Dissertationen zum Thema „Optical field manipulation“

Optical trapping techniques have become an important and irreplaceable tool in many research disciplines for reaching non-invasively into the microscopic world and to manipulate, cut, assemble and transform micro-objects with nanometer precision and sub-micrometer resolution. Further advances in optical trapping techniques promise to bridge the gap and bring together the macroscopic world and experimental techniques and applications of Microsystems in areas of physics, chemistry and biology. In order to understand the optical trapping process and to improve and tailor experimental techniques and applications in a variety of scientific disciplines, an accurate knowledge of trapping forces exerted on particles and their dependency on environmental and morphological factors is of crucial importance. Furthermore, the recent trend in novel laser trapping experiments sees the use of complex laser beams in trapping arrangements for achieving more controllable laser trapping techniques. Focusing of such beams with a high numerical aperture (NA) objective required for efficient trapping leads to a complicated amplitude, phase and polarisation distributions of an electromagnetic field in the focal region. Current optical trapping models based on ray optics theory and the Gaussian beam approximation are inadequate to deal with such a focal complexity. Novel applications of the laser trapping such as the particle-trapped scanning near field optical microscopy (SNOM) and optical-trap nanometry techniques are currently investigated largely in the experimental sense or with approximated theoretical models. These applications are implemented using the efficient laser trapping with high NA and evanescent wave illumination of the sample for high resolution sensing. The proper study of these novel laser trapping applications and the potential benefits of implementation of these applications with complex laser beams requires an exact physical model for the laser trapping process and a nanometric sensing model for detection of evanescent wave scattering. This thesis is concerned with comprehensive and rigorous modelling and characterisation of the trapping process of spherical dielectric particles implemented using far-field and near-field optical trapping modalities. Two types of incident illuminations are considered, the plane wave illumination and the doughnut beam illumination of various topological charges. The doughnut beams represent one class of complex laser beams. However, our optical trapping model presented in this thesis is in no way restricted to this type of incident illumination, but is equally applicable to other types of complex laser beam illuminations. Furthermore, the thesis is concerned with development of a physical model for nanometric sensing, which is of great importance for optical trapping systems that utilise evanescent field illumination for achieving high resolution position monitoring and imaging. The nanometric sensing model, describing the conversion of evanescent photons into propagating photons, is realised using an analytical approach to evanescent wave scattering by a microscopic particle. The effects of an interface at which the evanescent wave is generated are included by considering the scattered field reflection from the interface. Collection and imaging of the resultant scattered field by a high numerical aperture objective is described using vectorial diffraction theory. Using our sensing model, we have investigated the dependence of the scattering on the particle size and refractive index, the effects of the interface on the scattering cross-section, morphology dependent resonance effects associated with the scattering process, and the effects of the incident angle of a laser beam undergoing total internal reflection to generate an evanescent field. Furthermore, we have studied the detectability of the scattered signal using a wide area detector and a pinhole detector. A good agreement between our experimental measurements of the focal intensity distribution in the back focal region of the collecting objective and the theoretical predictions confirm the validity of our approach. The optical trapping model is implemented using a rigorous vectorial diffraction theory for characterisation of the electromagnetic field distribution in the focal region of a high NA objective. It is an exact model capable of considering arbitrary amplitude, phase and polarisation of the incident laser beam as well as apodisation functions of the focusing objective. The interaction of a particle with the complex focused field is described by an extension of the classical plane wave Lorentz-Mie theory with the expansion of the incident field requiring numerical integration of finite surface integrals only. The net force exerted on the particle is then determined using the Maxwell stress tensor approach. Using the optical trapping model one can consider the laser trapping process in the far-field of the focusing objective, also known as the far-field trapping, and the laser trapping achieved by focused evanescent field, i.e. near-field optical trapping. Investigations of far-field laser trapping show that spherical aberration plays a significant role in the trapping process if a refractive index mismatch exists between the objective immersion and particle suspension media. An optical trap efficiency is severely degraded under the presence of spherical aberration. However, our study shows that the spherical aberration effect can be successfully dealt with using our optical trapping model. Theoretical investigations of the trapping process achieved using an obstructed laser beam indicate that the transverse trapping efficiency decreases rapidly with increasing size of the obstruction, unlike the trend predicted using a ray optics model. These theoretical investigations are in a good agreement with our experimentally observed results. Far-field optical trapping with complex doughnut laser beams leads to reduced lifting force for small dielectric particles, compared with plane wave illumination, while for large particles it is relatively unchanged. A slight advantage of using a doughnut laser beam over plane wave illumination for far-field trapping of large dielectric particles manifests in a higher forward axial trapping efficiency, which increases for increasing doughnut beam topological charge. It is indicated that the maximal transverse trapping efficiency decreases for reducing particle size and that the rate of decrease is higher for doughnut beam illumination, compared with plane wave illumination, which has been confirmed by experimental measurements. A near-field trapping modality is investigated by considering a central obstruction placed before the focusing objective so that the obstruction size corresponds to the minimum convergence angle larger than the critical angle. This implies that the portion of the incident wave that is passed through the high numerical aperture objective satisfies the total internal reflection condition at the surface of the coverslip, so that only a focused evanescent field is present in the particle suspension medium. Interaction of this focused near-field with a dielectric micro-particle is described and investigated using our optical trapping model with a central obstruction. Our investigation shows that the maximal backward axial trapping efficiency or the lifting force is comparable to that achieved by the far-field trapping under similar conditions for either plane wave illumination or complex doughnut beam illumination. The dependence of the maximal axial trapping efficiency on the particle size is nearly linear for near-field trapping with focused evanescent wave illumination in the Mie size regime, unlike that achieved using the far-field trapping technique.

