Force analysis of optical fiber tweezers with different endface configurations

The optical tweezers (OT) use optical force to trap micro/nano particles. This technology has widely been employed in manipulating cells, viruses and atoms, etc. Optical fiber tweezers (OFT), as a novel type of OTs, possess excellent characteristics of easy fabrication, strong anti-interference ability, high compatibility with chip devices, and flexible operation with a compact structure. Usually, the OFT probes are tapered, but they have a low optical trap strength, which makes it difficult to capture smaller particles (sub-micron size). In this study, we demonstrate theoretical analyses of the fields and optical forces with different fiber structures, including the tapered, spherical, hemispherical, zone plate, and gold-overlapped endface. Numerical simulations are carried out to investigate the magnitude and direction of the optical force exerted on microparticles in water. The results reveal that the shape of the fiber endface structure significantly alters the distribution of the optical field. The hemispherical and tapered endface have a similar ability to trap microparticles. The spherical and zone plate endface is better than the two endface mentioned above. Meanwhile, the use of gold-overlapped fiber tips can strongly constrain the dielectric microparticles on the endface, achieving effective two-dimensional trapping. This research offers theoretical guidance for designing and optimizing fiber endface structures for efficient optical micro-manipulations.