Le piégeage optique se présente maintenant, depuis quelques décennies, comme une thématique majeure à l'intersection de diverses disciplines. Depuis les résultats d'Ashkin, de nombreux travaux ont été effectués dans le piégeage et le guidage d'objets physiques (particules, molécules, bactéries, etc.) de toutes tailles. Ces derniers caractériseront alors, devant la longueur d'onde, le domaine optique dans lequel nous nous placerons (Rayleigh, Mie, Optique géométrique).Notre travail porte donc sur l'étude des propriétés de chaînes linéaires périodiques de gouttelettes (huile), placées dans l'eau, et soumises à deux faisceaux laser horizontaux contra-propageants de profil gaussien. Nous démontrons qu'il est possible d'établir un ordre spatial sur un ensemble de grosses gouttes (devant la longueur d'onde) suivant une structure périodique. L'originalité d'un tel système réside dans le fait que la lumière peut alors être refocalisée par l'ensemble des gouttes espacées périodiquement. Cette périodicité peut ainsi, dans certains cas, conférer au faisceau une refocalisation périodique au sein du réseau. Cette première étude, en limite statique, nous permet ainsi de mettre en évidence les conditions de couplage des modes liés aux chaînes de gouttes. En particulier, nous caractérisons la présence de modes de Bloch où le faisceau se propage avec une périodicité équivalente à celle du réseau. Cela nous amène à remarquer que ces conditions modales sont soumises au paramètre de phase gaussien "Thêta" (phase de Gouy). Ainsi, bien que structuré à une échelle largement supérieure, nous mettons en évidence théoriquement des propriétés analogues à celle des cristaux photoniques, conférées par la périodicité des chaînes de gouttes. Ce qui nous permet, en conséquence, de démontrer l'existence de bandes interdites, nous amenant à définir un ensemble de modes guidants/nonguidants de cette chaîne. Cette étude statique est, par la suite, étendue d'un point de vue dynamique en considérant l'effet des forces optiques sur les gouttes. Nous démontrons ainsi qu'il est possible de piéger optiquement de telles gouttes sur des états d'équilibres stables. Au-delà desquels nous mettons en évidence, à travers une étude paramétrique, l'existence de modes oscillants périodiques ou pseudo-périodiques. Enfin, nous prenons en compte les phénomènes de collisions par coalescence, entraînant une réorganisation des répartitions de champs optiques qui peuvent se traduire par de nouvelles configurations de piégeage / Optical trapping appears now, since a few decades, as a major theme at the intersection of variousdisciplines. Since the results of Ashkin, many works were made in the trapping and the guidance of physical objects (particles, molecules, bacteria, etc.) of any sizes. The latter will characterize then, in front of the wavelength, the optical domain in which we shall take place (Rayleigh, Mie, Geometrical Optics).Our work thus concerns the study of the properties of periodic linear chains of droplets (oil), placed in water, and submitted to two counter-propagating horizontal laser beams of gaussian profile. We show that it is possible to establish a spatial order of a set of large drops (in front of the wavelength) in a periodic structure. The originality of such a system lies in the fact that the light can then be refocused by the set of periodically spaced drops. This periodicity may thus, in some cases, confer on the beam a periodic refocusing within the network. This first study, in static limit, allows us to identify the conditions of coupling modes associated with drop channels. In particular, we characterize the presence of Bloch modes where the beam propagates with similar frequency to that of the network. This leads us to note that these modal conditions are submitt to the gaussian phase parameter "Thêta" (Gouy phase). Thus, although structured at a widely higher scale, we highlight theoretically similar properties to that of the photonic crystals, conferred by the periodicity of the chains of drops. This allows us, consequently, to demonstrate the existence of bandgaps, leading us to define a set of guiding/not-guiding modes of this chain. This static study, thereafter, is extended from a dynamic point of view by taking into account the effect of the optical forces on the drops. We show that it is possible to optically trap such drops on stable equilibrium states. Beyond of which we highlight, through a parametric study, the existence of periodic or pseudo-periodic oscillating modes.Finally, we take into account the phenomena of collisions by coalescence, involving a reorganization of the distributions of optical fields which can result in new configurations of trapping.

