For measuring force one needs two things: a spring and calibration. Both are provided by our “dual beam optical tweezers setup”.

Due to momentum conservation it is possible to trap microscopic particles using sharply focused laser beams. The object trapped in the laser focus obeys Hooke’s law, where the restoring force felt by the object is linear to ist displacement from the trap centre. Thus, the focused laser beam can be seen as a spring for microscopic objects.

We use this effect to trap two 1µm-diameter silica beads, functionalized in such a way that we can hold a single protein molecule between them. By moving one of the laser beams by very small, accurately-controlled nm-scale distances, we can apply piconewton (pN) forces to individual protein molecules.