Since then the technique of optical laser trapping has been used for many applications in biology spanning the range from trapping and moving cells and bacteria to measuring the minute forces acting between single molecules. One particularly well studied system is the force generating interaction of actin and myosin in muscle. In order to understand these precise, single molecule measurements one has to look at the nature of optical forces.
Optical forces are very small and are in the range of picoNewton.
There are two major forces acting on a particle in an intense laser beam which can result in stable trapping when they both are equilibrated. The more obvious one is the scattering force, which is due to the scattering and absorption of photons by the particle. The force is proportional to the polarizability of the particle and to the intensity gradient. To achieve a steep intensity gradient the laser beam is focused through a microscope objective of high numerical aperture to a diffraction limited spot. This marks the condition for a high intensity electromagnetic field right in the focus of the objective with a rapidly decaying intensity profile in its vicinity.