Optical tweezers have been widely used by biophysicists to measure forces in single molecular processes, such as the force of a motor molecule walking and the force of a DNA molecule winding and unwinding. In these and similar force measurements, the usual assumption is that the force applied to a particle inside the tweezers is proportional to the displacement of the particle away from the trap center like Hookean springs, which would imply that the force field is conservative. However, the Gaussian beam model has indicated that the force field generated by optical tweezers is actually nonconservative, yet no experiments have measured or accounted for this effect. We introduce an experimental method -- the local drift method -- that can measure the force field in optical tweezers with high precision without any assumptions about the functional form of the force field. The force field is determined by analyzing the Brownian motion of a trapped particle. We successfully applied this method to different sizes of particles and measured the three dimensional force field with 10 nm spatial resolution and femtonewton precision in force. We find that the force field is indeed nonconservative. The nonconservative contribution increases radially away from the optical axis for both small and large particles. The curl vector field -- a measurement of the nonconservative force field -- reverses direction from counter-clockwise for small particles in the Rayleigh regime to clockwise for large particles in the ray optics regime, consistent with the different scattering force profiles in the two distinct scattering regimes. Together with the thermal fluctuations of the trapped particle, the nonconservative force can cause a complex flux of energy into the system. Optically-confined Brownian motion is further used to probe nanostructures such as a biopolymer network. This technique -- thermal noise imaging -- uses a Brownian particle as a "natural scanner" to explore a biopolymer network by moving the Brownian particle through the network with optical tweezers. The position fluctuations of the probe particle reflect the location of individual filaments as excluded volumes. The resolution of thermal noise imaging is directly coupled to the size of the probe particle. A smaller probe is capable of exploring smaller pore sizes formed by dense network. Previously, a 200 nm polystyrene particle had been used to probe an agar network. In this work, 100 nm gold probe particles are used to enhance the resolution. A 100 nm particle explore a network with mesh 2³ times smaller and therefore enhance the network resolution by 2³ times. A 100 nm particle also improves the imaging speed by a factor of 2 because of its faster diffusion. Three-dimensional thermal noise images of agarose filaments are obtained and a resolution of 10 nm for the position of the filaments is achieved. In addition, a gold particle is trapped with significantly less power than a polystyrene particle of the same size, indicating the possibility for using even smaller gold particles to further improve the resolution. / text

Optical trapping enables the non-contact manipulation of micro and nanoparticles with extremely high precision. Recent research on integrated optical trapping using the evanescent fields of photonic devices has opened up new opportunities for the manipulation of nano- and microparticles in lab-on-a-chip devices. Considerable interest has emerged for the use of optical microcavities as “sensors-on-a-chip”, due to the possibility for the label-free detection of nanoparticles and molecules with high sensitivity. This dissertation focuses on the demonstration of an on-chip optical manipulation system with multiple functionalities, including trapping, buffering, sorting, and sensing. We demonstrate the optically trapping of polystyrene particles with diameters from 110 nm to 5.6 \(\mu m\) using silicon microrings and photonic crystal cavities. By integrating multiple microrings with different resonant wavelengths, we show that tuning the laser wavelength to the resonance wavelengths of different rings enables trapped particles to be transferred back and forth between the rings in a controllable manner. We term this functionality “buffering”. We furthermore demonstrate an integrated microparticle passive sorting system based on the near-field optical forces exerted by a 3-dB optical power splitter that consists of a slot waveguide and a conventional channel waveguide. In related work, we demonstrate an ultra-compact polarization splitter design leveraging the giant birefringence of silicon-on-insulator slot waveguides to achieve a high extinction ratio over the entire C band. We demonstrate trapping-assisted particle sensing, using the shift in the microcavity resonance induced by the trapped particle. We show that this permits the sensing of proteins via a binding assay approach, in which the presence of green fluorescent protein causes the particles to bind. By detecting the size distribution of particles clusters using the microcavity, we quantitatively detect the GFP concentration. In a complementary approach, we demonstrate a reusable and reconfigurable surface-enhanced Raman scattering (SERS) sensing platform. We use a photonic crystal cavity to trap silver nanoparticles in a controllable manner, and measure SERS from molecules on their surfaces. We anticipate that the on-chip sensing approaches we introduce could lead to various applications in nanotechnology and the environmental and life sciences. / Engineering and Applied Sciences

