Single beam trap

The single beam trap (SBT) is now a classical tool to trap objects of diameters from tens of nanometers to tens of micrometers. SBT is used in a very popular optical micromanipulation tool - optical tweezers. 

Principles

SBT uses single tightly focused laser beam that provides 3D confinement of dielectric particle only if its refractive index is higher than refractive index of the surrounding medium (usually water). Due to the tight focusing the spatial field distribution is highly inhomogeneous and it causes strong optical forces (frequently called gradient or dipole forces) in radial and axial directions pushing the object to high intensity part of the field. 

The total effect of absorption of many photons from one direction and their scattering to all directions push the object in the direction of the  incident beam. This effect is also called radiation pressure and the force is called scattering force. 

Only if the axial gradient force is stronger than the scattering one, the total force done by sum of them is equal to zero and creates equilibrium position where the object is trapped. 

Experimental set-up

The smallest spot of the beam focus is obtained if a lens with the highest numerical aperture is used. Therefore oil or water immersion microscope objective is usually used. 

At the same time the diameter of the laser beam entering the objective must as wide or even wider as the exit aperture of the objective. Therefore, the beam coming out of the laser must be enlarged by a telescopes. Movable lenses of one or two telescopes can provide axial and radial positioning of the optical trap and consequently also the manipulation with the trapped object. 

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Last modification: 30 Mar 2007