108 research outputs found

    Optical super-resolution and periodical focusing effects by dielectric microspheres

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    Optical microscopy is one of the oldest and most important imaging techniques; however, its far-field resolution is diffraction-limited. In this dissertation, we proposed and developed a novel method of optical microscopy with super-resolution by using high- index dielectric microspheres immersed in liquid and placed on the surface of the structures under study. We used barium titanate glass microspheres with diameters of D~2-220 µm and refractive indices n~1.9-2.1 to discern minimal feature sizes ~?/4 (down to ~?/7) of various photonic and plasmonic nanostructures, where ? is the illumination wavelength. We studied the magnification, field of view, and resolving power, in detail, as a function of sphere sizes. We studied optical coupling, transport, focusing, and polarization properties of linear arrays of dielectric spheres. We showed that in arrays of spheres with refractive index n=v3, a special type of rays with transverse magnetic (TM) polarization incident on the spheres under the Brewster’s angle form periodically focused modes with radial polarization and 2D period, where D is the diameter of the spheres. We showed that the formation of periodically focused modes in arrays of dielectric spheres gives a physical explanation for beam focusing and extraordinarily small attenuation of light in such chains. We showed that the light propagation in such arrays is strongly polarization- dependent, indicating that such arrays can be used as filters of beams with radial polarization. The effect of forming progressively smaller focused beams was experimentally observed in chains of sapphire spheres in agreement with the theory. We expanded the concept of periodically focused modes to design a practical device for ultra-precise contact-mode laser tissue-surgery, with self-limiting ablation depth for potential application in retina surgery. By integrating arrays of dielectric spheres with infrared hollow waveguides and fibers, we fabricated prototypes of the designs and tested them with an Er:YAG laser. Furthermore, we proposed another design based on conical arrays of dielectric spheres to increase the coupling efficiency of the probe

    Development of laser sources and interferometric approaches for polarization-based label-free microscopy

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    The project developed in this thesis describes the design and the experimental realization of optical methods which can probe the anisotropy of semitransparent media. The ability to manipulate polarized light enables a label-free imaging approach that can retrieve fundamental information about the sample structure without introducing any alteration within it. Such a potential is of great importance and methods like the ones based on polarization analysis are gaining more and more popularity in the biomedical and biophysical fields. Moreover, when they are coupled with fluorescence microscopy and nanoscopy, they may provide an invaluable tool for researchers. The optical method I developed mainly exploits the laser radiation emitted from tailored optical oscillators to dynamically generate polarization states. The realization of such states does not comprise any external active device. The resulting time-evolving polarization state once properly coupled to an optical system enables probing a sample to retrieve its anisotropies at a fast rate. The development of two different laser sources is presented together with the characterizations of their optical properties. One of them consists of a Helium-Neon laser modified by applying an external magnetic field to trigger the Zeeman effect in its active medium. The other one is a Dual-Comb source, that is a mode-locked (ML) laser generating a pair of mutually coherent twin beams. Moreover, the thesis delivers the theoretical model and the experimental realization of the optical method to probe the optical anisotropies of specimens. Finally, the technical realization of a custom laser scanning optical microscope and its imaging results obtained with such methods are reported

    Contrast Mechanisms & Wavefront Control in Coherent Nonlinear Microscopy

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    This Thesis deals with theoretical and experimental aspects of coherent nonlinear microscopy, with a special attention given to contrast mechanisms in Third Harmonic Generation (THG) microscopy.Ce travail de thèse présente une étude théorique et expérimentale de la microscopie non-linéaire cohérente pour l’étude de systèmes biologiques.Dans un premier temps, nous avons étudié l’origine des signaux et les mécanismes de contrastes en microscopie non-linéaire cohérente. En particulier, nous avons analysé l’influence de la microstructure de l’échantillon sur le signal de troisième harmonique rayonné. Nous illustrons cette discussion par une étude sur les contrastes endogènes de la cornée humaine. Dans un second temps, nous nous sommes intéressés à diverses méthodes de contrôle de front d’onde afin de moduler le contraste des images. Nous avons effectué par une étude théorique du signal obtenus à l’aide de modes d’excitation spatiale non-Gaussiens, puis avons implémenté deux systèmes de modulation du front d’onde : un modulateur acousto-optique permettant d’obtenir une profondeur de champ étendue en microscopie par fluorescence excitée à deux photons, et un miroir déformable permettant d’effectuer une correction dynamique des aberrations en microscope par génération de troisième harmonique. Enfin, nous nous sommes intéressés à l’application de la microscopie non-linéaire cohérente pour l’observation du développement précoce d’organismes modèles en embryologie: la drosophile et le poisson zèbre