Thesis (PhD) - Swinburne University of Technology, Faculty of Engineering and Industrial Sciences, Centre for Micro-Photonics, 2005.
A thesis submitted for the degree of Doctor of Philosophy, Centre for Micro-Photonics, Faculty of Engineering and Industrial Sciences, 2005. Typescript. Includes bibliographical references (p. 164-177). Also available on cd-rom.

In recent times, near-field optical microscopy has received increasing attention for its ability to obtain high-resolution images beyond the diffraction limit. Near-field optical microscopy is achieved via the positioning and manipulation of a probe on a scale less than the wavelength of the incident light. Despite many variations in the mechanical design of near-field optical microscopes almost all rely on direct mechanical access of a cantilever or a derivative form to probe the sample. This constricts the study to surface examinations in simple sample environments. Distance regulation between the sample surface and the delicate probe requires its own feedback mechanism. Determination of feedback is achieved through monitoring the shift of resonance of one arm of a 'tuning fork', which is caused by the interaction of the probes tip with the Van der Waals force. Van der Waals force emanates from atom-atom interaction at the top of the sample surface. Environmental contamination of the sample surface with additional molecules such as water makes accurate measurement of these forces particularly challenging. The near-field study of living biological material is extremely difficult as an aqueous environment is required for its extended survival. Probe-sample interactions within an aqueous environment that result in strong detectable signal is a challenging problem that receives considerable attention and is a focus of this thesis. In order to increase the detectible signal a localised field enhancement in the probing region is required. The excitation of an optically resonant probe by morphology dependent resonance (MDR) provides a strong localised field enhancement. Efficient MDR excitation requires important coupling conditions be met, of which the localisation of the incident excitation is a critical factor. Evanescent coupling by frustrated total internal reflection to a MDR microcavity provides an ideal method for localised excitation. However it has severe drawbacks if the probe is to be manipulated in a scanning process. Tightly focusing the incident illumination by a high numerical aperture objective lens provides the degree of freedom to enable both MDR excitation and remote manipulation. Two-photon nonlinear excitation is shown to couple efficiently to MDR modes due to the high spatial localisation of the incident excitation in three-dimensions. The dependence of incident excitation localisation by high numerical aperture objective on MDR efficiency is thoroughly examined in this thesis. The excitation of MDR can be enhanced by up to 10 times with the localisation of the incident illumination from the centre of the microcavity to its perimeter. Illuminating through a high numerical aperture objective enables the remote noninvasive manipulation of a microcavity probe by laser trapping. The transfer of photon momentum from the reflection and refraction of the trapping beam is sufficient enough to exert piconewtons of force on a trapped particle. This allows the particle to be held and scanned in a predictable fashion in all three-dimensions. Optical trapping removes the need for invasive mechanical access to the sample surface and provides a means of remote distance regulation between the trapped probe and the sample. The femtosecond pulsed beam utilised in this thesis allows the simultaneous induction of two-photon excitation and laser trapping. It is found in this thesis that a MDR microcavity can be excited and translated in an efficient manner. The application of this technique to laser trapped near-field microscopy and single molecule detection is of particular interest. Monitoring the response of the MDR signal as it is scanned over a sample object enables a near-field image to be built up. As the enhanced evanescent field from the propagation of MDR modes around a microcavity interacts with different parts of the sample, a measurable difference in energy leakage from the cavity modes occurs. The definitive spectral properties of MDR enables a multidimensional approach to imaging and sensing, a focus of this thesis. Examining the spectral modality of the MDR signal can lead to a contrast enhancement in laser trapped imaging. Observing a single MDR mode during the scanning process can increase the image contrast by up to 1:23 times compared to that of the integrated MDR fluorescence spectrum. The work presented in this thesis leads to the possibility of two-photon fluorescence excitation of MDR in combination with laser trapping becoming a valuable tool in near- field imaging, sensing and single molecule detection in vivo. It has been demonstrated that particle scanned, two-photon fluorescence excitation of MDR, by laser trapping 'tweezers' can provide a contrast enhancement and multiple imaging modalities. The spectral imaging modality has particular benefits for image contrast enhancements.

