Abstract

The laser beam trapping method is considered to be useful for manipulation of microscopic objects and biological cells because of noninvasivc technology. Its operation principle is based upon the electromagnetic momentum preservation. Generally, the force on a micro-sphere divides itself naturally into two components which always act through the center of the sphere: one in the axial direction of the light, denoted by F_ax, and the other a transverse force, denoted by F_tr [1]. Wright et al. have developed an analytical method[1] that accurately described the forces exerted on a dielectric micro-sphere and showed that when the spot size at the beam waist was larger than 0 7 µm, for example, an optical trap was not created at all. The same phenomenon could be seen in our trapping experiments using a tapered hemispherically lensed optical fiber A sphere was always pushed in the direction of the beam away from the fiber end independent of the distance between the fiber end and sphere, because the beam emerging from optical fiber end could not be strongly focused In the fiber trapping system, the focused spot size at the beam waist was ~6 µm in ethanol solution, which was measured using an 1R camera. On the other hand, we have already developed an optical fiber trapping system[2] and verified that optical trapping and manipulation of a micro object were easily achieved by a focused laser beam emerging from an optical fiber inserted into a sample chamber at angle of 35 degrees. In this paper, we investigated the optical forces acting on a micro-sphere to corroborate the optical trapping using a tapered hemispherically lensed optical fiber. A semiconductor laser module at 1.48 µm was used for experiments.

© 2000 IEEE