Optical Tweezers

Optical tweezers are a powerful tool for manipulating microscopic particles such as glass or latex beads, but also biological cells. Individual molecules can be manipulated by attaching a handle, usually a polystyrene bead. When a particle is forced out of the center of the tweezers the restoring force increases linearly with the distance from the center (up to distances of ~200 nm). The optical tweezers act like a spring, with a spring constant that depends on the laser intensity, but which is usually in the range of 0.01 - 1.0 pN/nm. pN-sized force exerted on a particle can be measured by determining the position of the particle in the tweezers.

In addition to measuring forces, the optical trap can also be used to exert very precisely controlled forces on single molecules. The figure on the right shows a molecular force clamp, a feedback-controlled optical tweezers that maintains a constant force on a single moving kinesin molecule by keeping the attached bead at a fixed distance from the center of the tweezers.

Watch a movie (~8.5 MB) of a force clamp in action.

How do optical tweezers work?

Although a ray diagram-based explanation is not strictly correct it can provide intuitive insight into how tweezers work. Refraction of light at the bead causes a change in momentum of the light (the direction of the outgoing ray does not equal that of the incoming ray), which results, by Newton's Law, in a force on the particle. By sharply focusing the laser beam a change in momentum can be achieved such that the particle is pulled against the direction of propagation of light towards the focus such that it becomes trapped at a position near the focus.

Watch a movie (~8 MB) of beads trapped in multiple optical tweezers created by rapidly scanning a single trap along different locations.

Other movies:
Isolation of Yeast Cells (~28 MB)
3D manipulation of two beads in two traps (~5 MB)