Optical sorting and manipulation of microscopic particles

Abstract

Over the last few decades, the use of light to control and manipulate microscopic particles has become widespread. These methods are enabling new areas of research to flourish across the physical and biological sciences.

This thesis describes investigations into both optical trapping and the closely related field of optical sorting. It documents the development of a variety of new techniques.

The thesis begins with a short review of optical trapping and existing methods for sorting mixtures of microscopic particles. The first half of this chapter highlights some of the reasons behind optical trapping's rapid growth in popularity. By reviewing an array of methods for sorting particles and discussing the relative merits of each, the case for optical sorting is established.

The second chapter describes research into using a spatial light modulator to create three-dimensional optically trapped colloidal structures using the time-sharing technique. Limiting factors inherent in the technology are discussed in detail.

The third chapter reviews a sophisticated particle-tracking software package that has proved to be a considerable success. It was developed explicitly with colloidal microscopy in mind and experimental plots produced by the software are used throughout the thesis.

Experimental studies have been performed into the behaviour of microscopic particles moving under the influence of two classes of propagation-invariant beams: Mathieu beams and Bessel beams. The Bessel beam studies have been complimented by a theoretical model and have led ultimately to a new method for the static optical sorting of both solid particles and biological cells, with particular emphasis on human blood.

The fifth and final chapter describes how re-configurable optical devices can be implemented to spatially separate different colloidal species. A new method for creating arbitrary optical landscapes using an acousto-optic modulator is reported. This new technique is then used to optically sort four particle species simultaneously - the first experimental demonstration of polydisperse optical fractionation. Additionally, experiments are reported that demonstrate controlled, static optical sorting using a spatial light modulator.