New trapping and optical pinching device for manipulating and moving tiny objects

When you shine a ray of light on your hand, you do not feel much except a little heat generated by the ray. When you project the same light into a world measured at the nanoscale or micro scale, light becomes a powerful manipulative tool that you can use to move objects – trapped securely in the light.

Researchers from the Structured Light Group of the Faculty of Physics at the University of the Witwatersrand in Johannesburg, South Africa, have found a way to use the full beam of a laser light to control and manipulate tiny objects such as individual cells in a human being. bodies, tiny particles in small volume chemistry, or working on future on-chip devices.

Although the specific technique, called trapping and holographic optical pinching, is not new, Wits researchers have found a way to optimally utilize the full force of light, including vector light, which n & # 39; 39; was not available before for this application. This is the first vector holographic trap.

"Previously, holographic traps were limited to certain clbades of light (scalar light), so it is very exciting to be able to reveal a holistic device covering all light clbades, including replication of all previous trapping devices." said Professor Andrew Forbes, collaborating leader and distinguished professor of the faculty of physics where he directs the Wits Structured Light Lab.

"What we did, is present the first vector holographic optical capture and pinching system.The device allows particles of the size of a micrometer, such as biological cells, to be used. to be captured and manipulated only with light. "

The final device could trap multiple particles at once and simply move them with vector light states. The experiments for this study were done by Nkosi Bhebhe as part of his PhD studies. The work is published in Nature's online journal, Scientific reports.

In conventional optical trapping and optical pinching systems, the light is very narrowly focused in a small volume containing small particles, such as biological cells. On this small scale (typically micro or nanometers), the forces that light can exert are important, so that particles can be trapped by light and then controlled. When the light is moved, the particles move with it. This idea has earned the American scientist Arthur Ashkin the 2018 Nobel Prize in physics. Originally, the light was mechanically controlled using decks and mirrors, but the idea was improved by a holographic shift of light, ie by the use of computer-generated holograms to control light without moving parts, thus controlling particles. Until now, only special clbades of laser beams, called scalar beams, could be used in such holographic traps.

In their article entitled A Vector Holographic Optical Trap, Wits researchers have shown how to create and control any type of light by holography, before using it to form a new optical trapping and pinching device.

"In particular, the device could work with traditional laser beams (scalar beams) as well as with more complex vector beams." The vector beams are very current and have found many applications, but no vector holographic trap. it was possible until now, "explains Forbes.

Wits researchers demonstrate their new trap by holographically controlling scalar and vector beams in the same device, advancing the state of the art and introducing a new device to the community. The group expects the new device to be useful for controlled experiments in micro and nano-worlds, including unicellular studies in biology and medicine, small volume chemical reactions, fundamental physics and future physics. devices on a chip.

Having already shown that it is possible to create hundreds of custom light patterns from a hologram, the research works bring together their earlier work on the holographic control of light with the application of trapping optical and optical pinch.

How does a holographic optical trap work?

In a conventional optical trap, the light is very closely focused to be able to exert forces on the material. Matter, say a small particle, remains trapped in the light. When the light is moved by mirrors or mechanical stages, the particle moves with it. This is called optical trapping (capture of the particle) and optical pinching, moving the particle as with tweezers, but in this case, a tweezers made of light. To make control less mechanical, researchers used holograms to control light. With spatial light modulators, it is possible to encode structured light patterns and move them within the trap, so that many particles can be trapped and moved simultaneously. This opened up many exciting new fields, but the latest holographic optical (HOT) traps were limited to scalar light beams, a tiny fraction of what was possible. The other clbad of optical beams, the vector states, was found to be impossible to control by holography. With the new HOT vector, all light states can now be used. Time will determine what it means for the community in general.