Optical Tweezers & Micromanipulation

The usage of strongly focused light to trap particles was first demonstrated by Ashkin in 1986.
Since then, optical tweezing has become popular in many application fields.
In recent years new methods to create the optical traps employing various types of diffractive elements have been shown.
One method which offers the most flexibility is by using LCOS - Spatial Light Modulators.

This method provides flexibility and simplicity in creating multiple traps from a single laser beam. The input phase holograms are changed in real time. This allows trapping and moving of multiple microscopic (10 nm – 50 µm) particles simultaneously. Using multiple Computer Generated Holograms (CGH) allows 3D optical micro-manipulation and can be used for studying biological samples (DNA, unicellular algae), stem cells and carbon nanotubes among others.
As well as using more traditional Gaussian Beam profiles, new trap geometries have been created using novel light modes such as Laguerre-Gaussian or Bessel Beams.Furthermore, excellent optical systems can be realized by correcting all of the optical aberrations in parallel within the experimental setup.

Recently it was shown that Holographic Optical Tweezers (HOT’s) have also been used in quantitative experiments in precise force measurements.

Pointing stabilities of the LCOS-SLM created traps of better than 1 nm ["Positional stability of holographic optical traps", Opt. Express 19, 21370-21384 (2011)] and are now as efficient to that of regular non-holographic optical traps.

The Optical Trapping community report new applications such as in the field of Microfluidics, Cell sorting (FACS), Atom-Trapping, Femto Newton-Force measurements, DNA stretching and Molecular motors among others. Please see attached link to the report for further details: Optical Tweezers

Available software such as Labview or Matlab calculates the traps in real time and easy control has been made possible by using tablet computers such as the ipad or force-feedback joysticks.

An example of a typical optical configuration of a Holographic Laser Tweezers experiment is shown below along with an example of how individual particles can be independently controlled.


Advantages SLM can offer and why

The benefits that Spatial Light Modulators offer in the field of optical trapping are:

Multiple traps

No time-sharing/multiplex of the laser beam

Fully 3D trap positioning

Trap geometries with novel light modes such as Laguerre-Gaussian, Bessel and non-diffracting Airy-Beams as well as Optical Vortices

Simultaneous compensation of wave front aberrations in the optical setup

Features of the Hamamatsu X10468 LCOS-SLM Series

High laser power handling capability Enables the creation of numerous traps, no time-multiplexing
High reflectivity up to 95% High efficiency
High pixel fill factor resulting in high diffraction efficiency Fewer diffraction losses from the surface, excellent quality of novel light modes
Very low phase noise Pointing stability of less than 1 nm, stable trapping forces and precise force measurements
Linear phase response and corrected wave front response Easy implementation, no calibration needed

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