Optical Trapping


It was known from physics and the early history of optics that light carries linear and angular momentum, and therefore exerts radiation pressure and torques on physical objects. However, its strength was not recognized until recently to be large enough for practical uses. With the advent of laser radiation, Arthur Ashkin showed, in 1970, that one could use the radiation pressure from laser beams to significantly affect the dynamics of small transparent micro and nanometer sized neutral particles. In 1986, A. Ashkin and S. Chu managed the trapping of neutral particles with a focused laser beam, giving birth to the optical tweezers technique and enabling the optical manipulation of atoms, molecules, cells and nanoparticles.


A laser can be viewed for many purposes as a gun, shooting photons in almost linear trajectories. These light bullets go through the physical objects pushing or modifying their trajectories in complex ways. By focusing the light rays it is possible to trap a neutral particle in three dimensions. More in depth, the total force on a particle can be split into two main components: the scattering force, which pushes the particle along the incident beam, and the gradient force, which points towards the centerline of the beam and is responsible for the trapping.

Optical forces on a neutral sphere. Left diagram illustrates the origin of the scattering and gradient components of the optical force. Right diagram shows the axial stability generated by the gradient force along the centerline of the beam, towards the focal region.