Optical tweezers are a top-end research tool that employ a highly focused laser beam to trap and manipulate incredibly small particles. Once exclusive to physics, optical tweezers are now widely used to study biological systems and are perhaps best known for their role in determining DNA’s elasticity.
Arthur Ashkin first described optical tweezers in a paper in Physical Review Letters in January 1970: “Micron-sized particles have been accelerated and trapped in stable optical potential wells using only the force of radiation pressure from a continuous laser.”
Capabilities and Applications of Optical Tweezers
His optical tweezers were capable of holding microscopic particles stable in three dimensions. Ashkin, who won the Nobel Prize in Physics in 2018 for this work, predicted that similar acceleration and trapping would be possible with atoms and molecules using laser light tuned to specific optical transitions, with implications for isotope separation.
By 1986, Ashkin’s colleague Steven Chu was using optical tweezers in his work on cooling and trapping neutral atoms, work which won him a share of the Nobel Prize in Physics in 1997. By the late 1980s, optical tweezers had been utilized in biological sciences to trap tobacco mosaic virus and E. Coli in aqueous solution with no damage from the laser.
Since their inception, optical tweezers have found many uses, including in synthetic biology to construct tissue-like networks of artificial cells and to fuse synthetic membranes together; in cell sorting on the basis of a cell’s intrinsic optical characteristics; to probe the cytoskeleton, measure visco-elastic properties of biopolymers and study cell motility. They have also been utilized in research on single-molecule force measurements, to study DNA and RNA and may have uses in determining blood type or guiding sperm towards the egg in in vitro fertilization.
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Image Credits: Block Lab at Stanford University
Fundamentals of Optical Tweezers
Optical tweezers work on the basis of momentum transfer associated with light. Light carries a momentum proportional to its energy and direction of movement, any change in direction caused by reflection or refraction leads to a change in momentum of light. Light at the center of the beam is brighter than at the edges and when it interacts with a particle the light bends. It creates a scattering force in the direction of the laser, and a gradient force in the XY plane towards the center of the beam. If the scattering force is larger than that of the reflected rays the result is a restoring force along the z-axis and a stable optical trap.
The tweezers are often built by modifying a standard optical microscope; the basic setup includes:
- A laser, normally neodymium-doped yttrium aluminum garnet (Nd:YAG)
- A beam expander
- Optics to steer the beam location in the sample plane
- High-quality microscope objective and condenser to create a trap in the sample plane
- Position detector to measure beam displacement
- Microscope illumination source coupled to a camera.
The tool is able to manipulate nanometer and micron-sized dielectric particles by exerting extremely small forces via the highly focused laser. The laser beam is focused by the microscope objective to a spot in the sample plane; this spot creates an optical trap which can hold a small particle in the canter. The particle is pushed by the momentum from the laser beam and trapped by gradient forces of the beam’s refracted light. Trapping forces upon the particle include light-scattering and gradient forces due to the particle’s interaction with the light. These forces act against thermal fluctuations and in biological samples, motility.
Shortcomings
In practice, optical tweezers are expensive and often need to be custom built. Although they might start with a commercial optical microscope, they have extensive add-ons to turn them into the sensitive tools they need to be.
Optical tweezers have advanced from a simple tool to manipulate micron-sized objects to an exceedingly sophisticated device under computer control which can measure displacement and forces with high accuracy and exactness.
Sources and Further Reading
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