This thesis describes the experimental and theoretical work that investigated the transfer of orbital angular momentum from light to matter. This was achieved by combining two established areas of laser physics which were "optical tweezers" and Laguerre-Gaussian laser modes. The optical tweezers are essentially a tightly focussed laser beam from a high numerical aperture microscope objective lens, which traps particles in three dimensions just below the beam focus. By incorporating a Laguerre- Gaussian laser mode into the tweezers system, the trapping efficiency was doubled. These improved optical tweezers have been successfully demonstrated both theoretically and experimentally. In addition to the spin angular momentum which is associated with the polarisation state, the Laguerre-Gaussian laser modes also possess orbital angular momentum. The "optical spanners" utilised this property by transferring orbital angular momentum from the laser beam to the trapped particle, causing it to rotate whilst being held in the optical trap. This effect was theoretically modelled and experimentally observed. Using the optical spanners, the spin angular momentum of the laser was used to directly cancel the orbital angular momentum in the beam, which was observed as a cessation in rotation of the trapped particle. This demonstrated the mechanical equivalence of the spin and orbital components of angular momentum in a light beam, and gave experimental evidence for the well defined nature of the orbital angular momentum present in Laguerre-Gaussian laser modes.

Since its inception in 1970, optical manipulation has evolved into a versatile tool across many fields of science. Notably, the now widely employed optical tweezers invented in 1986 is a good example, which is in essence a strongly focused fundamental Gaussian beam. Although the optical tweezers remained as an important tool in optical manipulation, the shaped structured light such as an optical vortex beam also provides unusual light patterns and promotes exciting discoveries. This thesis is devoted to some unsolved theoretical aspects of optical manipulation. Since optical force acting on a micro-particle is typically on the order of pN and seldom larger than nN, it is a common belief that optical force is relevant in particle manipulation only when all other forces are comparable or smaller than the optical force. In chapter 2, surprisingly we showed that this is not always the case. Here, we find that under appropriate condition, optical vortices can make a sphere orbit around the beam center owing to the non-conservative optical force. If the sphere is attached to a mechanical spring, the spring can be stretched significantly even when the mechanical spring is orders of magnitude stronger than the optical force. Since its inception in 1970, optical manipulation has evolved into a versatile tool across many fields of science. Notably, the now widely employed optical tweezers invented in 1986 is a good example, which is in essence a strongly focused fundamental Gaussian beam. Although the optical tweezers remained as an important tool in optical manipulation, the shaped structured light such as an optical vortex beam also provides unusual light patterns and promotes exciting discoveries. This thesis is devoted to some unsolved theoretical aspects of optical manipulation. Since optical force acting on a micro-particle is typically on the order of pN and seldom larger than nN, it is a common belief that optical force is relevant in particle manipulation only when all other forces are comparable or smaller than the optical force. In chapter 2, surprisingly we showed that this is not always the case. Here, we find that under appropriate condition, optical vortices can make a sphere orbit around the beam center owing to the non-conservative optical force. If the sphere is attached to a mechanical spring, the spring can be stretched significantly even when the mechanical spring is orders of magnitude stronger than the optical force

Raman microscopy is a tool used by physicists to collect molecular information from a wide variety of samples. Biophysicists have increasingly made use of Raman microscopy in combination with optical tweezers to identify the molecular makeup of structures inside cells. There are high levels of background and noise in Raman spectra from cells, however, that obscure low intensity scattering peaks and prevent complete molecular characterization. We have designed and built a Mechanical Modulation Raman Microscope(MMRM) that is capable of background subtraction and noise reduction for Raman spectra from cells in vivo. There are two mechanisms of modulation: (1) three-axis stage modulation for objects fixed to the coverslip and (2) separate optical trap modulation for objects in solution. In both cases, objects of interest are modulated in and out of the Raman excitation volume while spectra are collected. Difference spectra are created by subtracting the spectrum without the object from the spectrum including the object. These difference spectra are averaged over the number of cycles of modulation. With the mechanical modulation technique, the background in Raman spectra is removed, and the signal-to-noise ratio is improved by two orders of magnitude. This technique was applied to fission yeast cells. Mechanical modulation Raman spectra of exponentially growing cells and starved cells were collected in three dimensions, and spatial differences were observed in the molecular composition for different metabolic states of individual yeast cells. / text

