Nobel Prize in Physics – 2018

Nobel Prize in Physics - 2018



Fig. one. 2018 Nobel Prize Winners in Physics. From left to right: Arthur Ashkin, Gérard Mourou and Donna Strickland. Photo from voanews.com

On October 2, the Royal Swedish Academy of Sciences announced the award of the next Nobel Prize in Physics – “For revolutionary inventions in the field of laser physics” (“For groundbreaking inventions in the field of laser physics”). The new laureates were the American Arthur Eshkin, the Frenchman Gerard Mouroux and the Canadian Donna Strickland. Eshkin is noted for the invention of optical tweezers and their use to study biological systems. Moore and Strickland received an award for developing a method for generating ultrashort optical pulses of extremely high intensity.

This year’s laureates were awarded for their work more than thirty years ago. Accordingly, they are all far from young. Arthur Ashkin, who until 1992 headed the Physical Optics and Electronics Division of Bell Labs, turned 96 a month before the award. He turned out to be the oldest of the winners of this award in its entire history and, moreover, the first and so far the only one to receive it in his tenth decade of life (with the exception of Leonid Gurvich, who in 2007, at the age of 90, became a laureate of a non-Nobel Prize proper , and the Nobel Prize in Economics). By the way, Eshkin’s father, Isador Ashkenazi, moved to the United States from Odessa during the tsarist era. Gerard Moore, a professor at the Ecole Polytechnique in Paris and professor emeritus at the University of Michigan, is 74 years old, and his former graduate student and now associate professor at the University of Waterl Canada, Donna Strickland, will turn 60 next May. So the achievements of all three scientists, now celebrated by the Stockholm Areopagus, have long since become a scientific classic.

The official formulations of the merits of the new laureates show that we are talking about applied research with a clearly expressed technological focus. The last time this happened was in 2014, when three Japanese scientists were awarded for inventing (again inventing!) Blue LEDs. In 2015, 2016 and 2017, the Nobel Prizes in Physics were awarded for fundamental research.

The prizes in 2014 and 2018 are recognized for work in physical optics, which in recent decades has greatly enriched both pure physics and technology. As for the works of Eshkin, Moore and Strickland, they have a concrete common core. The remarkable inventions of these scientists greatly expanded the practical application of light pressure, which became possible thanks to the progress of quantum optical generators – lasers. This is what unites them.

The hypothesis of the existence of light pressure is by no means new – next year it will be half a thousand years old. It first appeared in Johannes Kepler’s book “De Cometis Libelli Tres”, which was published in 1619. With the help of this hypothesis, Kepler explained why the tails of comets are directed not towards the Sun, but in the opposite direction. On the whole, his guess turned out to be correct (with the clarification that cometary tails are also formed under the influence of the solar wind). In 1873, James Clerk Maxwell showed that the existence of light pressure (like the pressure of any electromagnetic radiation) follows directly from the equations of electrodynamics. In 1899-1901, Maxwell’s formula for the magnitude of light pressure was confirmed (in precision and very laborious experiments!) By Moscow University professor Pyotr Nikolaevich Lebedev and American physicists Ernest Fox Nichols and Gordon Ferrie Hull.

The pressure of ordinary light is extremely low. The force with which sunlight repels our planet is sixty trillion times less than the sun’s gravity. It is no coincidence that in 1905, the English physicist John Henry Poynting, in his presidential message to the British Physical Society, noted that experiments to determine the magnitude of light pressure demonstrated the extremely smallness of this effect, “excluding it from consideration in earthly affairs.” And until the advent of lasers, this conclusion remained completely valid.

As you know, laser light has such remarkable properties as exceptional spectral purity (that is, the ability to generate almost perfect monochromatic radiation) and high spatial coherence. Therefore, the laser beam can be focused into a spot with a diameter of only slightly more than one wavelength. With a laser emitter power of only a few watts, radiation intensity can be obtained that is thousands of times higher than the total intensity of the visible spectrum of the sun. Hence, in particular, it follows that with its help, in principle, it can accelerate very small particles to accelerations, in a million times greater than the acceleration of gravity at the earth’s surface. And this is just one of the gigantic variety of applications imaginable.

Arthur Eshkin appreciated the unique capabilities of lasers almost immediately after their invention. Since the early 1960s, he has conducted many ingenious experiments at Bell Laboratories, the result of which was the emergence of light traps that reliably hold the smallest objects of various natures. These studies took a quarter of a century – the first article by Ashkin and his collaborators describing the optical trapping of dielectric particles ranging in size from tens of nanometers to tens of micrometers appeared in 1986 (A. Ashkin et al., 1986. Observation of a single-beam gradient force optical trap for dielectric particles). Interestingly, it fits into three magazine pages – fundamental scientific works are often very compact.

