Dynamic Hyperspectral and Polarized Endoscopic Imaging

The health of rich, developed nations has seen drastic improvement in the last two centuries. For it to continue improving at a similar rate new or improved diagnostic and treatment technologies are required, especially for those diseases such as cancer which are forecast to constitute the majority of disease burden in the future. Optical techniques such as microscopy have long played their part in the diagnostic process. However there are several new biophotonic modalities that aim to exploit various interactions between light and tissue to provide enhanced diagnostic information. Many of these show promise in a laboratory setting but few have progressed to a clinical setting. We have designed and constructed a flexible, multi-modal, multi-spectral laparoscopic imaging system that could be used to demonstrate several different techniques in a clinical setting. The core of this system is a dynamic hyperspectral illumination system based around a supercontinuum laser and Digital Micromirror Device that can provide specified excitation light in the visible and near infra-red ranges. This is a powerful tool for spectroscopic techniques as it is not limited to interrogating a fixed range of wavelengths and can switch between excitation bands instantaneously. The excitation spectra can be customised to match particular fluorophores or absorption features, introducing new possibilities for spectral imaging. A standard 10 mm diameter rigid endoscope was incorporated into the system to reduce cost and demonstrate compatibility with existing equipment. The polarization properties of two commercial endoscopes were characterised and found to be unsuited to current polarization imaging techniques as birefringent materials used in their construction introduce complex, spatially dependent transformations of the polarization state. Preliminary exemplar data from phantoms and ex vivo tissue was collected and the feasibility and accuracy of different analysis techniques demonstrated including multiple class classification algorithms. Finally, a novel visualisation method was implemented in order to display the complex hyperspectral data sets in a meaningful and intuitive way to the user

Development of Multiscale Spectro-microscopic Imaging System and Its Applications

A novel multi-modality spectro-microscopic system that combines far-field interferometry based optical microscopy imaging techniques (differential interference contrast microscopy and cross-polarized light microscopy), total internal reflection microscopy (total internal reflection fluorescence and scattering microscopy) and confocal spectroscopy (Raman spectroscopy and photoluminescence spectroscopy) is developed. Home-built post treatment stages (thermal annealing stage and solvent annealing stage) are integrated into the system to realize in situ measurements. Departing from conventional characterization methods in materials science mostly focused on structures on one length scale, the in situ multi-modality characterization system aims to uncover the structural information from the molecular level to the mesoscale. Applications of the system on the characterization of photoactive layers of bulk heterojunction solar cell, two-dimensional materials, gold nanoparticles, fabricated gold nanoparticle arrays and cells samples are shown in this dissertation

Novel up-conversion concentrating photovoltaic concepts

This thesis summarises a set of experiments towards the integration of concentrating optics into up-conversion photovoltaics. Up-conversion in rare earths has been investigated here. This optical process is non-linear therefore a high solar irradiance is required. High solar irradiance is achievable by solar concentration. Two concentrating approaches were investigated in this thesis: The first approach involved the concentration of the incident solar irradiance into optical fibres. An optical system with spherical lenses and dielectric tapers was designed accordingly. A solar concentration of 2000 suns was realised at the end of a single optical fibre. In addition to the total solar concentration, the spectral dependence was characterised to account for the effect of chromatic aberrations. Then, the solar concentration could be transferred into rare earth-doped fibres. For this reason, a series of experiments on double-clad erbium-doped silicate fibres was carried out. Although up-conversion in this type of fibre is minimised, the measured power dependence agrees with up-conversion via excited state absorption. In the second approach, concentrating optics were integrated in up-conversion solar cells. The role of the optics was to couple the photons transmitted by the solar cell to the rare earth up-converter. Therefore, imaging and non-imaging optics were investigated, with the latter exhibiting ideal coupling characteristics; concentration and high transmission of the incident irradiance, but also efficient collection of the up-converted emission. Out of the non-imaging optics, the dielectric compound parabolic concentrator fulfilled these characteristics, indicating its novel use in up-conversion solar cells. Two erbium-doped up-converters were utilised in this approach, beta-phase hexagonal sodium yttrium tetrafluoride (β-NaYF4:25%Er3+) and barium diyttrium octafluoride (BaY2F8:30%Er3+). The latter performed best, with an external quantum efficiency (EQE) of 2.07% under 1493 nm illumination, while the former exhibited an EQE of 1.80% under 1523 nm illumination both at an irradiance of 0.02 W/cm2. This corresponds to a relative conversion efficiency of 0.199% and 0.163% under sub-band-gap illumination, respectively, for a solar cell of 17.6% under standard AM1.5G conditions. These values are among the highest in literature for up-conversion solar cells and show the potential of the concentrating concept that can be important for future directions of photovoltaics.Engineering and Physical Sciences Research Council (EPSRC)European Community's Seventh Framework Program (FP7/2007-2013

