Particle manipulation using ultrathin optical fibres

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Maimaiti, Aili
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University College Cork
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Optical manipulation in the vicinity of ultrathin optical fibres has shown potential across several fields including control and delivery of dielectric, metallic and biological microscopic objects, and cold atom probing and trapping. The unique properties of ultrathin fibres, such as strong field confinement, large evanescent fields in the transverse plane, and interaction lengths greater than the Rayleigh range, are key factors needed for the study of fibre-based lightmatter interactions. Differences in the evanescent fields between the various tapered fibre modes give rise to differing interactions with particles. In this thesis, we studied the propulsion of polystyrene particles (diameters of 1 μm ~ 5 μm) in the evanescent field of higher order modes and the fundamental mode in aqueous solutions. For a power of 25 mW, particles were propelled eight times faster in the case of the higher order modes when compared to the fundamental mode. The dependency of particle speed on particle type was also investigated. We further explored the longitudinal optical binding interactions of chains of 3 μm polystyrene spheres in the evanescent fields of the fundamental and the higher order modes of a microfibre. Using a theoretical model based on a scattering-matrix approach, the optical forces, optical binding interactions, and the velocity of bounded particle chains were calculated and compared with experimental observations. Additionally, we examined the transfer of angular momentum of the fundamental mode and the higher order modes of a microfibre on polystyrene microparticles. For a fundamental mode with circular polarisation, the transfer of spin angular momentum was evident via the rotation of particles around the fibre. Furthermore, we investigated the angular momentum of selectively excited higher order modes and its transfer to trapped particles. The results are useful in understanding the evanescent behaviour of different tapered fibre modes.
Optical trapping , Tapered fibre , Fibre modes , Angular momentum , Higher order modes , Evanscent fields
Maimaiti, A. 2017. Particle manipulation using ultrathin optical fibres. PhD Thesis, University College Cork.