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In the 1966 science fiction film Fantastic Voyage, medical personnel are reduced to the size of microbes and enters the body of a scientist to perform a brain operation. Thanks to the work of this year's Nobel Prize winners in physics, laser tools are now working on this scale.
Arthur Ashkin has won for his development of optical tweezers that use a laser to grab and manipulate objects like a small molecule. And Gerard Mourou and Donna Strickland have been rewarded for finding a way to produce high-intensity ultra-short laser pulses, now used to perform millions of laser eye correction operations each year.
Here is an overview of these inventions, their inventors and the applications that made them important enough to win a Nobel Prize.
How do optical tweezers work
Here's how the optical tweezers work. Laser light passes through an objective lens as it focuses it to a point within the particle to be held. Explanations of what happens next depend on the diameter of the particle.
If the diameter is significantly greater than the wavelength of light, you can use the ray optics to explain its operation. Due to refraction, the light changes direction as it passes from the surrounding environment, eg water, to the particle and vice versa. Light carries speed and the change of direction involves a change in the speed of light. This change of moment produces a net force on the particle that moves it towards the focal point.
If, instead, the particle has a diameter much smaller than the wavelength of light, then it can be treated as an electric dipole. The narrow area around the focal point of light has a strong electric field that weakens as it moves away from this point, forming electric field gradients on both sides. These gradients move the particle towards the focal point.
However, in both cases, it is also necessary to take into account the force of light on the particle, called diffusion force. Photons have a momentum and when the photons impact the particle, they transmit a part of this momentum. This makes the center of the particle slightly ahead of the focal point.
The result is something that looks like sci-fi trunk beams, but on a much smaller scale.
Arthur Ashkin: The man behind the tweezers
Arthur Ashkin, born in 1922, earned a BS degree in physics from Columbia University and a PhD from Cornell. During the Second World War, he worked on magnetrons for US military radar systems and met Hans Bethe and Richard Feynman through his brother, Julius, who was working on the Manhattan project.
In 1952 he went to work at Bell Labs where he stayed, retiring in 1992. There he worked in the microwave field until 1961, when he switched to laser research.
This radiation could exert pressure, was experimentally demonstrated in the early 1900s but the effect was weak. Lasers, however, allowed more powerful effects, and in 1970 Ashkin showed that narrow – beam focused laser light could produce scattering forces displacing small dielectric particles in the air and water. . He also showed that the particles could be drawn transversely to the center of the beam due to the surrounding intensity gradients. These discoveries led him to use two opposing beams to trap the particles, then a single beam effect in which the beam force on the particle was countered by the force of gravity. Finally, in 1986, he introduced the all-beam single-beam trap, now known as the optical tweezers.
Use of optical tweezers
A few years after their invention, Ashkin and other researchers had used optical tweezers to trap and manipulate neutral atoms, viruses, bacteria, yeast cells, human red blood cells, and so on. They have been widely used for biological research since.
One method of handling molecules is to attach beads of micron size polystyrene or silica to the beads and handle the easily trapped bead. A good example of the application of this is the understanding of the kinetics and mechanics of molecular motors, engines that convert chemical energy into linear motion or rotation. The kinesin protein, for example, uses these motors to move along microtubule filaments in cells to carry cellular cargoes. By attaching a pearl to kinesin, you can measure its size and the strength it produces.
Sometimes, double traps are used to stand at the opposite ends of a protein or DNA molecule. Early DNA testing included the measurement of elasticity and relaxation and the stimulation of life forces to move to an extended form of DNA.
The resolution has been improved so that one can track the one-step motion of the RNA polymerase engine along DNA base pairs. The RNA polymerase engine copies DNA into mRNA at one stage of protein production in cells. Subsequent processing of the mRNA by the ribosomes was also followed with the aid of an optical forceps.
It is for the invention of optical tweezers and their application to the study of biological systems that Arthur Ashkin received the 2018 Nobel Prize in Physics.
Ultra-short and high-intensity optical pulses
This prize was shared with Gérard Mourou and Donna Strickland for having developed a method for generating ultra-short optical pulses with the aid of lasers, but at high intensity in 1985. This technique is called pulsed pulsed amplification (CPA). Previously, it was possible to produce ultra-short pulses at a relatively low cost, but the increase in their intensity was hindered by the fact that it was damaging the amplifier and the optical components. To solve this problem, it was necessary to use a large beam diameter, but this increased the costs and only required a few shots a day to allow the amplifier time to cool down.
The solution proposed by Mourou and Strickland, which is illustrated in the diagram, consists in stretching the pulse at first several orders of magnitude. This then allowed amplification without damaging the amplifier. The wide and amplified pulse was then compressed to its original duration, which gave it a higher intensity in the terawatts.
Strickland And Mourou: The Laser Jock And The Teacher
Donna Strickland is the first woman Nobel laureate to become Maria Goeppert-Mayer in 1963 and the only other was Marie Curie in 1903.
Strickland would suit here with the readers of Hackaday. When she attended McMaster University in Canada, she and her other graduate students qualified as "laser jocks." She thinks that's because they were good with their hands.
As an experimenter, you have to understand physics, but you
You also have to be able to make something work, and the lasers were very capricious at the time.
After graduating, she moved to the United States to pursue her Ph.D. at the University of Rochester.
Gérard Mourou is a French scientist who has obtained his doctorate. from the Pierre and Marie Curie University in Paris in 1973. In 1977 he became a professor at the University of Rochester where he met Strickland.
CPA drew inspiration from the long radio waves of the radar, but it took a few years for these two people to work with the light. At the time, they stretched the light by passing it through a long fiber optic cable. At one time, when you were working with a 2.5-mile (1.5-mile) cable, no light was coming from the other end. There was a break in the cable somewhere. They settled instead on 1.4 km. In 1985, they managed to make it work and published their article. Over the next few years, the fiber has been replaced by a pair of diffraction gratings and amplification has been increased from nanojoules to joules.
CPA apps
The CPA has found a number of uses, including the detection of electron activities. This is made possible by sufficiently powerful femtosecond and attosecond lasers.
A development still in progress but one that will eventually have a medical application is plasma laser acceleration, which is currently the domain of radio frequency (RF) accelerators. This works by injecting electrons into a plasma channel and using a laser to accelerate them into the channel. Recently, a Petawatt class laser from the Lawrence Berkeley National Laboratory accelerated electrons up to 4.2 GeV over a distance of only 9 cm.
But the application in which CPA has had the greatest effect is laser eye surgery, of which millions are performed each year. Very short cuts minimize damage due to heating and of course, extreme accuracy is also beneficial.
Noble Nobel Winners
According to the will of Alfred Nobel, who created the prize, this prize was designed to reward those who serve humanity. Thus, while we may fancifully talk about how this year's winners have made inventions that have made science fiction a reality, they have been selected for the applications in which these inventions were used. For our complex understanding of the amazing inner workings of the cells and for the millions of people who can now better see, we believe that this year's award has been well won.
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