Recent Advances in Adaptive Liquid Crystal Lenses

An adaptive-focus lens is a device that is capable of tuning its focal length by means of an external stimulus. Numerous techniques for the demonstration of such devices have been reported thus far. Moving beyond traditional solutions, several new approaches have been proposed in recent years based on the use of liquid crystals, which can have a great impact in emerging applications. This work focuses on the recent advances in liquid crystal lenses with diameters larger than 1 mm. Recent demonstrations and their performance characteristics are reviewed, discussing the advantages and disadvantages of the reported technologies and identifying the challenges and future prospects in the active research field of adaptive-focus liquid crystal (LC) lenses.This work was supported by Comunidad de Madrid and FEDER Program under grant S2018/NMT-4326 and the Ministerio de Economía y Competitividad of Spain (TEC2013-47342-C2-2-R)

Computational Light Transport for Forward and Inverse Problems.

El transporte de luz computacional comprende todas las técnicas usadas para calcular el flujo de luz en una escena virtual. Su uso es ubicuo en distintas aplicaciones, desde entretenimiento y publicidad, hasta diseño de producto, ingeniería y arquitectura, incluyendo el generar datos validados para técnicas basadas en imagen por ordenador. Sin embargo, simular el transporte de luz de manera precisa es un proceso costoso. Como consecuencia, hay que establecer un balance entre la fidelidad de la simulación física y su coste computacional. Por ejemplo, es común asumir óptica geométrica o una velocidad de propagación de la luz infinita, o simplificar los modelos de reflectancia ignorando ciertos fenómenos. En esta tesis introducimos varias contribuciones a la simulación del transporte de luz, dirigidas tanto a mejorar la eficiencia del cálculo de la misma, como a expandir el rango de sus aplicaciones prácticas. Prestamos especial atención a remover la asunción de una velocidad de propagación infinita, generalizando el transporte de luz a su estado transitorio. Respecto a la mejora de eficiencia, presentamos un método para calcular el flujo de luz que incide directamente desde luminarias en un sistema de generación de imágenes por Monte Carlo, reduciendo significativamente la variancia de las imágenes resultantes usando el mismo tiempo de ejecución. Asimismo, introducimos una técnica basada en estimación de densidad en el estado transitorio, que permite reusar mejor las muestras temporales en un medio parcipativo. En el dominio de las aplicaciones, también introducimos dos nuevos usos del transporte de luz: Un modelo para simular un tipo especial de pigmentos gonicromáticos que exhiben apariencia perlescente, con el objetivo de proveer una forma de edición intuitiva para manufactura, y una técnica de imagen sin línea de visión directa usando información del tiempo de vuelo de la luz, construida sobre un modelo de propagación de la luz basado en ondas.<br /

Investigations of the Physical and Magnetic Microstructure of CoCr Thin Film Perpendicular Magnetic Recording Media

The work presented in this thesis is concerned with improving the understanding of the relationship between the physical and magnetic microstructure in CoCr thin film perpendicular magnetic recording media. This was investigated using a combination of transmission electron microscope (TEM) techniques to study a series of CoCr films with systematically adjusted growth conditions. The first chapter begins with an outline of basic ferromagnetism and the energy considerations governing the domain configuration in ferromagnetic thin films. General principles of magnetic recording are then discussed, with a more detailed treatment of media, high density recording formats and the properties of CoCr for perpendicular recording. Chapter 2 introduces basic TEM imaging theory and describes the conventional transmission electron microscopes (CTEM's) and the dedicated scanning transmission electron microscope (STEM) used in this project. The chapter then discusses techniques for imaging magnetic structures and the use of high resolution energy dispersive x-ray (EDX) microanalysis in the STEM for microcompositional investigations. The first sections in chapter 3 discuss the planar and cross-sectional specimen preparation techniques adopted for all the work in this thesis. The remainder of the chapter describes the study of a series of CoCr layers grown to various thicknesses and with different compositions. Bulk physical and magnetic characterisation are combined with microstructural investigations in the CTEM. The work demonstrates that bulk measurements alone are insufficient to predict the physical, and therefore the magnetic microstructure, of CoCr thin films. It also illustrates the usefulness of such a study as part of any investigation of the microscopic properties of CoCr recording media. The next three chapters contain the experimental results which form the core of the thesis. Chapter 4 describes the experiments conducted on CoCr films using EDX microanalysis. Planar sections were investigated to allow correct positioning of the probe on a particular region of specimen and thus reveal, directly, the local elemental composition associated with features of the microstructure. Study was made of films deposited at different substrate temperatures both with and without a Ge underlayer. The results obtained using this technique provided detailed quantitative data on the extent and pattern of Cr segregation in each film. Chapter 5 describes the improvements in the differential phase contrast (DPC) mode of Lorentz microscopy in the STEM which facilitated simultaneous imaging of the physical and magnetic microstructure of thinned cross-sections of CoCr films. The results from the application of this technique to the study of the effect on the microstructure of substrate temperature during deposition are then interpreted in conjunction with the compositional information from chapter 4. The importance of direct microscopic study of recorded tracks in CoCr perpendicular recording media forms the introduction for chapter 6. The chapter then explains the development and results of a successful method for direct observation of the tracks with the Fresnel mode of Lorentz microscopy in the JEOL 2000FX TEM. The final chapter draws conclusions on the results of the project and presents proposals for possible future investigations of CoCr perpendicular magnetic recording media

