Abstract

Laser tweezers applications are extremely challenging in turbid biological media and in thick samples, as it is very difficult to achieve a tightly focussed laser beam, which is needed for optical trapping [1]. This problem could be solved using optical fibers to carry the light close to the object to be trapped. Moreover, fiber-based optical tweezers could be easily integrated to a standard microscope, increasing the ease and the versatility of use of the optical tweezers. Nevertheless, up to nowadays, three dimensional (3-D) trapping using a single fiber and all-optical forces has never been reported. The main limitation comes from the fact that the numerical aperture (N.A.) of the fiber is usually quite low: considering the small refractive index difference between the fiber and the surrounding medium (usually water in biological applications) the gradient forces that can be generated in the focal region, by creating a hemispherical microlens on standard axial core fibers, are not sufficiently intense to trap the particle by counterbalancing the axial scattering forces [2]. Here we present a new, simple and low-cost, approach that allows obtaining an all optical 3-D single-fiber tweezers. The above mentioned limitations are overcome by using an annular core fiber (or a multifiber system characterised by an annular distribution of the cores) and by properly shaping the fiber-end, as a truncated cone. Exploiting the total reflection at the interface between the fiber and the surrounding medium, beam convergence at considerable distance from the fiber end and with high numerical aperture is achieved (Fig. 1).

© 2007 IEEE