    Spatiotemporal Nonlinear Optical Effects in Multimode Fibers

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    The advent of the optical fiber in the second half of the 20th century has had numerous consequences not only for the advancement of telecommunication and information transfer technologies, but also for humanity as a whole. At the time of writing of this thesis, we live in a world that is defined by unprecedented and unparalleled access to information made possible by fiber optic cables that line the ocean beds. As the world becomes increasingly reliant upon the internet, the demand for access is outgrowing the pace at which the capacity of our fiber optic networks can be scaled up. In the past decade, a consensus has emerged among researchers in academia and the industry that we are now approaching the fundamental classical capacity limit of a conventional single mode fiber, namely the Shannon limit. This has spurred interest in multimode fibers that allow for hundreds of spatial modes to co-propagate, potentially allowing for at least an order of magnitude increase in capacity per fiber. While multiplexing in the spatial domain has the potential to offer significantly higher capacities, linear and nonlinear mixing between the spatial modes of a fiber are expected to play an important role in determining the capacity and performance of spatially multiplexed telecommunication systems. So far, multimode fibers have mostly been relegated to low-power short- distance links, as a result of which nonlinear propagation effects in the presence of multiple spatial modes has received relatively little attention. This thesis adds to a growing body of literature that is increasingly interested in uncovering the physics of multimodal propagation in the nonlinear regime. Although the need for spatial multiplexing is important factor driving research interest in this topic, experiments in recent years have revealed a plethora of complex spatiotemporal non- linear phenomena occurring in multimode fibers, including Kerr-induced beam self-cleaning, parametric instability processes and the existence of multimode solitons. This has sparked great interest in understanding multimodal nonlinearity from a fundamental and applied physics per- spective. Nonlinear multimode fiber optics is also of central importance for the development of high power fiber-coupled lasers as the larger core size of multimode fibers allow for far higher power throughput than current state-of-the-art lasers based on single mode fibers. Most literature reported thus far in multimodal nonlinear optics focuses on complex phe- nomena occurring when hundreds of spatial modes co-propagate in the nonlinear regime. While that has proven to be a fascinating field of study, there have not been many studies on experimental investigation of intermodal nonlinear effects in the presence of a small number of spatial modes. Furthermore, nonlinear phenomena in multimode fibers are ‘spatiotemporal’ in nature, meaning that the spatial and time-domain waveforms are intertwined, and the two degrees of freedom cannot be separated. Conventional measurement techniques are not capable of resolving such a multimodal beam in space and time simultaneously. Finally, most research involving nonlinear optical effects has thus far focused on linearly polarized modes in conventional fibers, and nonlinear effects involving vector orbital angular momentum modes remains relatively understudied. In this thesis, we seek to study nonlinear optical effects involving a small number of selectively excited scalar as well as vector spatial modes, and to develop experimental techniques capable of resolving the output in both space, frequency and time. To this end, we design, prototype and fabricate devices and methods aimed at exciting a small number of spatial modes of a fiber. In particular, we adopt methods from integrated photonics such as focused ion beam milling and metasurface devices to selectively excite modes of a fiber in an efficient manner. Spatial and temporal resolution of the output beam is achieved by the development of a new technique that involves raster-scanning of a near-field scanning optical microscopy probe, coupled with a high speed detector, along the output end-face of the fiber. Using these methods, we uncover and report our observations of spatiotemporal nonlinear phenomena that are unique to multimodal systems. We first demonstrate nonlinear intermodal interference of radially symmetric modes in step-index and parabolic index fibers. We then apply the same spatiotemporal measurement technique to observe the Kerr-induced beam self-cleaning phenomenon in a parabolic index fiber. And finally, we discuss our discovery of a spin-orbit coupled generalization of the well-known nonlinear polarization rotation phenomenon

