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Curated by RSF Research Staff

An approach to manipulate small objects with light

Suspected from the outset Kepler’s observations of comet tails, the fact that light exerts forces on matter, and therefore on objects is now well established. Thanks to the work of Arthur Ashkin among many others, optical traps are now a reality. Using laser beams optical levitation of microspheres is used nowadays in many applications from stretching DNA to nanotechnology, spectroscopy, stochastic thermodynamics and critical Casimir forces.

Structuring light makes optical manipulation techniques possible, like using the Spatial Light Modulators (SLMs) to produce holographic optical traps (HOTs). These Spatial Light Modulators are liquid crystal technology with a fast and precise control of the beam shape used to control multiple particles in 2D and 3D configurations.

Previously holographic traps were limited to particular classes of light (scalar light), so it is very exciting that we can reveal a holistic device that covers all classes of light, including replicating all previous trapping devices.

For a holographic optical tweezer, an essential element is the Spatial Light Modulators trapping beams with a low incidence conditions. The optical tweezers use forces exerted by the focused beam to trap and move the matter. This technique allows computer-generated holograms to dynamically reconfigure a single optical tweezer into hundreds of independent optical traps.

 

The invention of holographic optical tweezers is allowing simultaneous manipulation of many particles, traditionally done with arrays of scalar beams.

Recently, a team from the Structured Light group at the University of the Witwatersrand in Johannesburg, South Africa, have published a paper on how to use a laser light, to control and manipulate minute objects such as single cells in a human body, tiny particles in small volume chemistry, or working on future on-chip devices. They demonstrated the concept of a holographic optical trap for the delivery of 2D arrays of traps using Higher-Order Poincaré Sphere (HOPS) beams.

Optical tweezers configurations used to study RNA polymerase. A schematic of a dual-trap configuration is shown. A RNA polymerase–DNA complex is trapped by two optical traps simultaneously.

What we have done is that we have demonstrated the first vector holographic optical trapping and tweezing system. The device allows micrometer sized particles, such as biological cells, to be captured and manipulated only with light.

- Professor Andrew Forbes, University of the Witwatersrand

The researchers employed a simple set-up using a spatial light modulator and showed that each beam in the array can be manipulated independently and set to an arbitrary HOPS state, including replicating traditional scalar beam HOTs. They demonstrated trapping and tweezing with customized arrays of HOPS beams.

 

By Dr. Olivier Alirol, RSF Research Scientist

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