Microscopic applications of holographic beam shaping and studies of optically trapped aerosols

Abstract

This thesis has two themes. Firstly, it concerns the original application of holographic beam shaping, employed through the methods associated with optical manipulation, to three microscopic fields of research. Secondly, it studies the optical trapping of aerosol droplets through experimentation and computational modelling.

The aims are to not only provide an account of the work carried out but also a base for future researchers and students.

Chapter 1 provides an introduction to the field of optical manipulation and the relevance of my studies. Chapter 2 outlines the construction of an optical tweezers which is the basis of advanced experimental work described in later chapters. It also overviews how optical tweezers operate and are quantified.

In chapter 3 I describe how beam shaping is implemented for my investigations with a spatial light modulator and phase-only holograms. I detail the algorithms and software written before discussing their performance and finally the optimisation of the apparatus.

Chapter 4 describes three original applications of beam shaping, including the trapping and coagulation of multiple aerosols, the manipulation of filamentous fungi hyphal tips and novel digital microfluidic operations using thermocapillary forces. I also lay down preliminary results for observing orbital angular acceleration using beams carrying orbital angular momentum.

To study single optically trapped aerosols I use two methods. Firstly, their Brownian motion is investigated through sub diffraction limit position detection. Unique results in optical tweezers are shown with liquid droplets behaving as under-damped Brownian oscillators. Through these studies I demonstrate a new technique for sizing trapped aerosols, with significant advantages over current methods. I also show that the droplets can be be parametrically excited which can result in trap failure.

Secondly, in chapter 6, I use a theoretical model to describe the forces imparted to a trapped droplet. I extend current theories to include the effects of a three medium focal region to accurately describe airborne optical traps. The work qualitatively explains the phenomena observed experimentally.

The work contained here leaves much scope for future investigations, for which I provide an overview in chapter 7.