Manipulation of particles on optical waveguides

Abstract

A theoretical and experimental study on the optical trapping and propulsion of latex, gold aggregate and colloidal gold particles with average radius of 1.5µm, 250nm and 10nm respectively, in the evanescent region of an illuminated ion-exchange channel waveguide is documented in this thesis.
Optimisation of light-induced forces exerted on a particle on a waveguide relies on two important factors, firstly a maximisation of the intensity and intensity gradient present in the guide-cover interface and secondly, an optimisation of the polarizability of a particle. To this end, a transcendental equation was established and was used to generate design curves for the normalised waveguide thickness required for achieving a maximum gradient force on the guide-cover interface of a waveguide for a specific set of indices.
A study based on Mie theory for the investigation of morphology dependent resonance exhibited by a spherical particle is described. The dependence of resonances on particle radius, index of the sphere with respect to the surrounding medium, absorption, plasmon resonance and symmetry of the incident beam has been investigated. In particular, a simplification of the Mie model was carried out to derive Rayleigh expressions of cross sections from which particle polarizability originates. The validity of the Rayleigh model was assessed with respect to the limiting particle radius.
Based on a semi-classical approach, a derivation of light-induced forces applying to a Rayleigh sphere in the cover region of a waveguide is detailed. The three main optical force components produced are (i) a forward scattering and absorption force due to the intensity of the incident radiation which accounts for propulsion of particles, (ii) a transverse gradient force due to an intensity gradient generated by a decaying evanescent field and finally (iii) a lateral gradient force which arises from the near-Gaussian intensity distribution on a channel waveguide. A comparison of the relative magnitude of each component is described, with additional forces due to gravity, buoyancy and Brownian motion studied.
Factors affecting the propulsion of a gold nanoparticle were investigated. It was shown that the particle velocity is linearly dependent upon the waveguide modal power, increases with a wavelength closer to plasmon resonance in the case of a Rayleigh gold particle, is stronger for TM polarized light, increases with a larger change in the waveguide refractive index and is maximum for a minimum modesize.
For the first time, under the action of light-induced forces generated on the surface of an optical waveguide, colloidal gold particles are propelled in the direction of wave propagation reaching at a maximum velocity of 10µm/s for a modal power of 500mW at lambda=1.047µm. Results obtained will be useful for future applications in particle sorting, fluorescence sensing and surface enhanced Raman sensing of chemical species.

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