This dissertation details our experiments on studying the Brownian motion of an optically trapped microsphere with ultrahigh resolution, and cooling of its motion towards the quantum ground state. We have trapped glass microspheres in water, air and vacuum with optical tweezers. We developed a detection system that can monitor the position of a trapped microsphere with Angstrom spatial resolution and microsecond temporal resolution. We studied the Brownian motion of a trapped microsphere in air over a wide range of pressures. We measured the instantaneous velocity of a Brownian particle. Our results provide direct verification of the Maxwell-Boltzmann velocity distribution and the energy equipartition theorem for a Brownian particle. For short time scales, the ballistic regime of Brownian motion is observed, in contrast to the usual diffusive regime. We are currently developing a new detection system to measure the instantaneous velocity of a Brownian particle in water. In vacuum, we have used active feedback to cool the three center-of-mass vibration modes of a trapped microsphere from room temperature to millikelvin temperatures with a minimum mode temperature of 1.5 mK, which corresponds to the reduction of the root mean square (rms) amplitude of the microsphere from 6.7 nm to 15 pm for that mode. The mean thermal occupation number of that mode is reduced from about 6.8$\times 10^8$ at 297 K to about 3400 at 1.5 mK. / text

This dissertation presents subwavelength optics with focus on the theory and applications of subwavelength apertures in a metal film. Two main issues regarding the optics with subwavelength apertures are investigated. As the first issue, the extraordinary optical transmission (EOT) through a single hole in a metallic waveguide is presented. A total transmission through a single subwavelength aperture is theoretically predicted for a perfect electric conductor regardless of the aperture size, without relying on aperture arrays and surface corrugations as presented in previous works. The waveguide EOT is then applied to boost the optical throughput of an apertured near-field scanning optical microscope (NSOM) probe. Using a new structure for the apertured NSOM probe which allows for waveguide EOT, the optical throughput and the damage threshold are boosted by 100× and 40× as compared to a conventional structure, and the experimental findings are backed-up by comprehensive finite-difference time-domain (FDTD) simulations. Single fluorescent molecules are scanned using the EOT apertured NSOM probe, and a spatial resolution of 62 nm is achieved. As the second issue, subwavelength apertures are found useful for optical trapping. A small dielectric particle can significantly change the optical transmission through an aperture by dielectric loading, and subsequently, a large optical force is induced which favors trapping. A self-induced back-action (SIBA) optical trap is designed using a circular nanohole in a gold film. Trapping of 50 nm polystyrene particle is experimentally achieved, which is not possible using a conventional single beam optical tweezers. The circular nanohole SIBA trap works beyond the perturbative regime, as proven by FDTD simulations and a Maxwell stress tensor analysis. We further improve the nanohole trapping using a double-nanohole, which is more sensitive for small dielectric changes due to the intense local field enhancement between its two sharp tips. A single 12 nm silica sphere is experimentally trapped using the double-nanohole, as the smallest trapped dielectric particle reported. We also achieve the trapping of a single protein – a bovine serum albumin (BSA) protein with a hydrodynamic radius of 3.4 nm in the folded form. The trapped BSA is also unfolded by the large optical force, as confirmed by experiments with changing optical power and changing pH. The high signal-to-noise ratio of 33 in monitoring single protein trapping and unfolding shows a tremendous potential for using the double-nanohole as a sensor for protein binding events at a single molecule level. / Graduate

Este trabalho é uma contribuição às análises de forças de aprisionamento óptico exercidas sobre partículas esféricas por superposições discretas de feixes de Bessel. Um estudo teórico-numérico foi realizado no regime de óptica geométrica, completando as pesquisas já realizadas com tais feixes e no caso escalar, tanto no regime de Rayleigh quanto através de um formalismo eletromagnético completo. Investigam-se padrões longitudinais de intensidade de possível interesse prático, com potenciais de fornecer múltiplas armadilhas ópticas simultâneas utilizando dois métodos de análise. O primeiro parte da observação de que no regime paraxial todos os raios associados aos feixes se encontram quase paralelos entre si quando se toma como aproximação uma superposição de raios paralelos que incidem completamente sobre um hemisfério do espalhador. Tal método, entretanto, é naturalmente restrito pelas forças transversais. O segundo, que torna mais confiáveis e precisas as predições acerca da componente longitudinal de força, adota procedimentos mais robustos que levam em consideração tanto a contribuição da pressão de radiação (força de espalhamento) quanto a força gradiente devido a gradientes locais de intensidade associados a raios não-paralelos. Assim, acredita-se que este trabalho traz como contribuição um reforço a esta classe específica e promissora de feixes não difrativos como feixes de luz interessantes para aplicações em aprisionamento e micromanipulação óptica. / This work is a contribution to the analyses of optical trapping forces exerted on spherical particles by discrete superpositions of Bessel beams. A theoretical-numerical study has been carried out in the ray optics regime, completing the pre-existing research performed with such beams, in the scalar case, both in the Rayleigh regime and through a complete electromagnetic formalism. We investigate longitudinal intensity patterns of possible practical interest with potential to provide multiple simultaneous optical traps, by using two methods of analysis. The first assumes that, in the paraxial regime, all rays associated to the beams are almost parallel to each other, taking a superposition of parallel rays that are completely incident on a hemisphere of the scatterer as a suitable approximation. Such a method, however, is naturally constrained by the transverse forces. The second one, which makes the predictions about the longitudinal force component more reliable and accurate, adopts more robust procedures that take into consideration the contribution of the radiation pressure (scattering force) as well as the gradient force due to local intensity gradients associated to non-parallel rays. Thus it is hoped that this work will contribute to reinforce this specific and promising class of non-diffractive beams as interesting light beams for applications in optical trapping and micromanipulation.

