The control of gold and latex particles on optical waveguides

Abstract

The trapping of microparticles by optical methods using a focussed laser beam, known as optical tweezers, has evolved rapidly in the last thirty years. However this process is limited to the trapping of a small number of particles. Evanescent wave trapping allows simultaneous trapping of many particles due to the long length over which a strong intensity gradient is present. A channel waveguide used to produce such an evanescent wave can be photolithographically defined on a flat substrate and thus can be integrated with other micron scale processes. This therefore has potential applications in the lab-on-a-chip field that is currently proving so successful, particularly in biochemical areas where evanescent wave manipulation is ideal for its properties of being cheap, robust, contaminant-free and designed for use with aqueous solutions.

This thesis describes both a theoretical and experimental study into the optical trapping and propulsion of gold nanoparticles and latex microparticles above caesium ion-exchanged waveguides. Gold particles of radii varying from 50nm to 250nm and latex particles varying from 1.5µm to 7.5µm were propelled above a waveguide in an aqueous medium. The evanescent field of the channel waveguide was used to both optically trap and propel these particles and speeds of up to 500µm/s were achieved, a full order of magnitude faster than has previously been reported.

The optical forces on the particles were derived and used to predict the trapping ability and the speed of particles and physical, electric, and thermophoretic forces, that also affect the particles were described. In addition, modelling allowed the theoretical optimisation of the waveguides for this process.

Using a counter-propagating wave it is demonstrated that it is possible to both render particles stationary and position them at any point along the waveguide. In addition, devices were fabricated that allow particles to be automatically sorted down either branch of a Y-junction waveguide. These results demonstrate, that evanescent wave based, integrated optical devices for trapping are feasible and it is anticipated that this will lead to devices for real-life applications being realised.

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Identifiers

ORCID for James Wilkinson: orcid.org/0000-0003-4712-1697

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