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New optical capture and tweezing devices for manipulation, moving tiny objects

When you light a beam of light on your hands, you do not feel much, except for some heat generated by the beam. When you shine the same light into a world that is measured on a nano or microwave, light becomes a powerful manipulation tool that you can use to move objects around – safely caught in the light.

Scientists from the group of structured light from Physics School at Witwatersrand University in Johannesburg, South Africa, have found a way to use a full beam of laser light, control and manipulate miniature objects such as individual cells in the human body, small particles in small volume chemistry, or work on future devices on the chip.

While the specific technique, called holographic optical capture and tweezing, is not new, Wits scientists have found a way to make the most of the full power of light – including vector light that was not available for that application before. This is the first vector holographic passport.

"The earlier holographic traps were limited to certain light classes (scalar light), so it is very exciting to discover a holistic device that covers all light classes, including the replication of all previous hunting equipment," Professor Andrew Forbes, Team Leader and Major a professor at the Physics School, where she is headed by the Wits Structured Light Laboratory.

"What we have done is that we have demonstrated the first vector holographic system of optical capture and tweezing." The device allows micrometre-sized particles, such as biological cells, to be captured and manipulated only by light. "

The end device can capture multiple particles at once and displace them only with vector light conditions. Nkosi Bhebhe's attempts at this study were conducted as part of a PhD study. The work is published in the online journal Nature, Scientific reports.

In conventional optical capture and tweezing systems, light is very strongly focused on a small volume that contains small particles such as biological cells. At this small scale (typically a micro or nanometer), the forces that the light can develop are significant so that the particles can be captured by the light and then checked. As the light moves, the particles move with it. This idea was won by American scientist Arthur Ashkin in 2018 in the Nobel Prize in Physics. Originally, the light was mechanically controlled by degrees and mirrors, but the idea was later improved by the fact that the light moved holographically, that is to say using computer-generated holograms to control light without moving parts, thereby controlling the particles. Until now, only the special classes of laser beams, called scalar rays, have been used in such holographic traps.

In his paper entitled: Vector Holographic Optical Trap, Wits scientists have demonstrated how to create and control any light pattern holographically, and then use them to create new optical capture and tweezing.

"The device could work both with traditional laser beams (scalar beams) and with more complex vector beams. Vector beams are highly up to date and have found many applications, but no vector holographic trap has ever been," says Forbes.

Wits scientists demonstrate their new trap by controlling both scalar and vector beams in the same device, improving the state of the art, and introducing new devices into the community. The Group expects the new facility to be useful in controlled experiments in microprocesses and nano-worlds, including single-cell studies in biology and medicine, small volume chemical reactions, basic physics, and future chip devices.

Previously, hundreds of light samples from one hologram could be created, and research links their previous work on holographic light control using optical capture and tweezing.

How does a holographic optical trap work?

In a conventional optical trap, the light focuses very tightly so it can develop forces on the mass. A thing, say, a small particle, caught in the light. When the light moves with mirrors or mechanical degrees, it moves with it. This is referred to as optical capture (particle capture) and optical tweezing, moving particles as in tweezers, but in this case tweezers from light. To make the controls less mechanical, scientists used holograms to control the light. With spatial light modulators, it would be possible to encode structured light patterns and move these patterns inside the trap so that many particles could be trapped and shifted simultaneously. This has opened up many new exciting fields, but the final holographic optical traps (HOTs) have been limited to scalar light rays, which is only a small fraction of what was possible. The second class of optical beams, vector states, was not considered to be holograms. All the light conditions can now be used with the new HOT vector. Time will be what it means for the entire community.


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