Desde a sua origem nos laboratórios do Bell Labs, durante a década de 70, pinças ópticas vêm sendo investigadas, desenvolvidas e diversificadas por vários grupos nacionais e internacionais incluindo, nos últimos anos, o nosso departamento (Departamento de Engenharia Elétrica e de Computação - SEL/EESC/USP) e, em particular, o Grupo de Telecomunicações a ele vinculado, com propostas de aplicações em óptica biomédica e medicina. Dada a importância deste assunto, o principal objetivo deste trabalho é estender a análise teórica de forças ópticas utilizando uma descrição simples, porém eficaz, e analítica de um feixe de Bessel truncado, isto é, gerado por abertura finita. Até onde sabemos, esta é a primeira tentativa de introduzir esta investigação dos feixes originais de Durnin na literatura, e, deste modo, temos a possibilidade de contribuir com uma análise inédita de um feixe de Bessel com características físicas reais. Com a modelagem matemática dos feixe de Bessel truncados em mãos, aplicamos o cálculo de forças nos regimes de óptica geométrica e de Rayleigh e analisamos o seu comportamento quando os feixes incidem em espalhadores dielétricos esféricos. Como resultado direto deste trabalho, publicamos dois artigos descrevendo as forças ópticas no regime de óptica geométrica e de Rayleigh que um feixe de Bessel truncado causa em uma partícula dielétrica esférica. Como resultados indiretos, publicamos dois artigos com os mesmos cálculos de forças, adotando classe de feixes chamada Frozen-Waves, que é uma superposição de feixes de Bessel de mesma frequência. / Since its origins at Bell Labs laboratories, during the 1970s, optical tweezers have been researched, developed and diversified by several research groups, both nationally and internationally, including in our department (Department of Electrical and Computing Engineering - SEL/EESC/USP), in particular the Telecom Group associated to it, with several proposals with applications of optical tweezers in different fields from medicine to biomedical optics. Given the importance of this field, our main goal is to extend the theoretical analysis of optical forces using a simplest - but effective - analytical description of truncated Bessel beams (i.e. beams that are generated by finite apertures). As far we know, this is the first attempt to introduce such analysis of the originals Durnin beams in the literature, thus, we may have the chance to contribute to an unpublished study of a truncated Bessel beam with real characteristics. With such mathematical model at hands, we calculate the forces exerted by the truncated Bessel beam in ray optics and Rayleigh regime and analyze their behavior when hits dielectric spherical particles. As direct result of this work, we have published two papers describing the optical forces in ray optics and Rayleigh regime that a truncated Bessel beam causes in a spherical dielectric particle. As indirect results, we have published two papers with the same force considerations, adopting a beam class called Frozen-Waves, which is a superposition of same frequency Bessel beams.

No description available.

No description available.

The two main themes in this thesis are the implementation of interference methods with optically trapped particles for measurements of position and optical phase (optical interferometric metrology) and the optical manipulation of nanoparticles for studies in the assembly of nanostructures, nanoscale heating and nonlinear optics (nanophotonics). The first part of the thesis (chapter 1, 2) provides an introductory overview to optical trapping and describes the basic experimental instrument used in the thesis respectively. The second part of the thesis (chapters 3 to 5) investigates the use of optical interferometric patterns of the diffracting light fields from optically trapped microparticles for three types of measurements: calibrating particle positions in an optical trap, determining the stiffness of an optical trap and measuring the change in phase or coherence of a given light field. The third part of the thesis (chapters 6 to 8) studies the interactions between optical traps and nanoparticles in three separate experiments: the optical manipulation of dielectric enhanced semiconductor nanoparticles, heating of optically trapped gold nanoparticles and collective optical response from an ensemble of optically trapped dielectric nanoparticles.

