The Nobel Prize in Physics was awarded to Arthur Ashkin, Gérard Mourou and Donna Strickland this year, with one half to Ashkin and other half jointly to Mourou and Strickland. The award honours the inventions in the field of laser physics. 

Arthur Ashkin was awarded the Nobel Prize for his invention of optical tweezers that grab particles, atoms and molecules with laser beam. Viruses, bacteria and other living cells can be held too without being damaged.

Gérard Mourou and Donna Strickland developed a technique to create high intensity ultra-short optical pulses. This technique has broad industrial and medical applications. 

Let’s first understand what optical tweezers mean,  its brief history and applications followed by ultrashort high intensity beams and its applications.

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Optical Tweezers

Tweezers literally means some sort of instrument which is used to grab very small objects. Optical tweezers means light based method to grab something very small. Arthur Ashkin used laser beam to trap/grab/manipulate particles which are as small as atoms. Optical tweezers take advantage of ability of light to exert force, or radiation pressure.

Radiation pressure is the pressure exerted by light on matter. There’s an interesting video by Vsauce about radiation pressure click here to watch!

The idea that light could exert pressure was put forward by Johannes Kepler in 1619, who postulated that pressure of light explains why comet tails always point away from Sun. In 1873, James Clerk Maxwell showed theoretically that light can exert pressure, based on his theory of electromagnetism. In 1900s, the existence of radiation pressure was experimentally confirmed by Pyotr Lebedev, Ernest F. Nicholas and Gordan F. Hull. Radiation pressure is extraordinarily weak under everyday circumstances.

Lasers were invented in the year 1960 and soon after Ashkin began to experiment with it. In lasers light moves coherently, unlike ordinary white light in which the beams are mixed in all the colours of the rainbow and scattered in every direction. Ashkin did an experiment designed to look for particle motion from force due to radiation pressure of laser light on small particle. A sample of transparent latex spheres suspended in water was used to avoid any heating or radiometric forces. With just milliwatts of power, particle motion was observed in the direction of mildly focused Gaussian beam. However, an additional unanticipated force component was soon discovered that strongly pulled particles located in the fringes of the beam into the high intensity region on the beam axis. The understanding of the magnitude and properties of these two force components made it possible to devise the first stable three-dimensional optical trap for single neutral particles.

The trap consists of two opposing moderately diverging Gaussian beams focused at points A and B as shown in the figure above (Fig 1 (B))

The next advance in optical trapping and manipulation was the demonstration of the optical levitation trap in air, under conditions in which gravity plays a significant role. In the levitation trap, as shown in Fig.2, a single vertical beam confines a macroscopic particle at a point E where gravity and the upward scattering force balance. 

Optical tweezers are now a widely used tool in biological physics and related areas, and continue to find new applications. The method is used for non-invasively trapping and manipulating objects such as single cells and organelles and for performing single-molecule force and motion measurements. The study of single molecules is made possible by linking them to “handles” that can be easily trapped with the tweezers, such as micron-sized polystyrene or silica beads. The beads also act as probes to monitor motion and force. It is also used to trap living bacteria. One important breakthrough was the ability to investigate the mechanical properties of molecular motors, large molecules that perform vital work inside cells. The first one to be mapped in detail using optical tweezers was a motor protein, kinesin, and its stepwise movement along microtubules, which are part of the cell’s skeleton. 

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High Intensity Ultra-short Beams

Laser light is created through a chain reaction in which the particles of light, photons, generate even more photons. These can be emitted in pulses. Ever since lasers were invented, almost 60 years ago, researchers have endeavoured to create more intense pulses. However, by the mid-1980s, the end of the road had been reached. For short pulses it was no longer practically possible to increase the intensity of the light without destroying the amplifying material. 

Strickland and Mourou’s new technique is known as Chirped Pulse Amplification (CPA). They took a short laser pulse, stretched it in time, amplified it and squeezed it together again. When a pulse is stretched in time, its peak power is much lower so it can be hugely amplified without damaging the amplifier. The pulse is then compressed in time, which means that more light is packed together within a tiny area of space – and the intensity of the pulse then increases dramatically. The CPA-technique invented by Strickland and Mourou revolutionised laser physics. It became standard for all later high-intensity lasers and a gateway to entirely new areas and applications in physics, chemistry and medicine.

There are some interesting applications of this. Things happen so quickly at the molecular and atomic levels that it was difficult to describe the process. Only before and after picture was possible to be described. But with pulses as short as a femtosecond, one million of a billionth of a second, it is possible to see events that previously appeared to be instantaneous. Ultra-sharp laser beams also make it possible to cut or drill holes in various materials extremely precisely – even in living matter. Many applications for these new laser techniques are waiting just around the corner – faster electronics, more effective solar cells, better catalysts, more powerful accelerators, new sources of energy, or designer pharmaceuticals.

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To read the published papers click on the links below:

Arthur Ashkin 

Gérard Mourou and Donna Strickland