One half of this year’s Nobel Prize in physics has celebrated the optical tweezers, says S.Ananthanarayanan.

No physical instrument can hope to manipulate things at the dimensions of atoms. This is equally a limitation in working with very minute objects, like biological cells and the smallest organisms. The solution, to use forceps that consist of light beams, has opened the doors to new research in many areas

Arthur Ashkin, who cornered half the 2018 Nobel Prize for physics, for this work, has engaged with the effect of lasers on micro-particles, at the Bell Laboratories, New Jersey, since the 1960s. And he first demonstrated that a light beam can be used to exert a mechanical force, in a paper that was published nearly 50 years ago, in 1970. The principle was used by Steven Chu and others to coax atoms to get cooler by losing energy to particles of light. The Steven Chu group got the Nobel Prize in 1997, but Ashkin had to wait till October 2018 for the Nobel Committee to recognize his seminal work.

That rays of light exert a force was noticed by Johann Kepler as early as the 17th Century. Kepler noticed that the plume, or the vapour trail, of comets was blown in the direction facing away from the sun. In the mathematical formulation of electromagnetic waves, of which ordinary light consists, Clerk Maxwell, 19th Century, showed that the waves had momentum, or the inertia of a massive object in motion. This was later confirmed in experiment and there are ideas of using starlight to propel spacecraft on long flights where fuel cannot be carried.

Ashkin’s 1970 paper, carried by the journal, Physical Review Letters, explains that with very small and lightweight objects, the acceleration produced by light pressure can be sizeable. The principle is that when a photon of light reflects off an object, its momentum is reversed and transferred to the object. As the mass of an object falls very fast when its dimensions are reduced, in the case of very small objects, the mass is drastically low and acceleration is rapid. An orange-red beam from the argon laser, with power of just 1 Watt, the paper says, can accelerate an object of the size of the wavelength of the laser, and one that reflects only a tenth of the light that falls on it, thousands of times more than the force of gravity.

The problem with getting anything out of this force, however, has been that there are other, many times more powerful forces that are in action, and the effect of the light beam is completely obscured. These forces are the effects of temperature differences in the surrounding medium and the heating effect of the laser light itself.

In the work that Ashkin reported, warming effects of the laser beam were avoided by using particles, spheres of latex, and the medium, water, in which they were suspended, which were transparent to the laser light used. The beam thus traversed the cell without any part being absorbed and the only effect on the particles was because of the light that they reflected.

A narrow argon laser beam was focused horizontally through a glass cell and manipulated to strike single particles. Particles off the axis of the beam were then drawn inwards, while simultaneously being moved along the path of the beam, as fast as microns per second, in the water medium, the paper said.

A second version of the effect is when the cell is illuminated by two, opposing beams of laser light. The particle is then constrained in all directions and is effectively ‘trapped’, and yet not subjected to any force other than the light pressure of the lasers.

Later, in 1986, Ashkin, with others, including Steven Chu, found a way to trap a particle with a single laser beam. As the picture shows, while the light that passes through the particle exerts no force, the light beam bends when it enters the particle and is internally reflected, leading to a backward force. The dimensions can be arranged so that, while there is a force due to backward reflection that pushes the particle forward, the backward force due to other reflections and the force towards the axis of the beam trap the particle to be motionless! The method was good with a range of particle sizes and the paper of 1986 mentions the possible use to study minute biological particles, as well as cooling atoms by constraining their thermal motion.

Steven Chu went on to develop a way to get atoms to cool down to temperatures not possible by conventional means, which employ cooling by expansion or by demagnetization. The method was by selective transfer of energy of atoms to photons of light, at the time of reflection. In any sample of a material, the atoms are in constant motion. A photon of light, on reflection, would either gain energy and slow the atom, or lose energy and speed up the atom, depending on which way the atoms is moving. Chu used laser light of wavelength just below a characteristic wavelength at which reflection, as opposed to transmission, would take place. Now, reflection would occur only when the photon and atom were moving in opposite directions, and the atoms would lose momentum. All other atoms would be left unaffected. The result was that although only a fraction of the incident photons were reflected, the reflections led to slowing of the atoms, which amounts to cooling. Unprecedented low temperatures could be attained and Steven Chu, with Claude Cohen-Tannoudji and William D Philips, got the Nobel Prize for physics in 1997, for this work.

The basis, however, was the work of Arthur Ashkin and his work on constraining the motion of smallest particles. As suggested in his paper of 1986, the techniques developed, which have been called ‘optical tweezers’, have enabled detailed study of biological specimens in a way that was not possible using conventional methods of capturing and fixing samples for study. Typically, apart from trapping single atoms, the method has enabled trapping single cells, viruses and bacteria, measuring the forces exerted by components within cells, the dynamics of DNA. The properties of light and its effect on nanoparticles has become an important science and newer applications, using features like light waves where the plane of vibration of the waves being made to rotate, are enabling investigation of the most fragile states of matter.

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