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Longitudinal optical binding refers to the light induced self organisation of micro particles in one dimension. In this thesis I will present experimental and theoretical studies of the separation between two dielectric spheres in a counter-propagating (CP) geometry. I will explore the bistable nature of the bound sphere separation and its dependency on the refractive index mismatch between the spheres and the host medium, with an emphasis on the fibre separation. The physical under pining principle of longitudinal optical binding in the Mie regime is the refocusing effect of the light field from one sphere to its nearest neighbour. In a second set of experiments I developed means to visualise the field intensity distribution responsible for optical binding using two-photon fluorescence imaging from fluorescein added to the host medium. The experimental intensity distributions are compared to theoretical predictions and provide an in situ method to observe the binding process in real time. This coupling via the refocused light fields between the spheres is in detailed investigated experimentally and theoretically, in particular I present data and analysis on the correlated behaviour of the micro spheres in the presence of noise. The measurement of the decay times of the correlation functions of the modes of the optically bound array provides a methodology for determining the optical restoring forces acting in optical binding. Interestingly micro devices can be initiated by means of the light-matter interaction. Light induced forces and torques are exerted on such micro-objects that are then driven by the optical gradient or scattering force. I have experimentally investigate how the driving light interacts with and diffracts from the motor, utilising two-photon imaging. The micromotor rotation rate dependence on the light field parameters is explored and theoretically modelled. The results presented will show that the model can be used to optimise the system geometry and the micromotor.

Optical trapping and tweezing has been around for the last 30 years and since found its place in the fields of physics and biology. Over the years this technique has advanced exceedingly and is a unique tool to carry out research in the micrometre and nanometre scale regime. The aim of this dissertation was to illustrate that an optical trapping and tweezing system is an effective tool for the manipulation of micron sized particles and that using such a system allows one the ability to accurately and precisely measure optical forces in the piconewton scale. A custom built single gradient optical trapping system was built to illustrate the manipulation of micron sized particles. Here we will highlight some of the key components of such a system and give an explanation of how these components affect the optical trap. To enhance this system, we exploit the ability to shape light and in particular laser light to generate novel laser beams. This was achieved using a diffractive optical element known as a spatial light modulator (SLM). A spatial light modulator is an electronically addressed optical element which when incorporated into an optical system effectively manipulates the phase of light in order to generate various novel laser beams. In particular these novel laser beams include Laguerre-Gaussian, Bessel and recently proposed Bessel-like beams. Each of these beams contains interesting properties which can be beneficially exploited. Laguerre-Gaussian beams are particularly known as ‘donut’ shaped beams since they have a central dark hole. Increasing the order of these Laguerre-Gaussian beams leads to an increase in the central dark region. These beams are of particular interest since they carry orbital angular momentum. This is not easily observed; however, when incorporated into the optical trapping system, leads to the rotation of trapped particles due to the transfer of photons carrying orbital angular momentum. Bessel and Bessel-like beams on the other hand are classes of beam that possess interesting non-diffracting and self-reconstructive properties upon encountering an obstacle. Here the generation and properties of these novel laser beams will be discussed in detail. Furthermore it is well known that these novel laser beams prove highly useful when incorporated into an optical trapping system hence we will illustrate the effects on a trapped particle when incorporating a Laguerre-Gaussian beam carrying a topological charge of one. It is expected that the trapped particle should rotate due to the transfer of orbital angular momentum. The knowledge gained from beam shaping and the means to trap micron sized particles optically allows one the ability to incorporate this technique in a number of fields, including the promising field of microfluidics. This is an emerging field that deals with investigating fluid properties at the nano and microlitre regime. Optical tweezers integrated into a microfluidic device are beneficial since they are an adequate tool for measuring fluid flow using Stokes’ Law. / Thesis (M.Sc.)-University of KwaZulu-Natal, Durban, 2011.