Les nanocavités optiques sur puces sont devenues aujourd'hui des objets de base pour le piégeage et la manipulation d'objets colloïdaux. Nous étudions dans cette thèse des nanocavités comme briques de bases du piégeage et de la manipulation par forces optiques. La preuve de concept du piégeage de microsphères diélectriques apparaît comme le point de départ de l'élaboration d'un laboratoire sur puce. Dans le premier chapitre nous parcourons la bibliographie de l'utilisation des forces optiques en espace libre et en milieu confiné pour le piégeage de particules. Le second chapitre présente les dispositifs expérimentaux pour la caractérisation des nanocavités et les outils mis en place pour les mesures optiques en présence de particules colloïdales. Le troisième chapitre explique la preuve de concept du piégeage de particules de polystyrène de 500 nm, 1 et 2 µm. Dans le chapitre qui suit nous analysons le piégeage de particules en fonction de la puissance injectée dans la cavité. Le chapitre cinq décrit quelques exemples des possibilités de fonctions de manipulation de particules grâce à des cavités couplées. Enfin, dans le dernier chapitre nous montrons les assemblages de particules sur les différents types de cavités étudiées dans cette thèse
On chips optical nanocavities have become useful tools for trapping and manipulation of colloidal objects. In this thesis we study the nanocavities as building blocks for optical forces, trapping and handling of particles. Proof of concept of trapping dielectric microspheres appears as the starting point of the development of lab on chip. In the first chapter we go through the literature of optical forces in free space and integrated optics. The second chapter presents the experimental tools for the characterization of nanocavities and the set-up developed to perform optical measurements with the colloidal particles. The third chapter describes the proof-of-concept trapping of polystyrene particles of 500 nm, 1 and 2 µm. In the following chapter we analyze the particle trapping as function of the injected power into the cavities. The chapter five gives some examples of the possibilities of particles handling functions with coupled cavities. Eventually, in the last chapter we show assemblies of particles on different geometry of cavities studied in this thesis

