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.

This thesis details the design, fabrication, and testing of two optofluidic platforms, a square fused silica capillary and a MgF2-PDMS microfluidic chip to detect radiation-induced biochemical changes in cells during radiation treatment (radiotherapy). The platforms integrate a near-infrared Raman system of 785 nm excitation and a fiber-based optical trap at 1064 nm in a dual-beam configuration for the manipulation and subsequent examination of single polystyrene beads (5µm) and two breast carcinoma cell lines, MCF-7, and MDA-MB-23 (20-30 µm). Particular attention was paid to the role of MgF2 as a novel substrate for microfluidic fabrication and the device background contributions that could hinder spectral contributions from the samples. Successful optical trapping within the platforms was performed, which allowed the sample immobilization for the entire Raman acquisition time (10-30 s) via an orthogonally positioned objective for the excitation and collection of Raman signal. Data collected in the MgF2-PDMS microchip yielded high-quality spectra with no presence of PDMS characteristic Raman peaks in the spectral region of 450-1800 cm-1. / Graduate / 2023-04-08

One of the most intriguing properties of light-matter interaction is the ability of an electromagnetic field to exert mechanical forces on polarizable objects. This phenomenon is a direct consequence of the fact that light carries momentum, which in turn can be transferred to matter. Mediated by scattering, this interaction usually manifests itself as a “pushing force” in the direction of beam propagation. However, it is possible to judiciously engineer these optical forces, either by tailoring particle polarizability, and/or by structuring the incident light field. As a simple example, a tightly focused laser beam demonstrates strong gradient forces, which may attract and even trap particles with positive polarizability in the focal volume. The opposite occurs in the regime of negative polarizability, where particles are expelled from the regions of highest intensity. Based on this fundamental principle, one can actively shape the beam using spatial light modulators to manipulate individual objects as well as ensembles of particles suspended in a liquid. In the latter case, a modulation of the local particle concentration is associated with changes of the effective refractive index. The result is an artificial nonlinear medium, whose Kerr-type response can be readily tuned by the parameters of its constituent particles. In the course of this work, we introduce a new class of synthetic colloidal suspensions exhibiting negative polarizabilities, and observe for the first time robust propagation and enhanced transmission of self-trapped light over long distances. Such light penetration in strongly scattering environments is enabled by the interplay between optical forces and self-activated transparency effects. We explore various approaches to the design of negative-polarizability arrangements, including purely dielectric as well as metallic and hybrid nanoparticles. In particular, we find that plasmonic resonances allow for extremely high and spectrally tunable polarizabilities, leading to unique nonlinear light-matter interactions. Here, for the first time we were able to observe plasmonic resonant solitons over more than 25 diffraction lengths, in colloidal nanosuspensions.