Bacterial resistance to antibiotics is increasing at a high rate in both developing and developed countries. To circumvent the problem of drug-resistant bacterial pathogens, we need to develop new effective methods, substances, and materials that can disarm and prevent them from causing infections. However, to do this we first need to find new possible targets in bacteria to approach and novel strategies to apply.Escherichia coli (E. coli) bacteria is a normal member of the intestinal microflora of humans and mammals, but frequently cause diverse intestinal and external diseases by means of virulence factors, which leads to hundreds of million sick people each year with a high mortality rate. An E. coli bacterial infection starts with adhesion to a host cell using cell surface expressed adhesion polymers, called adhesion pili. Depending on the local environment different types of pili are expressed by the bacteria. For example, bacteria found in the gastrointestinal tract commonly express different pili in comparison to those found in the urinary tract and respiratory tract. These pili, which are vital for bacterial adhesion, thereby serve as a new possible approach in the fight against bacterial infections by targeting and disabling these structures using novel chemicals. However, in order to develop such chemicals, better understanding of these pili is needed.Optical tweezers (OT) can measure and apply forces up to a few hundred pN with sub-pN force resolution and have shown to be an excellent tool for investigating mechanical properties of adhesion pili. It has been found that pili expressed by E. coli have a unique and complex force-extension response that is assumed to be important for the ability of bacteria to initiate and maintain attachment to the host cells. However, their mechanical functions and the advantage of specific mechanical functions, especially in the initial attachment process, have not yet been fully understood.In this work, a detailed description of the pili mechanics and their role during cell adhesion is presented. By using results from optical tweezers force spectroscopy experiments in combination with physical modeling and numerical simulations, we investigated how pili can act as “shock absorbers” through uncoiling and thereby lower the fluid force acting on a bacterium. Our result demonstrate that the dynamic uncoiling capability of the helical part of the adhesion pili modulate the force to fit the optimal lifetime of its adhesin (the protein that binds to the receptor on the host cell), ensuring a high survival probability of the bond.iiiSince the attachment process is in proximity of a surface we also investigated the influence of tether properties and the importance of different surface corrections and additional force components to the Stokes drag force during simulations. The investigation showed that the surface corrections to the Stokes drag force and the Basset force cannot be neglected when simulating survival probability of a bond, since that can overestimate the probability by more than an order of magnitude.Finally, a theoretical and experimental framework for two separate methods was developed. The first method can detect the presence of pili on single cells using optical tweezers. We verified the method using silica microspheres coated with a polymer brush and E. coli bacteria expressing; no pili, P pili, and type 1 pili, respectively. The second method was based on digital holography microscopy. Using the diffraction of semi-transparent object such as red blood cells, we showed that this method can reconstruct the axial position and detect morphological changes of cells.

Nanopores are apertures of nanometric dimensions in an insulating matrix. They are routinely used to sense and measure properties of single molecules such as DNA. This sensing technique relies on the process of translocation, whereby a molecule in aqueous solution moves through the pore under an applied electric field. The presence of the molecule modulates the ionic current through the pore, from which information can be obtained regarding the molecule's properties. Whereas the electrical properties of the nanopore are relatively well known, much less work has been done regarding their fluidic properties. In this thesis I investigate the effects of fluid flow within the nanopore system. In particular, the charged nature of the DNA and pore walls results in electrically-driven flows called electroosmosis. Using a setup which combines the nanopore with an optical trap to measure forces with piconewton sensitivity, we elucidate the electroosmotic coupling between multiple DNA molecules inside the confined environment of the pore. Outside the pore, these flows produce a nanofluidic jet, since the pore behaves like a small electroosmotic pump. We show that this jet is well-described by the low Reynolds number limit of the classical Landau-Squire solution of the Navier-Stokes equations. The properties of this jet vary in a complex way with changing conditions: the jet reverses direction as a function of salt concentration, and exhibits asymmetry with respect to voltage reversal. Using a combination of simulations and analytic modelling, we are able to account for all of these effects. The result of this work is a more complete understanding of the fluidic properties of the nanopore. These effects govern the translocation process, and thus have consequences for better control of single molecule sensing. Additionally, the phenomena we have uncovered could potentially be harnessed in novel microfluidic applications, whose technological implications range from lab-on-a-chip devices to personalised medicine.

