11/10/2018

Nobel Prize in Physics recognizes research developed since the 1970s.

Reproduction from the IMPA Science & Mathematics blog, from O Globo, coordinated by Claudio Landim.

Paulo Américo Maia Neto – Full Professor at the Institute of Physics, UFRJ (Federal University of Rio de Janeiro)

Half of this year's Nobel Prize in Physics was awarded to American Arthur Ashkin for the invention of optical tweezers. With this choice, the Royal Swedish Academy of Sciences finally recognized Ashkin's pioneering contribution to the use of laser light to capture atoms and micro-particles.

This line of research, developed by Ashkin since the 1970s, explores a very unfamiliar aspect of light: its ability to exert forces on material bodies. In our daily lives, we frequently deal with the energy carried by light. When absorbed by an opaque material, a beam of light transfers all its energy to the material medium, producing heat. In addition to energy, light carries another physical quantity that is very important for understanding mechanical forces: linear momentum. When struck by a projectile, a material body is pushed because it inherits the linear momentum of the projectile. Similarly, a body is pushed when it absorbs or reflects a beam of light. This effect, too small to be observed in usual everyday situations, is responsible for the propulsion of the Japanese solar sail IKAROS, which passed by the planet Venus in December 2010.

At the microscopic scale, the effects of the linear momentum of light are much more important. In 1970, Ashkin showed how to push microscopic latex balls using a beam of light produced by a laser (the laser had been invented a few years earlier). In the optical tweezers, invented by Ashkin and collaborators in 1986, a single, strongly focused laser beam exerts an optical force on a transparent micro-particle in such a way as to trap it in the vicinity of the focus. In this way, the position of the particle can be controlled by performing a spatial translation of the focal point. Everything happens as in a pair of tweezers, but there is no mechanical contact with the particle – only light is responsible for its manipulation.

Optical tweezers also allow the capture and manipulation of living cells without altering their vital functions. In optical tweezers, there is no energy exchange because the trapped particles or cells are always transparent, thus preventing the possibility of heating. How is it possible to transfer linear momentum, and thus exert a force, and at the same time not transfer energy? This occurs because the momentum of light is a vector that points in the direction of beam propagation. When the beam is deflected, as in a reflection, there is an exchange of momentum between the light and the captured particle, giving rise to the optical force, even when there is no absorption.

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