A small glass bead (4.5 microns in diameter) can be trapped by a focussed laser beam, as originally shown by Ashkin and Dziedzic (Appl. Phys. Lett. 19, 283 (1971)). We have realised a trap using two counterpropagating beams from optical fibres, as in Constable et al. (Appl. Opt. 36, 6423-6433 (1997)). A schematic of the trap is shown below.


A photograph of the fibre ends meeting is shown below. The fibre core is 125 microns in diameter.

Fibre trap

A video of a microsphere trapped between the optical fibres is shown in “Bare Fibre trap”. The image is of the trapping light (1064nm) scattering from the sphere. You can also see light scattering from the end of one of the fibres. This suggests that the and of the fibre is not perfect, a problem when using this method. The movement of the bead is partly due to laser power fluctuations, but mainly due collisions between the bead and air molecules, i.e. Brownian motion. This scheme has been used to study Brownian motion by the Raizen group (Science 328, 5986 (2010)).

By varying the balance of power between the two trapping beams the bead can be moved side to side, as shown in “Bare fibre move”…

…we have even trapped two spheres at once!