to a diffraction-limited spot (see fig. 1). When a small dielectric object such as a
vesicle, lipid droplet, or polystyrene or silica bead is placed in the beam, it feels a force
pulling it towards the beams’ focus. The magnitude of the exerted force is a function of a
number of parameters: the size and shape of the object, the difference between the index
of refraction of the object and the surrounding medium, and the gradient in the intensity
of the laser beam. The Force, F, the trap exerts on an object is a linear function of the
object’s displacement, x, from the trap center, i.e. F=-kx, where k is the trap stiffness.
Deviations from this linear relationship are observed at the very edges of the beam,
typically approximately 200 nm from the beam’s center. The applied force increases
linearly with laser power. The significance of object size and index of refraction
differences are discussed below. Typically, a force of 10’s of pN’s can be applied to a
small (half micron) bead, though it is possible to exert more than 100 pN of force on a
few μm bead using a high-powered laser. While in principle any laser can be used to
make an optical trap, in practice infra-red lasers are employed, because they cause the
least optical damage