Le magnétisme induit par la lumière décrit l'effet par lequel un matériau est magnétisé par une impulsion optique. Dans les matériaux transparents, la magnétisation induite optiquement peut être réalisée directement par la lumière polarisée circulairement. Parfois, dans les matériaux métalliques, ce type de magnétisation existe également en raison du trajet solénoïdal microscopique des électrons entraînés par la lumière polarisée circulairement. Dans certains cas, la lumière crée des courants de dérive continus circulants macroscopiques, qui induisent également une magnétisation continue dans le métal. De manière générale, ces magnétismes induits par la lumière sont connus sous le nom d'effet Faraday inverse. Dans le projet de doctorat, j'ai étudié les courants de dérive induits par la lumière dans plusieurs nanoantennes en or. Nous avons réalisé des champs magnétiques stationnaires amplifiés plasmoniquement grâce à ces courants de dérive. L'étude est basée sur la méthode des différences finies dans le domaine temporel (FDTD) et les théories correspondantes du magnétisme induit par la lumière. Dans différents sujets de recherche, nous avons réalisé : 1) un champ magnétique stationnaire ultra-rapide, confiné et fort dans une nanoantenne en forme d'œil de taureau. 2) Un champ magnétique stationnaire à travers une polarisation linéaire dans un nanorod. 3) Un skyrmion de type Neel construit par un champ magnétique stationnaire dans un nanoring. Dans ces études, nous avons examiné les propriétés optiques de différentes nanoantennes et expliqué l'origine physique des courants de dérive induits par la lumière et des champs magnétiques stationnaires. Nous avons démontré la méthode pour obtenir des effets Faraday inverses amplifiés plasmoniquement et exploré la possibilité de réaliser une magnétisation par la lumière incidente polarisée linéairement. Enfin, nous avons étendu l'effet Faraday inverse à d'autres domaines de recherche physique, tels que la construction de skyrmions par des champs magnétiques stationnaires à travers l'effet Faraday inverse. L'effet magnétique de la lumière reste un domaine de recherche riche. Mes études pourraient trouver des applications dans de nombreux domaines, y compris les matériaux et dispositifs magnéto-optiques, le stockage de données optiques, les applications biomédicales, la spintronique, l'informatique quantique, la recherche fondamentale en électromagnétisme et la recherche sur les matériaux avancés
Light-induced magnetism describes the effect where a material is magnetized by an optical pulse. In transparent materials, optically-induced magnetization can be realized directly by circularly polarized light. Sometimes, in metallic materials, this type of magnetization also exists due to the microscopic solenoidal path of electrons driven by circularly polarized light. In some cases, the light creates macroscopic circulating DC drift currents, which also induce DC magnetization in metal. In a broad sense, these light-induced magnetisms are known as the inverse Faraday effect.In the PhD project, I studied light-induced drift currents in multiple gold nanoantennas. We realized plasmonically enhanced stationary magnetic fields through these drift currents. The study is based on the Finite-Difference Time-Domain (FDTD) method and the corresponding light-induced magnetism theories. In different research topics, we have realized: 1) an ultrafast, confined, and strong stationary magnetic field in a bull-eye nanoantenna. 2) A stationary magnetic field through linear polarization in a nanorod. 3) A Neel-type skyrmion constructed by a stationary magnetic field in a nanoring. In these studies, we examined the optical properties of different nanoantennas and explained the physical origin of light-induced drift currents and stationary magnetic fields. We demonstrated the method to achieve plasmonically enhanced inverse Faraday effects and explored the possibility of realizing magnetization through linearly polarized incident light. Finally, we extended the inverse Faraday effect to more physical research areas, such as constructing skyrmions by stationary magnetic fields through the inverse Faraday effect.The magnetic effect of light remains a rich area of research. My studies might find applications in many areas, including magneto-optical materials and devices, optical data storage, biomedical applications, spintronics, quantum computing, fundamental research in electromagnetism, and advanced materials research

The loading and guiding of a launched cloud of cold atoms with the optical dipole force are theoretically and numerically modelled. A far-off resonance trap can be realised using a high power Gaussian mode laser, red-detuned with respect to the principal atomic resonance (Rb 5s-5p). The optimum strategy for loading typically 30% of the atoms from a Magneto optical trap and guiding them vertically through 22 cm is discussed. During the transport the radial size of the cloud is confined to a few hundred microns, whereas the unconfined axial size grows to be approximately 1 cm. It is proposed that the cloud can be focused in three dimensions at the apex of the motion by using a single magnetic impulse to achieve axial focusing. A theoretical study of six current-carrying coil and bar arrangements that generate magnetic lenses is made. An investigation of focusing aberrations show that, for typical experimental parameters, the widely used assumption of a purely harmonic lens is often inaccurate. A new focusing regime is discussed: isotropic 3D focusing of atoms with a single magnetic lens. The baseball lens offers the best possibility for isotropically focusing a cloud of weak-field-seeking atoms in 3D.A pair of magnetic lens pulses can also be used to create a 3D focus (the alternate-gradient method). The two possible pulse sequences are discussed and it is found that they are ideal for loading both 'pancake' and 'sausage’ shaped magnetic/optical microtraps. It is shown that focusing aberrations are considerably smaller for double-impulse magnetic lenses compared to single- impulse magnetic lenses. The thesis concludes by describing the steps taken towards creating a 3D quasi- electrostatic lattice for 85Ilb， using a CՕշ laser. The resulting lattice of trapped atoms will have a low decoherence, and with resolvable lattice sites, it therefore provides a useful system to implement quantum information processing.