Advanced direct laser micro-structuring of polymers for optical and other applications

Excimer laser ablation is a very well researched field, and there is a vast amount of publications on this subject. Many of them focus on the fundamentals of the ablation process and try to find models to describe the ablation mechanisms. This doctoral thesis takes the existing research as basis to focus on a more applied approach. Using well-established knowledge of UV ablation of polymers this thesis presents the development of a novel technique for the ablation of repeating micro-structures into polymer surfaces. Applications for these structures include large area optical films, e.g. lenticulars for 3D effect displays.As motivation for this thesis chapter 1 gives an overview over the emerging market of advanced micro-structures for displays and novel lighting solutions. State-of-the-art methods of realising micro-structures other than by laser ablation are presented. A review of the history of UV ablation of polymers, mask imaging and industrial applications in this field lays the foundation to validate the potential of employing excimer lasers to ablate complex 3D micro-structures.In chapter 2 the so-called Synchronised Image Scanning (SIS), an advanced mask imaging technology, is introduced along with the basic hardware components. The fundamentals of SIS are presented and the evolution of the technique from simple 2D thin film patterning to complex 3D micro-structuring is described. Furthermore, the growing complexity of the mask design as well as considerations regarding the mask set-up in the system are expounded.Chapter 3 looks more closely at the required hardware and the potential in efficiency, quality and new feature geometries and compares SIS to classical mask imaging methods. For instance, it is outlined that a Step & Repeat approach is not a valid option to ablate millions of features into a surface as it would take far too long while SIS cuts down process times dramatically by its on-the-flight and parallel processing. Furthermore, a portfolio of a great variety of different 3D features realised with the SIS technology is presented.Moving on from just qualitative considerations to more quantitative investigations, chapter 4 describes how a specific micro-lens array design is realised by SIS and analysed in detail using various metrology equipment and optical performance tests. These tests reveal generally a good agreement between design and ablation result. The cause of the relatively high surface roughness of the ablated features is investigated in more detail and the influence of ablation debris on the processed features is discussed.Chapter 5 looks at artefacts created between individual scans when convex (positive) micro-structures are processed. The appearance of the artefacts is first modelled and then compared to experimental results in order to validate the model. Further it is shown which measures can be used to eliminate these artefacts.Chapter 6 presents experiments in which the surface quality is enhanced by a laser polish post process. It was found that with the right combination of fluence and number of pulses per area it is possible to reduce the RMS value from 56 nm to 12 nm.While in all the experiments in the preceding chapters the material ablated was Polycarbonate, in chapter 7 the SIS technique is applied to a wider range of polymers. It is demonstrated that by finding the etch rate data of the relevant polymers and adapting the process parameters accordingly the technique used for Polycarbonate machining is indeed transferable to other polymers as long as they show a suitable ablation behaviour.The conclusion and summary chapter 8 shows that SIS is a valid technology to produce a wide range of feature geometries on large area substrates. Enquiries from next-generation product developers in industries like displays, lighting and anti-counterfeiting show that this technology is indeed relevant for industrial applications

Phase control and measurement in digital microscopy

The ongoing merger of the digital and optical components of the modern microscope is creating opportunities for new measurement techniques, along with new challenges for optical modelling. This thesis investigates several such opportunities and challenges which are particularly relevant to biomedical imaging. Fourier optics is used throughout the thesis as the underlying conceptual model, with a particular emphasis on three--dimensional Fourier optics. A new challenge for optical modelling provided by digital microscopy is the relaxation of traditional symmetry constraints on optical design. An extension of optical transfer function theory to deal with arbitrary lens pupil functions is presented in this thesis. This is used to chart the 3D vectorial structure of the spatial frequency spectrum of the intensity in the focal region of a high aperture lens when illuminated by linearly polarised beam. Wavefront coding has been used successfully in paraxial imaging systems to extend the depth of field. This is achieved by controlling the pupil phase with a cubic phase mask, and thereby balancing optical behaviour with digital processing. In this thesis I present a high aperture vectorial model for focusing with a cubic phase mask, and compare it with results calculated using the paraxial approximation. The effect of a refractive index change is also explored. High aperture measurements of the point spread function are reported, along with experimental confirmation of high aperture extended depth of field imaging of a biological specimen. Differential interference contrast is a popular method for imaging phase changes in otherwise transparent biological specimens. In this thesis I report on a new isotropic algorithm for retrieving the phase from differential interference contrast images of the phase gradient, using phase shifting, two directions of shear, and non--iterative Fourier phase integration incorporating a modified spiral phase transform. This method does not assume that the specimen has a constant amplitude. A simulation is presented which demonstrates good agreement between the retrieved phase and the phase of the simulated object, with excellent immunity to imaging noise