Fig.  2. Scheme of confinement of a Mi particle

Fig. 2. Left – a scheme of confinement of a Mie particle (a small spherical body made of a dielectric material, the radius of which is many times the wavelength, see Mie scattering), located in water, in a laser trap. The refraction of the rays is arranged in such a way that the resulting force FA, arising from the transfer of momentum from the beams to the particle, is directed upward (towards the laser beam). On right – a photograph of a real experiment in which a sphere with a diameter of 10 μm is held in this way. You can see the path of the laser beam before and after scattering on the sphere. Figures from A. Ashkin et al., 1986. Observation of a single-beam gradient force optical trap for dielectric particles

Eshkin’s light traps were eventually called optical tweezers (or laser tweezers, optical tweezers, laser tweezers). In subsequent years, this technology has been greatly improved, and its capabilities have expanded significantly. Laser tweezers not only hold micro- and nano-objects, but can move them, rotate and cut them into pieces. They are widely used in molecular biology, genomics, virology and many other places. The most important field of application of optical tweezers has become the laser cooling of neutral atoms to ultra-low temperatures. For these works, former Eshkin’s employee and one of the co-authors of his famous article Steven Chu with his compatriot William Daniel Phillips and French physicist Claude Cohen-Tannoudji became Nobel laureates in 1997.

Fig.  3. Capillary erythrocyte in the capillary of a living mouse using optical tweezers

Fig. 3. Capture an erythrocyte in the capillary of a live mouse using optical tweezers. On the first frame (a) the outline of the capillary is visible, but individual cells are not visible due to the high speed of blood flow (its direction is indicated arrow). The following frames show how one of the erythrocytes gets stuck in the place where the laser radiation is focused (indicated white risks). This slows down the blood flow, so that another red blood cell can be seen passing by. The whole process can be seen in the video. Image from article M.-C. Zhong et al., 2013. Trapping red blood cells in living animals using optical tweezers

If Arthur Eshkin owes a laureate to the method of manipulating micro-objects with the help of laser light, then Gerard Mourou and Donna Strickland, so to speak, acted on a more serious energy scale. They have developed an extremely efficient way to increase the power of laser pulses (see: In Pursuit of Petawatts, Elements, 10/10/2018). To appreciate it, you need to go deep into the past.

The history of laser technology began in May 1960, when Theodore Maiman, an employee at the Hughes Research Laboratories, launched the first artificial ruby ​​laser. Six months later, in the laboratories of the IBM Corporation, an infrared laser based on calcium fluoride with the addition of uranium ions, built by Peter Sorokin and Mirek Stevenson, was launched – however, it operated at the temperature of liquid hydrogen and did not acquire practical significance. In December 1960, Bell Laboratories researchers Ali Javan, William Bennett and Donald R. Herriott demonstrated the world’s first gas laser based on a mixture of helium and neon, which is still used everywhere. After that, a worldwide race began, the goal of which was the creation of new lasers, which has not ended to this day.

I have already noted that focused laser light produces a very high radiation intensity. In the early 1960s, it was 1010 watt / cm2, and ten years later increased by five orders of magnitude. However, then its growth slowed down, and this trend continued until the mid-1980s. The situation changed radically in 1985, when Gerard Mourou and Donna Strickland (then they were working in the USA), researchers from the Laser Energy Laboratory of the University of Rochester, published a three-page (history repeats itself!) Article describing their method (D. Strickland, G. Mourou, 1985. Compression of amplified chirped optical pulses). The power of laser pulses started to grow again and has now reached 1023 watt / cm2

The essence of their method can be described in literally three sentences. An ultrashort laser pulse is passed through a pair of diffraction gratings, which stretch it by several orders of magnitude in time (in their first experiments, Moore and Strickland used a fiber-optic cable for this, but the gratings turned out to be more efficient). As a result, the peak energy of the electric fields of the laser pulse decreases so much that it passes through an optical amplifier (for this, sapphire doped with titanium ions is usually used) without disturbing its crystal structure. The multiply amplified pulse is passed through another pair of diffraction gratings, and they compress it to its original length. The output is a very short pulse of extremely high intensity (Fig. 4). Already the first experiments on the application of this method led to the creation of picosecond laser systems of terawatt power. The rest turned out to be a matter of technique – and, of course, ingenuity.


Fig.  4. Circuit for amplification of chirped laser pulses

Fig. four. Amplification scheme for chirped laser pulses. Initial short laser pulse (BUT) is stretched using a pair of diffraction gratings (B), as a result of which its intensity decreases (FROM) and can be amplified in a conventional amplifier (D). Backward compression (again with a pair of diffraction gratings, E) generates a short pulse of very high intensity (F). Figure from en.wikipedia.org, with changes

The field of application of ultrashort super-high-power laser pulses is extremely wide. Suffice it to mention that it ranges from experiments in fundamental physics to the surgical treatment of myopia and astigmatism.

In conclusion, one more curious detail. Donna Strickland at her university leads a group dedicated to ultrafast lasers. In 1997, she received the position of assistant professor, and over the years she has risen only one step in the university hierarchy. When asked by a BBC reporter on October 2 why she did not become a full professor, the new Nobel laureate replied, “I never applied.” Such is the person!

See also:
Chasing Petawatts, Elements, 10/10/2018.

Alexey Levin

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