“Optical Tweezers” Wins The 2018 Nobel Prize In Physics

  • 2018 Nobel Prize in physics goes to three scientists for their remarkable work in laser physics. 
  • They discovered optical tweezers and ultrashort, high-intensity optical pulses. 
  • Today, it’s used to measure phenomena in femtosecond timescales, treat deep-tissue tumors, and much more. 

The 2018 Nobel Prize in Physics is split among three scientists. They are awarded for their phenomenal work in manipulating microscopic objects via laser physics.

Half of the prize goes to Arthur Ashkin, a physicist from Bell Lab in the USA, who discovered a technique to trap nanoparticles, living cells, and viruses in between two light beams. The method is called optical tweezers.

The remaining half of the prize is equally shared between Donna Strickland from the University of Waterloo and Gerard Mourou from the University of Michigan. They developed methods to produce high-intensity laser pulses capable of stimulating target on femtosecond timescales.

This year’s Noble prize is quite special because Ashkin is the oldest scientist (96) to win this prize. He started working on this method after 1964 when Noble prize recognized the invention of the laser for the first time. Also, a woman scientist has received a physics Nobel prize after 5 decades. Strickland is the 3rd woman getting this prize, following Marie Curie (1903) and Maria Goeppert-Mayer (1963).

Read: 10 Noble Prize Winners Who Were School/College Dropouts

What Did They Do?

Ashkin was interested in using the light field to move tiny objects. It was well-known (in the 1960’s) that light exerts a radiation pressure on objects, but too small to be observed. He conducted a set of tests in which he suspended micrometer-wide latex spheres in water within a glass cell and pointed an argon laser at the cell.

He found that spheres were dragged to the distant end and trapped along the axis of the beam. This is exactly what he anticipated from radiation pressure model. It happens due to spatial variation in the intensity of the beam.

Due to the refraction, the light passing through the sphere is bent and comes out at a certain angle, causing the latex sphere to recoil. The net recoil would be zero if the beam is uniform. But since the most intense part of the laser beam is in the center, the radiation pressure onto the sphere pushes it towards the central axis.

Reference: Physical Review Letters | doi:10.1103/PhysRevLett.24.156 | APS Physics

The sphere is just an example in this experiment. It works for all tiny particles: the laser beam is strongly focused and pulls the nanoparticles towards the center of the beam. It keeps particles there, in steady state, thus the name optical tweezers.

This optical tweezers technique — a single-beam optical trap — can be used to guide and control living cells like bacteria and viruses.

In 1985, Strickland and Mourou’s research led to the development of a technique for generating the shortest, most intense laser pulses ever made. To do this, they slowed down the laser, amplified it and compressed it into a shorter time. When you compress a pulse in time, it gets shorter and more light is packed within the same space, increasing the intensity of the pulse. More importantly, it doesn’t damage the source material used to generate light.

Their method called Chirped Pulse Amplification enabled precise drilling/cutting through living cells. In fact, it has become a state-of-the-art technique, which is used by almost all high-power lasers (more than 100 terawatts) in the world.

How It’s Useful?

In recent years, optical tweezers technique has been particularly successful in investigating a wide range of biological systems. They are able to manipulate tiny particles, including absorbing and dielectric particles.

For instance, if you select a wavelength in a manner that the particle doesn’t absorb the light, it doesn’t heat up. Then you can move the cell wherever you need (within several hundred nanometers, based on the laser wavelength) without harming it.

Read: 15 Scientists That Are Not Rewarded Fairly For Their Contribution

The study has inspired scientists to enhance ultrashort intense laser pulses, leading to better data storage system, and measurement of events on attosecond and femtosecond timescales. Today, Chirped Pulse Amplification is used in numerous applications, such as Lasik eye surgery deep-tissue tumor treatments, and studying fundamental physics principles.

Written by
Varun Kumar

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