Methods to generate, manipulate, and measure optical and atomic fields with global or local angular momentum have a wide range of applications in both fundamental physics research and technology development. In optics, the engineering of angular momentum states of light can aid studies of orbital angular momentum (OAM) exchange between light and matter. The engineering of optical angular momentum states can also be used to increase the bandwidth of optical communications or serve as a means to distribute quantum keys, for example. Similar capabilities in Bose-Einstein condensates are being investigated to improve our understanding of superfluid dynamics, superconductivity, and turbulence, the last of which is widely considered to be one of most ubiquitous yet poorly understood subjects in physics. The first part of this two-part dissertation presents an analysis of techniques for measuring and manipulating quantized vortices in BECs. The second part of this dissertation presents theoretical and numerical analyses of new methods to engineer the OAM spectra of optical beams. The superfluid dynamics of a BEC are often well described by a nonlinear Schrodinger equation. The nonlinearity arises from interatomic scattering and enables BECs to support quantized vortices, which have quantized circulation and are fundamental structural elements of quantum turbulence. With the experimental tools to dynamically manipulate and measure quantized vortices, BECs are proving to be a useful medium for testing the theoretical predictions of quantum turbulence. In this dissertation we analyze a method for making minimally destructive in situ observations of quantized vortices in a BEC. Secondly, we numerically study a mechanism to imprint vortex dipoles in a BEC. With these advancements, more robust experiments of vortex dynamics and quantum turbulence will be within reach. A more complete understanding of quantum turbulence will enable principles of microscopic fluid flow to be related to the statistical properties of turbulence in a superfluid. In the second part of this dissertation we explore frequency mixing, a subset of nonlinear optical processes in which one or more input optical beam(s) are converted into one or more output beams with different optical frequencies. The ability of parametric nonlinear processes such as second harmonic generation or parametric amplification to manipulate the OAM spectra of optical beams is an active area of research. In a theoretical and numerical investigation, two complimentary methods for sculpting the OAM spectra are developed. The first method employs second harmonic generation with two non-collinear input beams to develop a broad spectrum of OAM states in an optical field. The second method utilizes parametric amplification with collinear input beams to develop an OAM-dependent gain or attenuation, termed dichroism for OAM, to effectively narrow the OAM spectrum of an optical beam. The theoretical principles developed in this dissertation enhance our understanding of how nonlinear processes can be used to engineer the OAM spectra of optical beams and could serve as methods to increase the bandwidth of an optical signal by multiplexing over a range of OAM states.

We present a study of the manipulation of micro-particles and the formation of optically bound structures of particles in evanescent wave traps. Two trapping geometries are considered: the first is a surface trap where the evanescent field above a glass prism is formed by the interference of a number of laser beams incident on the prism-water interface; the second uses the evanescent field surrounding a bi-conical tapered optical fibre that has been stretched to produce a waist of sub-micron diameter. In the surface trap we have observed the formation of optically bound one- and two-dimensional structures of particles and measured the binding spring constant by tracking particle motion and the extent of the particle’s Brownian fluctuations. Additionally, we have measured the inter-particle separations in the one-dimensional chain structures and characterised the geometry of the two-dimensional arrays. In the tapered optical fibre trap we demonstrated both particle transport for long distances along the fibre, and the formation of stable arrays of particles. We present the fabrication of tapered optical fibres using the 'heat-and-pull` technique, and evanescent wave optical binding of micro-particles to the taper. Calculations of the distribution of the evanescent field surrounding a tapered fibre are also presented. We show that the combination of modes can give control over the locations of the trapping sites. Additionally, we show how the plasmon resonance of metallic nano-particles can be exploited to enhance the optical trapping force, and suggest how a bi-chromatic nano-fibre trap for plasmonic particles may be implemented. In both experiments we implement video microscopy to track the particle locations and make quantitative measures of the particle dynamics. The experimental studies are complemented by light scattering calculations based on Mie theory to infer how the geometries of the particle structures are controlled by the underlying incident and scattered optical fields.

