Optical Forces in Complex Systems

Abstract

This thesis covers the development of a new Discrete Dipole Approximation (DDA) program MOSH-DDA. The program is then used to investigate optical forces and binding in simple and complex systems such as: homo-sphere pairs and triplets, hetero-sphere pairs and triplets, metasurfaces and optical conveyors and chiral particles.

In the first chapter we introduce the historical work performed in this area, and provide some of the motivation for performing this work. In the second chapter, the program is developed from the ground up to take advantage of improvements and methodologies for calculating the polarisabilities and forces of multiple, distinguishable, particles, while maintaining the speed of calculation to allow for large dipole numbers. Subsequent to this development the program is then validated on understood and explored systems, both experimental and simulated, to demonstrate it's validity. During this process the program will be compared against the existing code ADDA, both to assess validity and to demonstrate the changes that necessitated the creation of a new program. Following this in chapter three we take existing beam descriptions and convert them into field gradients to be able to generate forces with these beams.

The fourth and fifth chapters are both aimed at exploring the phenomenon of optical binding, with characterisation of the limitation of the weak binding approximation and demonstration of potentially novel instabilities in the Mie regime in chapter four. In chapter five we move onto more novel systems with starting with investigations into the motions of homo-sphere triplets before moving on to in-depth investigation of hetero-sphere systems, where we subsequently demonstrate that these systems do optically bind, and generate net motion. Furthermore we extend this investigation from hetero-sphere pairs to hetero-sphere triplets, demonstrating that we can generate orbital motion.

The final chapters will focus on a different thread of optical forces. In chapter six we investigate dielectric meta-surfaces and their use in optical conveyor type systems. Here we investigate the forces required to move a particle in a preferential direction using scattered light from a series of motifs that make up a dielectric meta-surface. These forces are extremely sensitive to both the motif shape and the wavelength of light, allowing us to tune these systems to our requirements. We investigate these systems predominantly with plane-wave physics with varying polarisations.

We finally wrap up the thesis with a summary of our key conclusions and a discussion on where we believe further investigation should be targeted.

Date of Award	1 Oct 2024
Original language	English
Awarding Institution	University of Bristol
Sponsors	Engineering and Physical Sciences Research Council & Arm Limited
Supervisor	Simon Hanna (Supervisor) & Annela M Seddon (Supervisor)