Abstract:We demonstrate how a Y-branched optical waveguide can be used for microparticle sorting. Polystyrene microparticles, optically guided in the waveguide's evanescent field, are directed down the desired, more strongly illuminated, output branch. The output of a fibre laser at a wavelength of 1066 nm is coupled to the waveguide by direct butting. The power distribution between the two output branches is selected by the relative position of the fibre to the waveguide input facet. This provides a simple method for reliable particle sorting with very high probability of success under appropriate conditions. The method can be easily combined with other particle manipulation techniques of interest for micro total analysis systems of the future.

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Abstract:We study the formation and the propulsion properties of chains of dielectric microspheres in the evanescent field of a channel waveguide made by Cs + ion-exchange. Particle chains are shown to move faster than single particles. We exploit counter-propagating waves for axial positioning of single and chains of microspheres. The particles can be propelled back and forth at will, and trapped at a given point for several minutes. We demonstrate that this technique can also be used to assemble a long, one-particle wide, chain.

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The velocity distributions of 250nm diameter gold nanospheres trapped in the evanescent fields of optical waveguides are studied. The automated analysis of a large number of particles and temporal frames is described. It is used to show that the envelope of the particles' speed follows the mode intensity profile of the evanescent field along a length of the waveguide and across its width. Modal beating in a dual-moded waveguide is mapped by analysis of nanoparticle distributions above the waveguide. A modal power of ∼150mW at λ =1066nm in a Cs + ion-exchanged monomode waveguide results in speeds of up to 500µm/s.