    Polarisation microscopy of single emitters

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    This thesis contains a report on the development of a new type of confocal microscope. The microscope aims to allow the user to be able to determine the three dimensional orientation of single fluorescent emitters. The microscope has at its heart a binary spatial light modulator that allows us to control the excitation electric field in the pupil of the microscope objective. This allows us to exploit the fact that the excitation of, and emission from, a single fluorescent emitter is polarisation and orientation dependent. By changing the field in the excitation pupil we can generate a set of images that when taken together can be analysed to find the emitter orientation. We show that the microscope allows us to resolve the orientation of single fluorescent molecules and nitrogen vacancy centres in nanodiamond. We designed the microscope from scratch using extensive mathematical modelling techniques. We anticipate that these models will be useful to other researchers. One example is that our model of the polarisation distortions introduced during scanning is relevant to any galvanometer-based scanning system. We also developed a full model of a confocal microscope that includes the dipole-like nature of many samples. We use this to calculate, amongst other things, the optical sectioning properties of confocal microscopes. This allows us to validate previous models that ignored polarisation distortions of high numerical aperture lenses and also to make calculations where previous models would have been inadequate, for example in calculating the sectioning strength of sheets of aligned dipoles. As well as developing numerical models, we invented a new method for controlling the polarisation of light using a binary spatial light modulator. This work has applications in materials science, and industrial applications.Open Acces

    Design and implementation of ultra-high resolution, large bandwidth, and compact diffuse light spectrometers

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    My research on the new concepts for spectrometer has been focused on the development of true multi-dimensional spectrometers, which use a multi-dimensional [two-dimensional (2D) or 3D] mapping of the spectral information into space. I showed that by combining a simple dispersive element (a volume hologram) formed in very inexpensive polymers with a basic Fabry-Perot interferometer, we can form a spectrometer with ultra-high resolution over a large spectral bandwidth, which surpasses all conventional spectrometers. I strongly believe that the extension of this mapping into three dimensions by using synthetic nanophotonic structures with engineered dispersion can further improve the performance and reduce the overall spectrometer size into the micron regime. The need for efficient modeling and simulation tools comes from the sophisticated nature of the new 3D nanophotonic structures, which makes their simple analysis using well-known simple formulas for the propagation of the electromagnetic fields in bulk materials impossible. In my Ph.D. research, I developed new approximate modeling tools for both the modeling of incoherent sources in nanophotonics, and for the propagation of such optical beams inside the 3D nanophotonic structures of interest with several orders of magnitude improvement in the simulation speed for practical size devices without sacrificing accuracy. To enable new dispersive properties using a single nanophotonic structure, I have focused in my Ph.D. research into polymer-based 3D photonic crystals, which can be engineered using their geometrical features to demonstrate unique dispersive properties in three dimensions that cannot be matched by any bulk material even with orders of magnitude larger sizes. I have demonstrated the possibilities of using a very compact structure for wavelength demultiplexing and also for spectroscopy without adding any other device.Ph.D.Committee Chair: Adibi, Ali; Committee Member: Bhatti, Pamela; Committee Member: Callen, William; Committee Member: Gaylord, Thomas; Committee Member: Zhou, Hao-Mi

    Roadmap on structured light

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    Structured light refers to the generation and application of custom light fields. As the tools and technology to create and detect structured light have evolved, steadily the applications have begun to emerge. This roadmap touches on the key fields within structured light from the perspective of experts in those areas, providing insight into the current state and the challenges their respective fields face. Collectively the roadmap outlines the venerable nature of structured light research and the exciting prospects for the future that are yet to be realized

    Recent Advances and Future Trends in Nanophotonics

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    Nanophotonics has emerged as a multidisciplinary frontier of science and engineering. Due to its high potential to contribute to breakthroughs in many areas of technology, nanophotonics is capturing the interest of many researchers from different fields. This Special Issue of Applied Sciences on “Recent advances and future trends in nanophotonics” aims to give an overview on the latest developments in nanophotonics and its roles in different application domains. Topics of discussion include, but are not limited to, the exploration of new directions of nanophotonic science and technology that enable technological breakthroughs in high-impact areas mainly regarding diffraction elements, detection, imaging, spectroscopy, optical communications, and computing

    Complex extreme nonlinear waves: classical and quantum theory for new computing models

