exerted. Why do you think it is important, for science and industry, to look at or employ techniques that tightly focus energy or pressure?

Are there situations where something opposite (that is, spreading out concentrated pressures) is desirable?

Journal Abstracts and Articles

(Researchers own descriptions of their work, summary or full-text, on scientific journal websites.)

Ashkin, Arthur. “Acceleration and trapping of particles by radiation pressure.” Physical Review Letters (January 26, 1970) [accessed October 8, 2018]: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.24.156.

Ashkin, Arthur and J.M. Dziedzic. “Optical levitation by radiation pressure.” Applied Physics Letters (October 15, 1971) [accessed October 8, 2018]: https://aip.scitation.org/doi/10.1063/1.1653919.

Ashkin, Arthur. “Trapping atoms by resonance radiation pressure.” Physical Review Letters (March 20, 1978) [accessed October 8, 2018]: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.40.729.

Ashkin, Arthur, Dziedzic, J.M., and Yamane, T. “Optical trapping and manipulation of single cells using infrared laser beams.” Nature (December 31, 1981) [accessed October 8, 2018]: https://www.nature.com/articles/330769a0.

Ashkin, Arthur, et al. “Observation of a single-beam gradient force optical trap for dielectric particles.” Optics Letters (May 1986) [accessed October 8, 2018]: https://www.osapublishing.org/ol/abstract.cfm?uri=ol-11-5-288.

Ashkin, Arthur and J.M. Dziedzic. “Optical trapping and manipulation of viruses and bacteria.” Science (March 20, 1987) [accessed October 8, 2018]: http://science.sciencemag.org/content/235/4795/1517.

Ashkin, Arthur and J.M. Dziedzic. “Internal cell manipulation using infrared laser traps.” Proceedings of the National Academy of Sciences (October 1989) [accessed October 8, 2018]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC298182/.

Ashkin, Arthur, et al. “Force generation of organelle transport measured in vivo by an infrared laser trap.” Nature (November 22, 1990) [accessed October 8, 2018]: https://www.ncbi.nlm.nih.gov/pubmed/2250707.

Ashkin, Arthur. “Force of a single-beam gradient laser trap on a dielectric sphere in the ray optics regime.” Biophysics Journal (February 1992) [accessed October 8, 2018]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1260270/.

Chu, Steven, et al. “Experimental observation of optically trapped atoms.” Physical Review Letters (July 21, 1986) [accessed October 8, 2018]: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.57.314.

Maine, P., et al. “Generation of ultrahigh peak power pulses by chirped pulse amplification.” IEEE Journal of Selected Topics in Quantum Electronics (February 1988) [accessed October 8, 2018]: https://ieeexplore.ieee.org/document/137.

Strickland, Donna and Gérard Mourou. “Compression of amplified chirped optical pulses.” Optics Communications (December 1, 1985) [accessed October 8, 2018]: https://www.sciencedirect.com/science/article/pii/0030401885901208.

Strickland, Donna, et al. “Picosecond pulse amplification using pulse compression techniques.” Conference on Lasers and Electro-Optics, OSA Technical Digest (1986) [accessed October 8, 2018]: https://www.osapublishing.org/abstract.cfm?uri=CLEO-1986-THL1.

Bibliography

Clery, Daniel and Cho, Adrian. “Turning lasers into versatile tools earns trio Nobel Prize in Physics.” Science (October 2, 2018) [accessed October 8, 2018]: http://www.sciencemag.org/news/2018/10/physics-nobel-three-scientists-who-turned-laser-light-tools.

Keywords

lasers, laser tools, 2018 Nobel in physics, optical tweezers, chirped pulse amplification (CPA), Arthur Ashkin, Donna Strickland, Gérard Mourou

Citation Information

Nisbett Becker, Catherine. “Shining a Light: Laser Tools Capture Nobel.” Today’s Science, Infobase Learning, Oct. 2018, http://tsof.infobaselearning.com/recordurl.aspx?wid=277843&ID=41369. Accessed 8 Nov. 2018.

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