Colloidal particles are small particles ranging in size from nanometres to micrometres suspended in a fluid. Amongst many scientific and biological applications, they have been used to model crystallisation, vitrification, and particle interactions along with the use of colloidal model systems for the study of the fundamental nature of the fluid-crystal and fluid-glass phase transitions. It has been shown that colloidal particles can be trapped and manipulated using strongly-focused light beams known as optical tweezers, and this has paved the way for research into the area of micromanipulation using optical trapping. Holographic elements can replace multiple lenses in creating large numbers of optical tweezers and this is known as holographic optical trapping (HOT). A computer generated hologram can be designed to create large structured light fields, consisting of multiple foci, to enable trapping of multiple particles in arbitrary configurations. The overall aim of this project was to design, develop and test the suitability of a simple, inexpensive optical trapping arrangement suitable for multiple optical trapping. To achieve this, a theoretically-exact expression for the wavefront of a single point source was implemented in the coding scheme, allowing for the fast creation of multiple point sources suitable for holographic optical trapping experiments. Compensation for the spherical aberration present in the focusing optics was implemented into the coding scheme. Kodalith photographic film was chosen as the holographic recording medium for its high contrast and availability. The film has proven to be a successful medium, when used to record photographically-reduced images of high-quality printouts of the computed diffraction pattern, as it was able to successfully reproduce complex light fields. It is believed that this will be the first time that this film has been implemented for optical trapping purposes. The main limitations concerning the performance of the holograms recorded on Kodalith were the phase nonuniformities caused by unevenness in the film thickness which resulted in a failure to separately resolve light traps separated by less than about 5 (Mu)m. Index matching of the film between sheets of flat glass helped to compensate for these limitations. Holographic optical trapping was successfully observed using a variety of different initial beam powers, holographic aperture settings and light field configurations. Trapping experiments on of two types of particles (PMMA and polystyrene) were successfully conducted, with as little as ~ 150 (Mu)W per trap being required for multiple polystyrene trapping. However, particles were weakly trapped and were easily dislodged at these powers, and a higher power per trap of around 1 mW is preferred. The use of a relatively low numerical aperture (NA) 50 mm SLR lens for focusing the holographic optical traps was successful, proving that optical trapping can be conducted without the use of high NA microscope-objective lenses commonly used in other set ups. Holographic trapping of colloidal particles was successfully conducted at RMIT University for the first time proving the validity of the coding scheme, the recording method and the trapping arrangement.

Optical tweezers using highly focussed laser beams can be used to exert forces and torques and thus drive micromachines. This opens up a new field of microengineering, whose potential has yet to be fully realized. Until now, methods that have been used for modelling optical tweezers are limited to scatterers that are homogeneous or that have simple geometry. To aid in designing more general micromachines, I developed and implemented two main methods for modelling the micromachines that we use. These methods can be used for further proposed structures to be fabricated. The first is a FDFD/T-matrix hybrid method that incorporates the finite difference frequency domain (FDFD) method, which is used for inhomogeneous and anisotropic media, with vector spherical wave functions (VSWF) to formulate the T-matrix. The T-matrix is then used to calculate the torque of the trapped vaterite sphere, which is apparently composed of birefringent unit crystals but the bulk structure appears to be arranged in a sheaf-of-wheat fashion. The second method is formulating the T-matrix via discrete dipole approximation (DDA) of complex arbitrarily shaped mesoscale objects and implementing symmetry optimizations to allow calculations to be performed on high-end desktop PCs that are otherwise impractical due to memory requirements and calculation time. This method was applied to modelling microrotors. The T-matrix represents the scattering properties of an object for a given wavelength. Once it is calculated, subsequent calculations with different illumination conditions can be performed rapidly. This thesis also deals with studies of other light scattering phenomena including the modelling of scattered fields from protein molecules subsequently used to model FRET resonance, determining the limits of trappability, interferometric Brownian motion and the comparison between integral transforms by direct numerical integration and overdetermined point-matching.

Optical tweezers using highly focussed laser beams can be used to exert forces and torques and thus drive micromachines. This opens up a new field of microengineering, whose potential has yet to be fully realized. Until now, methods that have been used for modelling optical tweezers are limited to scatterers that are homogeneous or that have simple geometry. To aid in designing more general micromachines, I developed and implemented two main methods for modelling the micromachines that we use. These methods can be used for further proposed structures to be fabricated. The first is a FDFD/T-matrix hybrid method that incorporates the finite difference frequency domain (FDFD) method, which is used for inhomogeneous and anisotropic media, with vector spherical wave functions (VSWF) to formulate the T-matrix. The T-matrix is then used to calculate the torque of the trapped vaterite sphere, which is apparently composed of birefringent unit crystals but the bulk structure appears to be arranged in a sheaf-of-wheat fashion. The second method is formulating the T-matrix via discrete dipole approximation (DDA) of complex arbitrarily shaped mesoscale objects and implementing symmetry optimizations to allow calculations to be performed on high-end desktop PCs that are otherwise impractical due to memory requirements and calculation time. This method was applied to modelling microrotors. The T-matrix represents the scattering properties of an object for a given wavelength. Once it is calculated, subsequent calculations with different illumination conditions can be performed rapidly. This thesis also deals with studies of other light scattering phenomena including the modelling of scattered fields from protein molecules subsequently used to model FRET resonance, determining the limits of trappability, interferometric Brownian motion and the comparison between integral transforms by direct numerical integration and overdetermined point-matching.