This dissertation presents and explores a technique to confine and manipulate single and multiple nano-objects in solution by exploiting the thermophoretic interactions with local temperature gradients. The method named thermophoretic trap uses an all-optically controlled heating via plasmonic absorption by a gold nano-structure designed for this purpose. The dissipation of absorbed laser light to thermal energy generates a localized temperature field. The spatial localization of the heat source thereby leads to strong temperature gradients that are used to drive a particle or molecule into a desired direction. The behavior of nano-objects confined by thermal inhomogeneities is explored experimentally as well as theoretically. The monograph treats three major experimental stages of development, which essentially differ in the way the heating laser beam is shaped and controlled. In a first generation, a static heating of an appropriate gold structure is used to induce a steady temperature profile that exhibits a local minimum in which particles can be confined. This simple realization illustrates the working principle best. In a second step, the static heating is replaced. A focused laser beam is used to heat a smaller spatial region. In order to confine a particle, the beam is steered in circles along a circular gold structure. The trapping dynamics are studied in detail and reveal similarities to the well-established Paul trap. The largest part of the thesis is dedicated to the third generation of the trap. While the hardware is identical to the second generation, using the real-time information on the position of the trapped object to heat only particular sites of the gold structure strongly increases the efficiency of the trap compared to the earlier versions. Beyond that, the optical feedback control allows for an active shaping of the effective virtual trapping potential by applying modified feedback rules, including e.g. a double-well or a box-like potential. This transforms the formerly pure trapping device to a versatile technique for micro and nano-fluidic manipulation. The physical and technical contributions to the limits of the method are explored. Finally, the feasibility of trapping single macro-molecules is demonstrated by the confinement of lambda-DNA for extended time periods over which the molecules center-of-mass motion as well as its conformational dynamics can be studied.

This thesis is concerned with the generation of non-classical quantum states of light, the photon-level manipulation of quantum states and the accurate tomography of both quantum states and quantum processes. In optics, quantum information can be encoded and processed in both discrete and continuous variables. Hybrid approaches combining for example homodyne detection with conditional state preparation and manipulation are gaining increasing prominence. The development and characterization of a time-domain balanced homodyne detector (BHD) is presented. The detector has a bandwidth of 80 MHz, a signal-to-noise ratio of 14.5 dB and an efficiency of 86% making it well-suited to pulse-to-pulse measurement of quantum optical states. The BHD is employed to perform quantum state tomography (QST) of non-classical multi-photon Fock states generated by spontaneous parametric down-conversion. A detailed investigation of the mode-matching between the local oscillator used for homodyne detection and the generated Fock states is presented. The one-, two- and three-photon Fock states are reconstructed with a combined preparation and detection efficiency exceeding 50%. Fock states have a number of applications in quantum state engineering, where non-classical ancilla states and conditional measurements enable photon-level manipulation of quantum states. Fock state filtration (FSF) is investigated - an example of a post-selected beam splitter which is a basic building block for many quantum state engineering protocols. A model is developed incorporating the effect of experimental imperfections. An experimental implementation of a Fock state filter is fully characterized by means of coherent-state quantum process tomography (QPT). The reconstructed process is found to be consistent with the model. The filter preferentially removes the single-photon component from an arbitrary input quantum state. Calibration of optical detectors in the quantum regime is discussed. Quantum detector tomography (QDT) is reviewed and contrasted with a new technique for performing QST with a calibrated detector known as the fitting of data patterns (FDP). The first experimental characterization of a BHD is performed by probing the detector with phase-averaged coherent states. The FDP method is shown to be applicable to the estimation of quantum processes, where a detector response is not assumed - thus demonstrating the versatility of the FDP approach as a new method in the quantum tomography toolbox.

Avec l'évolution rapide des techniques de nanofabrication et les besoins croissants d'intégration et d'utilisation à moindre cout énergétique, l'étude et la manipulation de résonances électromagnétiques d'objets de faibles dimensions représentent des enjeux cruciaux. Un des objectifs de ce travail de thèse a donc été d'approfondir nos connaissances de l'interaction entre le champ électromagnétique et la matière. Dans ce but la réalisation conjointe d'expériences en champ proche optique et le développement numérique de modèles associés nous ont permis d'étudier différentes résonances électromagnétiques basées sur l'interaction lumière-matière au sein d'objets de dimensions sub-micrométriques. Dans une première partie, ce manuscrit présente les phénomènes mis en jeu en microscopie champ proche optique et décrit le fonctionnement du microscope SNOM utilise. La seconde partie est dédiée à l'étude de l'interaction d'une sonde champ proche avec une nano-cavité Fabry-Perot en régime non-linéaire. Dans un premier temps, on présente l'étude des non-linéarités de nano-cavités en Silicium à grand facteur de qualité et faible volume modal, démontrant ainsi l'obtention d'un régime de fonctionnement bistable pour de très faibles puissances. Dans un deuxième temps, on démontre la modulation possible via la sonde champ proche de ce régime de bistabilité. Enfin, on étudie dans la troisième partie de ce manuscrit les résonances de films minces métalliques. L'étude complète des modes propres et de l'excitation optique de cette structure a permis de connaitre avec précision les modes résonants et les angles de Brewster du film mince. Cette étude a été prolongée expérimentalement par l'étude en champ proche optique des modes résonants de demi-films minces métalliques. Lors de ces deux études, les mesures expérimentales ont été systématiquement accompagnées par l'analyse théorique et le développement numérique de modèles que les expériences réalisées ont permis de valider et de discuter.

