Abstract
Particles trapped in the evanescent field of an ultrathin optical fibre interact over very long distances via multiple scattering of the fibre-guided fields. In ultrathin fibres that support higher order modes, these interactions are stronger and exhibit qualitatively new behaviour due to the coupling of different fibre modes, which have different propagation wave-vectors, by the particles. Here, we study one dimensional longitudinal optical binding interactions of chains of 3 μm polystyrene spheres under the influence of the evanescent fields of a two-mode microfibre. The observation of long-range interactions, self-ordering and speed variation of particle chains reveals strong optical binding effects between the particles that can be modelled well by a tritter scattering-matrix approach. The optical forces, optical binding interactions and the velocity of bounded particle chains are calculated using this method. Results show good agreement with finite element numerical simulations. Experimental data and theoretical analysis show that higher order modes in a microfibre offer a promising method to not only obtain stable, multiple particle trapping or faster particle propulsion speeds, but that they also allow for better control over each individual trapped object in particle ensembles near the microfibre surface.
Highlights
Attractive alternatives[18,19,20]
Preliminary experimental studies on the interaction of nanofibre higher order fibre modes (HOMs) with cold atoms[35] and particle manipulation[36] have been published. These works illustrate the advantages of HOM-supporting MNFs, such as achievable higher field amplitudes, larger field extensions from the fibre surface, and a larger fibre taper cut-off diameter compared to that needed for fibre mode (FM) propagation
While previously published work has shown that the speed of single particles for HOM compared to FM propagation is increased[36], our recent study on two-particle binding in a HOM37 field manifested new phenomena - the speed of two coupled particles and their inter-particle distances are clearly different from those of the fundamental mode case
Summary
Optical binding forces between trapped particles within a chain. In Fig. 2 we present the optical binding forces and their corresponding potentials as a function of the interparticle separation, d, for both the FM and HOM cases. We can make initial conclusions here that a pair of particles trapped in the HOM evanescent fields in a microfibre system (i) shows shorter stable interparticle distances, (ii) creates deeper potential wells along the fibre waist, and theoretically, (iii) exhibits stronger optical binding forces between the particles than compared to the FM propagating fields. Equivalent force lines for a three-particle chain formation as a function of the two independent interparticle distances in the FM (a) and HOM (b) evanescent fields Intersections of these equal force contours indicate the equilibrium configurations but only those denoted by the blue circles are under stable conditions. This smaller ratio could explain the case in which particles sometimes escape from a stable configuration in the HOM field This implies that, a stronger optical binding force is observed for the HOMs, it is still easier to control each individual particle within the particle chain using the scattering force. This study could be very useful for applications such as atom trapping and nanoparticle trapping, etc
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