The material that is covered in this thesis concerns the calibration and application of a set of optical tweezers to be used for one- and two-point micro-rheology experiments on hard sphere colloidal suspensions. The colloidal suspensions that were used in this study were all quasi-monodisperse density- and refractive index-matched PMMA particles that had a radii, a = 0:90 ± 0:05μm or a = 0:86 ± 0:07 for one-point microrheology experiments and radii a = 0:90 ± 0:05μm or a = 0:133 ± 0:010μm for the two-point micro-rheology experiments. By collecting the forward scattered light from a single optically trapped particle the particle's displacements in time were used to determine passive microviscosity, η(Passive) μ , for colloidal suspension in the range of 0:10 < Ø < 0:57 and comparison with literature data has been made and agreement found. Actively dragging an optically trapped particle through suspensions with volume fractions of the same range has yielded the active microviscosities, η(Active) μ , for both high and low shear regimes, displaying shear thinning behaviour. Comparison to literature data has been made and agreement found as well. Collecting the forward scattered light from two optically trapped particles has been used to determine the cross-correlated motion of the two particles in bare solvent and in suspensions with volume fraction Ø = 0:02. The friction coefficients ξ1;1 and ξ1;2 were extracted from the cross-correlated motion of the particles and agreement was found with theoretical predictions for bare solvent only. The suspensions with volume fraction Ø = 0:02 were found to have a friction coefficient ξ1;1 that was greater than what theory predicted with the suspension with bath particles a = 0:90 ± 0:05μm had the greater magnitude. The magnitude ξ1;2 was found to decrease for the suspension with bath particles of radius a = 0:133 ± 0:010μm and to increase for the suspension with bath particles a = 0:90 ± 0:05μm.

Nanoaperture optical tweezers are emerging as useful tools for the detection and identification of biological molecules and their interactions at the single molecule level. Nanoaperture optical tweezers provide a low-cost, scalable, straight-forward, high-speed platform for single molecule studies without the need to use tethers or labeling. This thesis gives a general description of conventional optical tweezers and how they are limited in terms of their capability to trapping biological molecules. It also looks at nanoaperture-based optical tweezers which have been suggested to overcome the limitations of conventional optical tweezers. The thesis then focuses on the double nanohole optical tweezer as a tool for trapping biological molecules and studying their behaviour and interactions with other molecules. The double nanohole aperture trap integrated with microfluidic channels has been used to detect single protein binding. In that experiment a double-syringe pump was used to deliver biotin-coated polystyrene particles to the double nanohole trapping site. Once stable trapping of biotin-coated polystyrene particle was achieved, the double-syringe pump was used to flow in streptavidin solution to the trapping site and binding was detected by measuring the transmission through the double nanohole aperture. In addition, the double nanohole optical tweezer has been used to observe the real-time dynamic variations in protein-small molecule interaction (PSMI) with the primary focus on the effect of single and multiple binding events on the dynamics of the protein in the trap. Time traces of the bare form of the streptavidin showed slower timescale dynamics as compared to the biotinylated forms of the protein. Furthermore, the double nanohole aperture tweezer has been used to study the real-time binding kinetics of PSMIs and to determine their disassociation constants. The interaction of blood protein human serum albumin (HSA) with tolbutamide and phenytoin was considered in that study. The dissociation constants of the interaction of HSA with tolbutamide and phenytoin obtained using our technique were in good agreement with the values reported in the literature. These results would open up new windows for studying real-time binding kinetics of protein-small molecule interactions in a label-free, free-solution environment, which will be of interest to future studies including drug discovery. / Graduate