An apparatus that combines single molecule fluorescence, optical trapping and bright-field microscopy is presented. Given the spread over orders of magnitude of the light intensities for the different techniques, special considerations in choosing the spectral regions for each were taken. Moreover, imaging single molecules in a background of intense light from the infra red laser used for the optical trap has been shown to result in enhanced photo-bleaching due to two-photon processes. A scheme for fast time-sharing was implemented in which the fluorescence excitation light and the trap light alternate in fast succession. This eliminates two-photon effects and enables stable manipulation using the optical trap. The simultaneous use of the bright-field imaging in differential interference contrast arrangement enables observation of refractile objects in the sample over large distances. The apparatus was designed for future use in studies of molecular motor regulation. However, to demonstrate the functionality of the system, the rotational diffusion of a micro-sphere fluorescently labelled at one spot was measured.
text

博士
國立交通大學
光電工程研究所
103
Optical force enables the contactless and nondestructive manipulation of tiny fragile objects which is unachievable by any mechanical tweezers. Recent research on integrated optical trapping using the forces induced by evanescent fields has opened up new opportunities for the manipulation in lab-on-a-chip systems. Due to the highly localized field distribution in near field region, particles of sub-micrometer and even nanometer size can be manipulated with precision higher than most conventional tools. However the evanescent field is always weak and therefore the input power must be high to generate force of sufficient strength. Considerable interest has emerged for the use of resonant mode in either optical microcavity or plasmonic structures as ways to enhance optical forces and lower power consumption. Moreover, the resonant characteristic will provide possibility for label-free detection of nanoparticles and molecules with high sensitivity. Such a multiple functional device, which is compatible with parallel manipulation, will be a key component in lab on chip development for bio-chemistry applications. This dissertation focuses right on demonstration of on-chip optical manipulation system with multiple functionalities, including trapping, transportation, detection, and sensing. By design of photonic crystal waveguide, we demonstrate a particle transportation system which is very efficient due to the implementation of slow light effect. For increasing the controllability of near-field optical manipulation, a brand new functionality called controllable transportation is proposed using tapered photonic crystal waveguide. For pursuing compactness of a trapping system we also integrate plasmonic structure on optical waveguide. To develop a compact and efficient trapping system with detection and sensing functionality at the same time, we turn to design one-dimensional photonic crystal cavity. We first propose a nanobeam photonic crystal cavity with waist structure which is accessible for particle of various sizes. This design reaches a trapping force record at nano-Newton level. At the end of this work we further push the record to the level of a few tens of nano-Newton by surface-like resonant mode of nano-fishbone photonic crystal cavity. Meanwhile its abilities in particle detection and surrounding medium sensing are quite remarkable. We anticipate these on-chip particle manipulation approaches we introduce could lead to various applications in nanotechnology and the environmental and life sciences.

