Breakthrough in ‘self-healing’ light beams

Wednesday 11 September 2002

Scientists in Scotland have performed new ‘optical trapping’ work using a light beam that repairs or self heals itself after encountering an obstacle.

The breakthrough will lead towards advanced studies in the microscopic world, such as advanced forms of micromachines and control of arrays of ‘lab-on-a- chip’ devices.

These latest advancements in ‘optical tweezing’ techniques developed by physicists at the University of St Andrews will been unveiled in tomorrow’s (Thursday 12th September, 2002) issue of the international journal ‘Nature’.

In their paper, ‘Simultaneous micromanipulation in multiple planes using a self-reconstructing light beam’, Dr Kishan Dholakia, and his research colleagues Veneranda Garcés-Chávez, David McGloin, Hannah Melville and Wilson Sibbett, describe how the ‘Bessel beam’ offers an exciting new form of light beam for optical tweezing work.

Unlike optical tweezing techniques using normal laser beams, in which the light beam is typically distorted when passing through a trapped object, the Bessel beam is not affected by such trapped objects or ‘obstacles’. Instead, it can repair itself after passing one object, enabling it to move on and grab and manipulate other objects, that are actually far removed from the original object.

“In optical tweezers the particle you grab is transparent to some extent. The light passing through it is bent, or refracts, and due to this change in momentum, is central to creating the minute forces that allows the particle to be held in the brightest part of the beam. Typically the laser light is distorted upon passing through the trapped object or ‘obstacle’, in a manner depending on the particle size and its properties. This makes it difficult if you try to grab lots of particles with different properties all along the direction of the incident beam,” explained Dr Dholakia.

The Bessel beam, however, comprises of light waves arranged on a “cone”. The summation, or “interference” of all these waves leads to a bright spot in the centre of the beam. If the bright centre of the beam is distorted (as it would through a tweezed particle) it creates a shadow after the distortion. Parts of the light waves on the cone, which are far removed from the centre, are able to move past the particle unhindered and are able to recreate the beam centre at some distance beyond the particle. Thus, the beam appears to “self-heal” or repair itself after passing beyond a tweezed object, enabling it to manipulate other particles.

The group, based at the University’s School of Physics and Astronomy, have demonstrated this method by trapping, moving and rotating particles in different sample chambers separated by millimetre distances. This cannot be achieved using conventional tweezers. The particles in different chambers were aligned with one another since it’s the same beam doing all the trapping.

“This means you can grab particles in many sample chambers all at the same time and move them in unison,” said Dr Dholakia.

“The technology may be used to realise advanced forms of optically driven micromachines and preparation of samples in multiple chambers but perhaps with different ambient conditions. This could be used for simultaneous parallel studies at the biological level,” he said.

The findings are the latest in ongoing advancements in optical tweezers technology, a powerful and non-contact technique pioneered in the 1970s in which microscopic particles are trapped and moved from one stop to another with a tightly focused laser beam.

Miniscule objects such as a glass particle can bend – or refract – the light, which acts like a lens, causing forces to be exerted on the object as the light passing through it changes direction. This leads to the particle being attracted, or sucked into, the brightest region of the beam and staying there. It does so without causing any damage to the particle, which means that several applications for optical tweezers techniques exist in biology.

Indeed, optical tweezers have become such an important bridge between physics and biology that numerous biology laboratories around the world now use them for a wide range of studies including the unravelling of DNA, the study of molecular motors and numerous other areas in biotechnology.



For further information contact Kishan Dholakia on tel 01334 463184 or email kd1@st-; David McGloin on 01334 463189, email dm11@st- or Veneranda Garcés- Chávez, tel 01334 463165, email [email protected]



Issued by Beattie Media On behalf of the University of St Andrews Contact Gayle Cook on 01334 467227, mobile 07900 050103, or email [email protected] Ref: Kishan Nature pr 100902 View the latest University news at

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