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    The historical role of nonlinear waves in developing the science of complexity, and also their physical feature of being a widespread paradigm in optics, establishes a bridge between two diverse and fundamental fields that can open an immeasurable number of new routes. In what follows, we present our most important results on nonlinear waves in classical and quantum nonlinear optics. About classical phenomenology, we lay the groundwork for establishing one uniform theory of dispersive shock waves, and for controlling complex nonlinear regimes through simple integer topological invariants. The second quantized field theory of optical propagation in nonlinear dispersive media allows us to perform numerical simulations of quantum solitons and the quantum nonlinear box problem. The complexity of light propagation in nonlinear media is here examined from all the main points of view: extreme phenomena, recurrence, control, modulation instability, and so forth. Such an analysis has a major, significant goal: answering the question can nonlinear waves do computation? For this purpose, our study towards the realization of an all-optical computer, able to do computation by implementing machine learning algorithms, is illustrated. The first all-optical realization of the Ising machine and the theoretical foundations of the random optical machine are here reported. We believe that this treatise is a fundamental study for the application of nonlinear waves to new computational techniques, disclosing new procedures to the control of extreme waves, and to the design of new quantum sources and non-classical state generators for future quantum technologies, also giving incredible insights about all-optical reservoir computing. Can nonlinear waves do computation? Our random optical machine draws the route for a positive answer to this question, substituting the randomness either with the uncertainty of quantum noise effects on light propagation or with the arbitrariness of classical, extremely nonlinear regimes, as similarly done by random projection methods and extreme learning machines

    Modelling diffraction in optical interconnects

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    Short-distance digital communication links, between chips on a circuit board, or between different circuit boards for example, have traditionally been built by using electrical interconnects -- metallic tracks and wires. Recent technological advances have resulted in improvements in the speed of information processing, but have left electrical interconnects intact, thus creating a serious communication problem. Free-space optical interconnects, made up of arrays of vertical-cavity surface-emitting lasers, microlenses, and photodetectors, could be used to solve this problem. If free-space optical interconnects are to successfully replace electrical interconnects, they have to be able to support large rates of information transfer with high channel densities. The biggest obstacle in the way of reaching these requirements is laser beam diffraction. There are three approaches commonly used to model the effects of laser beam diffraction in optical interconnects: one could pursue the path of solving the diffraction integral directly, one could apply stronger approximations with some loss of accuracy of the results, or one could cleverly reinterpret the diffraction problem altogether. None of the representatives of the three categories of existing solutions qualified for our purposes. The main contribution of this dissertation consist of, first, formulating the mode expansion method, and, second, showing that it outperforms all other methods previously used for modelling diffraction in optical interconnects. The mode expansion method allows us to obtain the optical field produced by the diffraction of arbitrary laser beams at empty apertures, phase-shifting optical elements, or any combinations thereof, regardless of the size, shape, position, or any other parameters either of the incident optical field or the observation plane. The mode expansion method enables us to perform all this without any reference or use of the traditional Huygens-Kirchhoff-Fresnel diffraction integrals. When using the mode expansion method, one replaces the incident optical field and the diffracting optical element by an effective beam, possibly containing higher-order transverse modes, so that the ultimate effects of diffraction are equivalently expressed through the complex-valued modal weights. By using the mode expansion method, one represents both the incident and the resultant optical fields in terms of an orthogonal set of functions, and finds the unknown parameters from the condition that the two fields have to be matched at each surface on their propagation paths. Even though essentially a numerical process, the mode expansion method can produce very accurate effective representations of the diffraction fields quickly and efficiently, usually by using no more than about a dozen expanding modes. The second tier of contributions contained in this dissertation is on the subject of the analysis and design of microchannel free-space optical interconnects. In addition to the proper characterisation of the design model, we have formulated several optical interconnect performance parameters, most notably the signal-to-noise ratio, optical carrier-to-noise ratio, and the space-bandwidth product, in a thorough and insightful way that has not been published previously. The proper calculation of those performance parameters, made possible by the mode expansion method, was then performed by using experimentally-measured fields of the incident vertical-cavity surface-emitting laser beams. After illustrating the importance of the proper way of modelling diffraction in optical interconnects, we have shown how to improve the optical interconnect performance by changing either the interconnect optical design, or by careful selection of the design parameter values. We have also suggested a change from the usual `square' to a novel `hexagonal' packing of the optical interconnect channels, in order to alleviate the negative diffraction effects. Finally, the optical interconnect tolerance to lateral misalignment, in the presence of multimodal incident laser beams was studied for the first time, and it was shown to be acceptable only as long as most of the incident optical power is emitted in the fundamental Gaussian mode
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