xii, 112 p. : ill. (some col.) / Biological molecular motors exist in an interesting regime of physics where momentum is unimportant and diffusive motion is large. While only exerting small forces, these motors still manage to achieve directed motion and do work. Brownian motors induce directed motion of diffusive particles and are used as models for biological and artificial molecular motors. A flashing ratchet is a Brownian motor that rectifies thermal fluctuations of diffusive particles through the use of a time-dependent, periodic, and asymmetric potential. It has been predicted that a feedback-controlled flashing ratchet has a center of mass speed as much as one order of magnitude larger than the optimal periodically flashing ratchet. We have successfully implemented the first experimental feedback ratchet and observed the predicted order of magnitude increase in velocity. We experimentally compare two feedback algorithms for small particle numbers and find good agreement with Langevin dynamics simulations. We also find that existing algorithms can be improved to be more tolerant to feedback delay times. This experiment was implemented by a scanning line optical trap system. In a bottom-up approach to understanding molecular motors, a synthetic protein-based molecular motor, the "tumbleweed", is being designed and constructed. This design uses three ligand dependent DNA repressor proteins to rectify diffusive motion of the construct along a DNA track. To predict the behavior of this artificial motor one needs to understand the binding and unbinding kinetics of the repressor proteins at a single-molecule level. An assay, similar to tethered particle motions assays, has been developed to measure the unbinding rates of these three DNA repressor proteins. In this assay the repressor is immobilized to a surface in a microchamber. Long DNA with the correct recognition sequence for one of the repressors is attached to a microsphere. As the DNA-microsphere construct diffuses through the microchamber it will sometimes bind to the repressor protein. Using brightfield microscopy and a CCD camera the diffusive motion of the microsphere can be characterized and bound and unbound states can be differentiated. This method is tested for feasibility and shown to have sufficient resolution to measure the unbinding rates of the repressor proteins. / Committee in charge: Dr. Raghu Parthasarathy, Chair; Dr. Heiner Linke, Research Advisor; Dr. Dan Steck; Dr. John Toner; Dr. Brad Nolan

The use of optical fibre as a mechanism for the delivery of the trapping laser beam to the sample chamber significantly reduces both the size and the build costs of “Optical Tweezers”. Furthermore, the use of fibre facilitates the decoupling of the optical trapping beam from the microscope optics, which provides further scope for the development of a portable optical trapping system, and the potential for uncomplicated integration with other advanced microscopy systems such as an atomic force microscope (AFM) for example. For use with an AFM, the optical fibre must be inserted at an angle of 10° with respect to the sample chamber floor. However, previous literature suggests that 3D optical trapping with a single fibre inserted at an angle ≤20° is not feasible. This thesis presents the design, development, build and test of a single beam optical fibre based gradient force optical tweezers system and its associated software. An investigation is conducted to ascertain why optical trapping, using single fibre systems, cannot be achieved at sub 20° insertion angles, the result of which formed the basis of a hypothesis that explains this limitation. This finding led to the development of tapered optical fibre tips that are cable of 3D optical trapping at an insertion angle of ≤10°. The optimised optical fibre tapers are presented and their ability to trap both organic and inanimate material in 3D at an insertion angle of 10° is demonstrated. The near-horizontal insertion angle introduced a maximum trapping range (MTR). The MTR of the tips is determined empirically, evaluated against simulated data, and found to be tuneable through taper optimisation. Optical trap characterisation has been undertaken in terms of the optical trapping forces acting on the trapping subjects. Finally, the fibre tapering devices ability to reproduce identical tapers, or not, using the same device parameters, was investigated and the results in terms of geometric profile and optical performance are presented.