Metallic nanostructures are one of the most versatile tools available for manipulating light at the nanoscale. These nanostructures support surface plasmons, which are collective excitations of the conduction electrons that can exist as propagating waves at a metallic interface or as localized excitations of a nanoparticle or nanostructure. Plasmonic structures can efficiently couple energy from freely propagating electromagnetic waves to localized electromagnetic fields and vice-versa, essentially acting as an optical antenna. As a result, the intensity of the local fields around and inside the nanostructure are strongly enhanced compared to the incident radiation. In this thesis, this ability to manipulate electromagnetic fields on the nanoscale is employed to control a wide range of optical phenomena. These studies are performed using structures based on metallic nanoshells, which consist of a thin Au shell coating a silica nanosphere. To investigate the parameters controlling the plasmonic response of metallic nanoshells, two changes to the nanoshell composition are studied: (1) the Au shell is replaced with Cu which has interband transitions that strongly influence the plasmon resonance, and (2) the silica core is replaced by a semiconducting Cu 2O core which has a significantly higher dielectric constant and non-trivial absorbance. The focusing of electromagnetic energy into intense local fields by plasmonic nanostructures is then directly investigated by profiling the nanoshell near field using a Raman-based molecular ruler. Next, plasmons supported by Au nanoshells are used to control the fluorescence of near-infrared fluorophores placed at controlled distances from the nanoshell surface. In this context, the analogy of an optical antenna is very relevant: the enhanced field at the surface of the nanoshell increases the absorption of light by the fluorophore, or equivalently couples propagating electromagnetic waves into a localized receiver, while the large scattering cross section enhances the coupling of energy from a localized source, the fluorophore, to far-field radiation. Excellent agreement with models based on Mie theory is achieved for both Raman and fluorescence. Experimentally measured enhancements of the radiative decay rate for fluorophores on Au nanoshells and Au nanorods are also consistent with this model. Plasmonic nanostructures can also control the flow of light into larger structures. This is observed by measuring the nanoparticle-induced enhancement and suppression of photocurrent in a silicon photodiode is at the single particle level for silica nanospheres, Au nanospheres, and two types of Au nanoshell Finally, the simultaneous physical manipulation of an individual plasmonic nanostructure on the few-nanometer scale using light and detection of the local electromagnetic field during this ongoing process with the same incident beam is performed. For this experiment, a Au nanoshell is separated from a metallic surface by a few-nanometer thick polymer layer to form a nanoscale junction, or nanogap Illuminating this structure with ultrashort optical pulses, exciting the plasmon resonance, results in a continuous, monitorable collapse of the nanogap. An easily detectable four-wave mixing (FWM) signal is simultaneously generated by this illumination of the nanogap, providing a continuous, highly sensitive optical monitor of the nanogap spacing while it is being optically reduced. The dramatic increase in this signal upon contact provides a clear, unambiguous signal of the gap closing.

博士
國立臺灣師範大學
物理學系
95
High local-field enhancement appears in the vicinity of the surface of metallic nanostructures due to surface plasmon excitation and therefore electromagnetic fields can be confined and controlled in nanoscale region. The controllable and tunable surface plasmon of metallic nanoparticles is studied analytically and numerically using Mie scattering theory and finite-difference time-domain method, respectively. For a single coated metallic nanoparticle, the surface plasmon excitation can be controlled by changing permittivity of the coated material, and furthermore the higher-mode enhancement of the coated metallic nanoparticle is observed. The surface plasmon due to near-field coupling of metallic nanoparticles is studied numerically by finite-difference time-domain method. For a silver nanocylinder pair with different radii of silver nanocylinders, asymmetric polarization charges are induced by the mutual interaction between nanocylinders. asymmetric confined charges are induced around the nanoscale gap by the mutual interaction between the nanocylinders. The field intensity in the gap associated with the density of confined charges around the gap and it can be controlled by interparticle distance, radius ratio of asymmetric pairs, and the illumination direction of incident light.Besides, the controllable and predictable surface plasmon resonance is demonstrated in three-pair array structures. A simplified open cavity model is proposed to understand the cavity-like resonant behavior of pair array structures. Recently, local-field enhancement of metallic nanoparticles has been applied to enhance the resolution and the sensitivity of near-field optical disks and fiber-optic biosensors, respectively. The random distributed silver nanoparticles which embedded in AgOx layer of an AgOx-type near-field optical disk become high scattering centers and can transfer the diffracted evanescent components from subwavelength recording marks into the propagating components that can be detected in far-field region. The numerical result shows that the recording marks smaller than wavelength/10 are distinguishable. A Fourier optics approach is used as a theoretical background to understand the subwavelength resolution capability of near-field optical disks. The influences of the distribution and density of metallic nanoparticles, and the illumination frequency of incident light on the resolution capability of near-field optical disks are also discussed. Finally, the mechanism of fluorescence signal enhancement of a localized surface plasmon coupled fluorescence fiber-optic biosensor with gold nanoparticles is studied by the scattering of evanescent waves by a single gold nanoparticle. Local-field enhancement appears in the vicinity of a gold nanoparticle when the nanoparticle is illuminated by evanescent waves from the surface of the uncladded fiber and the fluorescence signals of fluorophores which is bounded on the surface of the nanoparticle can be enhanced by the enhanced localfield. Calculated result shows that the averaged-field intensity around the gold nanoparticle is enhanced few times of the field intensity without nanoparticle and the field enhancement of the theoretical calculation is consistent with the experimental result.