Optical trapping of metal nanoparticles investigates phenomena at the interface of plasmonics and optical micromanipulation. This thesis combines ideas of optical properties of metals originating from solid state physics with force mechanism resulting from optical trapping. We explore the influence of the particle plasmon resonance of gold and silver nanospheres on their trapping properties. We aspire to predict the force mechanisms of resonant metal particles with sizes in the Mie regime, beyond the Rayleigh limit. Optical trapping of metal nanoparticles is still considered difficult, yet it provides an excellent tool to investigate their plasmonic properties away from any interface and offers opportunities to investigate interaction processes between light and nanoparticles. Due to their intrinsic plasmon resonance, metal nanoparticles show intriguing optical responses upon interaction with laser light. These differ greatly from the well-known bulk properties of the same material. A given metal nanoparticle may either be attracted or repelled by laser light, only depending on the wavelength of the latter. The optical forces acting on the particle depend directly on its polarisability and scattering cross section. These parameters vary drastically around the plasmon resonance and thus not only change the magnitude but also the direction and entire nature of the acting forces. We distinguish between red-detuned and blue-detuned trapping, that is using a trapping wavelength shorter or longer than the plasmon resonance of the particle. So far optical trapping of metal nanoparticles has focussed on a wavelength regime far from the particle’s resonance in the infrared. We experiment with laser wavelengths close to the plasmon resonance and expand the knowledge of metal nanoparticle trapping available to date. Existing theoretical models are put to the test when we compare these with our real experimental situations.

The mammalian hippo signalling pathway controls cell proliferation and apoptosis via transcriptional co-activators YAP and TAZ, and as such is a key regulator of organ and tissue growth. Multiple cellular components converge in this pathway, including the actin cytoskeleton, which is required for YAP/TAZ activity. The precise mechanism by which the mechanical actomyosin network regulates Hippo signalling, however, is unknown. Optical methods provide a non-invasive way to image and study the biomechanics of cells. In the past two decades, super-resolution fluorescence microscopy techniques that break the diffraction limit of light have come to the fore, enabling visualisation of intracellular detail at the nanoscale level. Optical trapping, on the other hand, allows precise control of micron-sized objects such as cells. Here, super resolution structured illumination microscopy (SIM) and elastic resonator interference stress microscopy (ERISM) were used to investigate a potential role for the FERM-domain protein FRMD6, or Willin, in the mechanical control of the Hippo pathway in a neuronal cell model. A double optical trap was also integrated with the Nikon-SIM with the aim of cell stretching. Willin expression was shown to modify the morphology, neuronal differentiation, actin cytoskeleton and forces of SH-SY5Y cells. Optical trapping from above the SIM objective, however, was demonstrated to be ineffective for manipulation of adherent cells. The results presented here indicate a function for Willin in the assembly of actin stress fibres that may be the result of an interaction with the Hippo pathway regulator AMOT. Further investigation, for example by direct cell stretching, is required to elucidate the exact role of Willin in the mechanical control of YAP/TAZ.

Exploiter le degré de liberté angulaire de la lumière pour contrôler les forces optiques ouvre une nouvelle voie pour la manipulation optique de systèmes matériels. Dans ce contexte, notre travail porte sur l’interaction lumière-matière en présence de chiralité, qu’elle soit matérielle ou ondulatoire. Expérimentalement, nous avons utilisé des gouttes de cristaux liquides cholestériques interagissant avec un ou plusieurs champs lumineux polarisés circulairement et nous avons apporté une description quantitative de nos observations. Notre principal résultat correspond à la démonstration que la pression de radiation optique peut être contrôlée par l’hélicité du photon. Ce phénomène est ensuite utilisé, d’une part pour faire une démonstration de principe du tri de la chiralité matérielle via une approche optofluidique et d’autre part pour réaliser un piège optique tridimensionnel sensible à la chiralité de l’objet piégé. / Exploiting the angular momentum degree of freedom of light to control the mechanical effects that result from light-matter exchanges of linear momentum is an intriguing challenge that may open new routes towards enhanced optical manipulation of material systems. In this context, our work addresses the interplay between the chirality of matter and the chirality of optical fields. Experimentally, this is done by using cholesteric liquid crystal droplets interacting with circularly polarized light and we provide with theoretical developments to quantitatively support our observations. Our main result is the demonstration of optical radiation force controlled by the photon helicity. This phenomenon is then used to demonstrate the optofluidic sorting of material chirality and the helicity-dependent three-dimensional optical trapping of chiral liquid crystal microspheres.