In this thesis, an approach for developing single-photon sources at the 1550nm wavelength will be demonstrated, based on optical trapping of luminescent upconverting nanoparticles. A single-photon source is a source that emits a single photon at a time, and hence it is a source of quantum bits that constitutes the basic building units in quantum computers and quantum communications. The approach exploits the plasmonic properties of gold films and the waveguiding characteristics of single mode optical fibers (SMFs). We start by planar nanofabrication of subwavelength nanoapertures in a thin gold film based on finite difference time domain simulations for a peak transmission at the wavelength in question. Subsequently, using ultraviolet curable epoxy adhesion material, a nanoaperture patterned on a gold film can be transferred to an SMF tip forming a nanoantenna enhanced optical fiber tweezer (NAFT). As a final step in building the optical tweezer, a test of the capability of the integrated optical fiber tweezer to trap 20 nm, and 30nm polystyrene nanospheres, as well as luminescent upconverting nanoparticles (UCNPs), has been experimentally realized with encouraging results. In addition to the optical trapping of the luminescent nanoparticles, the nano aperture antenna can improve light coupling into the low loss optical fiber guiding channel. Also, it could have a positive influence on enhancing the photon emission rate through the Purcell effect. Furthermore, we have combined NAFT with a low insertion loss wave splitter, a wavelength-division multiplexer (WDM), to allow measuring the 1550nm photon-emission statistics on a cooled superconducting nanowire single-photon detector (SNSPD) at ~ 2.4o K. Eventually, nanoantenna enhanced optical fiber tweezers can play an essential role in optical trapping towards developing single-photon sources and the emerging technology of quantum information processing, computation, and cryptography. / Graduate

Interest in rare-earth-doped crystalline materials, e.g., M(RE)F4 (M – alkali metal, RE – rare-earth metal), featuring unique optical properties such as light upconversion and downshifting is experiencing a surge due to the broad spectrum of applications that these photonic systems are facilitating. The development of reliable synthetic methods that grant rapid access to these materials is therefore of great importance. Microwave-assisted synthesis is appealing in this regard, because microwave radiation enables rapid and uniform heating of the reaction mixture and allows for rigid control of the reaction conditions, factors that facilitate the production of high-quality materials within minutes. Surprisingly, the investigation around microwave-assisted synthesis of M(RE)F4 materials featuring upconversion and downshifting luminescence is limited. Methods that have already been developed predominately target Na-based systems, despite the evidence that the Li-based analogues also display excellent optical properties. In fact, only a single microwave-assisted approach toward a nanoscale Li-based system has been reported to date, while to my knowledge, no report of a microwave-assisted synthesis of a microscale Li-based system existed prior to the commencement of the work presented in this thesis. The challenge lies in the fact that access to Li(RE)F4 is not easily achieved through a simple substitution of the alkali metal source in the established protocols that yield Na(RE)F4; rather, a complete re-optimization of the synthesis method is required. This particular challenge was successfully addressed in this work. Presented and discussed in Chapter 3 of this thesis is a rapid microwave-assisted solvothermal synthesis approach toward both upconverting and downshifting LiYF4:RE3+ microparticle systems. More specifically, it is detailed how the rigorous optimization of the reaction temperature/duration profile, initial reaction mixture pH, and ratio of the metal precursors was necessary in gaining control over the crystalline phase, morphology, and size of the microparticles under microwave-induced solvothermal conditions. Importantly, a materials growth mechanism involving the depletion of a Li-free crystal phase, followed by a particle ripening process is also proposed. Moreover, the versatility of the developed method is highlighted by showcasing how it can be extended toward the synthesis of other relevant Li- and Na-based M(RE)F4 nano- and microscale materials (i.e., LiYbF4, NaYF4, and NaGdF4) featuring upconversion luminescence. Lastly, potential challenges associated with microwave-assisted synthesis are discussed, and appropriate solutions are proposed. The upconversion and downshifting luminescence of the M(RE)F4 materials attained via the developed synthesis approach is investigated in Chapter 4. The first part of the chapter provides a general assessment of the characteristic luminescence generated by the M(RE)F4 materials featuring various RE3+ dopant systems. The second part of the chapter is devoted to a much more thorough single-particle investigation of the anisotropic luminescence behaviour exhibited by the LiYF4:RE3+ microparticles via hyperspectral imaging, polarized emission spectroscopy, and optical trapping. It is my hope that you, the reader, will find the work presented in this thesis stimulating from two vantage points – from the development of the most rapid microwave-assisted solvothermal synthesis of upconverting and downshifting M(RE)F4 nano/microscale materials reported to date, as well as from the utilization of specialized luminescence characterization techniques to provide fundamental insight into a seldom-considered luminescence property of crystalline materials such as LiYF4.