Phase control and measurement in digital microscopy

The ongoing merger of the digital and optical components of the modern microscope is creating opportunities for new measurement techniques, along with new challenges for optical modelling. This thesis investigates several such opportunities and challenges which are particularly relevant to biomedical imaging. Fourier optics is used throughout the thesis as the underlying conceptual model, with a particular emphasis on three--dimensional Fourier optics. A new challenge for optical modelling provided by digital microscopy is the relaxation of traditional symmetry constraints on optical design. An extension of optical transfer function theory to deal with arbitrary lens pupil functions is presented in this thesis. This is used to chart the 3D vectorial structure of the spatial frequency spectrum of the intensity in the focal region of a high aperture lens when illuminated by linearly polarised beam. Wavefront coding has been used successfully in paraxial imaging systems to extend the depth of field. This is achieved by controlling the pupil phase with a cubic phase mask, and thereby balancing optical behaviour with digital processing. In this thesis I present a high aperture vectorial model for focusing with a cubic phase mask, and compare it with results calculated using the paraxial approximation. The effect of a refractive index change is also explored. High aperture measurements of the point spread function are reported, along with experimental confirmation of high aperture extended depth of field imaging of a biological specimen. Differential interference contrast is a popular method for imaging phase changes in otherwise transparent biological specimens. In this thesis I report on a new isotropic algorithm for retrieving the phase from differential interference contrast images of the phase gradient, using phase shifting, two directions of shear, and non--iterative Fourier phase integration incorporating a modified spiral phase transform. This method does not assume that the specimen has a constant amplitude. A simulation is presented which demonstrates good agreement between the retrieved phase and the phase of the simulated object, with excellent immunity to imaging noise

The Micromagnetism of Video Recording Tracks Written on CoNi Metal Evaporated Tape (MET)

The main objective of the work described in this thesis was to study the micromagnetic structure of 8mm video recording tracks. We attempted to quantify the written magnetisation patterns and correlate the results with the bulk magnetic characteristics of each of the different samples analysed. We studied the micromagnetics as a function of the physical and magnetic characteristics of the films and also the wavelength of the recorded signal. The basic principles of ferromagnetism are outlined in chapter 1, and then in chapter 2 we describe the way in which the technology of magnetic recording harnesses these properties. Chapter 2 also provides details of the recording format used in video 8. This section concludes with the demands now placed on the physical and magnetic characteristics of the materials which these high storage density applications require. The high resolution required to examine the written tracks necessitates the use of electron microscopy and chapter 3 describes the basic beam specimen interactions and illustrates the ways in which contrast can be produced which relates to the magnetisation within the sample. Chapter 4 details the specific requirements of the Differential Phase Contrast (DPC) mode of Lorentz Electron Microscopy which have to be met if quantitative interpretation of the information is to be possible. This section also indicates the operational limits of this technique and the relevance of these on the results. The next 3 chapters (5-7) contain the results which form the main body of this thesis. The fabrication procedure of the films and the physical and compositional characterisation of the samples is detailed in chapter 5. Chapter 6 contains the results of the bulk magnetic characterisation of these MET films The quantitative micromagnetic results obtained from each film, at two recording densities, form the basis of chapter 7. We also detail small angle scattering experiments which were necessary to establish the validity of the quantitative results. The final chapter, chapter 8, suggests ways in which we feel that much of the information which we obtained, about the micromagnetic structure of the recorded samples, could be improved now that the DPC system has recently been modified. We also comment on the relevance of new techniques now available and how these might add to our existing knowledge of the micromagnetic structure produced within ferromagnetic materials when subjected to localised magnetic fields

Conference on Binary Optics: An Opportunity for Technical Exchange

The papers herein were presented at the Conference on Binary Optics held in Huntsville, AL, February 23-25, 1993. The papers were presented according to subject as follows: modeling and design, fabrication, and applications. Invited papers and tutorial viewgraphs presented on these subjects are included