Generating a stationary infinite range tractor force via a multimode optical fiber

TL;DR

This paper demonstrates how multimode optical fibers can generate a stationary tractor force on particles, enabling long-distance manipulation and assembly of particles using light-induced forces.

Contribution

It extends a scattering matrix approach to include multiple fiber modes, revealing conditions for stationary tractor forces and improving predictions for inter-particle forces in nanofiber fields.

Findings

Higher order modes induce tractor forces against injection direction.

Mode coupling dominates over backscattering and losses for tractor force.

Strong tractor forces can act on optically bound particle arrays.

Abstract

Optical fibers confine and guide light almost unattenuated and thus convey light forces to polarizable nano-particles over very long distances. Radiation pressure forces arise from scattering of guided photons into free space while gradient forces are based on coherent scattering between different fiber modes or propagation directions. Interestingly, even scattering between co-propagating modes induces longitudinal forces as the transverse confinement of the light modes creates mode dependent longitudinal wave-vectors and photon momenta. We generalize a proven scattering matrix based approach to calculate single as well as inter-particle forces to include several forward and backward propagating modes. We show that an injection of the higher order mode only in a two mode fiber will induce a stationary tractor force against the injection direction, when the mode coupling to the lower order mode dominates against backscattering and free space losses. Generically this arises for non-absorbing particles at the center of a waveguide. The model also gives improved predictions for inter-particle forces in evanescent nanofiber fields as experimentally observed recently. Surprisingly strong tractor forces can also act on whole optically bound arrays.