Subsurface optical microscopy of semiconductor integrated circuits

Thesis (Ph.D.)--Boston UniversityThe semiconductor industry continues to scale integrated circuits (ICs) in accordance with Moore's Law, and is currently developing the processing infrastructure at the 14nm technology node and smaller. In the wake of such rapid progress, a number of challenges have arisen for the optical failure analysis methods to meet the requirements of the advancing process technology. Most notably, complex circuits with shrinking critical dimensions will demand higher resolution signal localization currently beyond the capability of the existing optical techniques. This dissertation aims to develop novel optical systems to address the challenges of non-destructive circuit diagnostics at the 14nm technology node and beyond. Backside imaging through the silicon substrate has become an industry standard due to the dense multi-level metal wiring and the packaging requirements. The solid immersion lens is a plano-convex lens placed on the planar silicon substrate to enhance the subsurface focusing and collection of light in back-side imaging of ICs. The silicon and gallium-arsenide aplanatic solid immersion lenses (aSILs) were investigated in detail for the subsurface laser-scanning, voltage modulation, photon emission and dark-field IC imaging applications. Wave-front sensing and shaping techniques were developed to evaluate and mitigate optical aberrations originating from practical issues. Furthermore, the method of pupil function tailoring was explored for sub-diffraction spatial resolution. Super-resolving annular phase and amplitude pupil masks were developed and experimentally implemented. A record-breaking light confinement of 0.02 λ2 0(λ 0 refers to the free-space wavelength) was demonstrated using the vortex beams. The beam invasiveness is a critical issue in the optical circuit probing as the localized heat due to the absorption of the focused beams may unwittingly interfere with the circuit operation in the course of a measurement. A dual-phase interferometry assisted circuit probing was developed to enhance the signal extraction sensitivity by as much as an order of magnitude. Thus, the power requirement of the probe beam is significantly reduced to avert the consequences of the beam invasiveness. The optical systems and methods developed in this dissertation were successfully demonstrated using a number of modern ICs including devices of 14nm, 22nm, 28nm and 32nm technology nodes

Interference-based Investigation of Microscopic Objects Near Surfaces: a View From Below

Phenomena occurring when microscopic objects approach planar surfaces are challenging to probe directly because their dynamics cannot be resolved with a sufficiently high spatial/temporal resolution in a non-invasive way, and suitable techniques/methods involve complex instrumentation/computations of limited accessibility/applicability. Interference-based techniques can overcome these barriers. However, because most set-ups and analysis methods are ideal for planar-like geometries, their accurate application for studying microscopic objects has been difficult. Reflection interference contrast microscopy (RICM) has shown particular promise allowing objects in close proximity to a surface to be observed from below, producing interferograms that inherently embed detailed information about the objects’ topography near the substrate. Because precise extraction of this information has been challenging, this study seeks to develop analysis methods applicable to RICM to facilitate its practical implementation for accurate investigation of interfacial phenomena between microscopic objects and surfaces. The most sophisticated theory of RICM was significantly improved and coupled with a general method to simulate the interference pattern from arbitrary convex geometries. Experimental results revealed that accurate reconstruction of an object’s contour is possible by fitting its interferogram; however, this is computationally intensive and of limited applicability, motivating the formulation of a simplified and accurate RICM model. This facilitated a major breakthrough: an innovative analysis of RICM interferograms provides the inclination angles of the geometry under study and a mathematical procedure allows near-instantaneous reconstruction of the contour with nanometer-scale resolution, applicable to arbitrarily shaped convex objects under different experimental conditions. A method for extracting nanometer-scale topographic information from RICM interferograms has been proposed; in particular, microspheres can be conveniently analyzed to measure surface roughness based on fringe visibility. Also, precise and accurate measurements of microspheres’ size were performed by means of optimized and robust fringe spacing analysis. Finally, RICM’s distinctive “view-from-below” perspective was applied in simple experiments involving the deposition of microspheres on surfaces, directly revealing the existence of different scenarios depending on deposition media and unique femtoliter-scale capillary condensation dynamics underneath micron-sized glass beads. Results show that RICM has a clear potential for near real-time analysis of ensembles of objects near surfaces so that statistical/probabilistic behavior can be realistically captured

Emergence of colour by tunable surface wrinkling in one and multi-dimensions

Naturally occurring surface patterns often exhibit micro and nano-scale topography with various functionalities, including displaying optical and photonic effects. One such natural topography that contributes to structural colour is the presence of wrinkles. This thesis explores the controlled wrinkling of bi-layered materials as a powerful patterning method to create bioinspired topographies, and examines their optical properties. The primary method employed in this work involves the use of plasma oxidation of polydimethylsiloxane, combined with mechanical strain, to create wrinkles with varying topographies, ranging from nano to micronscales, by controlling the plasma and strain conditions, as well as the superposition of various generations of wrinkles with prescribed relative angle of orientation. This work firstly investigates the formation of one-dimensional uniaxial wrinkles acting as tunable sinusoidal phase gratings, and quantitatively models the diffractive behaviour of the wrinkled surfaces as a function of strain. Under white light, these wrinkles exhibit iridescent, structural colour on the surfaces that depends on the observation angles and incident light spectrum, and we explored the concept of optical and colour directionality by creating gradient and isotropic wrinkles. In addition to surface diffraction in reflection, we found that the wrinkled materials could act as transmission gratings, leading to multi-faceted structural colour through diffraction combined by total internal reflection. Finally, we investigated the potential and limitations of sequential two-dimensional wrinkling and its structural colour properties. The results of this study provide promising directions for using structural coloured wrinkles in applications such as sensors, displays, and packaging.Open Acces

Light Trapping Transparent Electrodes

Transparent electrodes represent a critical component in a wide range of optoelectronic devices such as high-speed photodetectors and solar cells. Fundamentally, the presence of any conductive structures in the optical path leads to dissipation and reflection, which adversely affects device performance. Many different approaches have been attempted to minimize such shadowing losses, including the use of transparent conductive oxides (TCOs), metallic nanowire mesh grids, graphene-based contacts, and high-aspect ratio metallic wire arrays. In this dissertation I discuss a conceptually different approach to achieve transparent electrodes, which involves recapturing photons initially reflected by highly conductive electrode lines. To achieve this, light-redirecting metallic wires are embedded in a thin dielectric layer. Incident light is intentionally reflected toward large internal angles, which enables trapping of reflected photons through total internal reflection (TIR). Light trapping transparent electrodes could potentially reach the holy grail of transparent electrodes: the simultaneous achievement of high conductivity and near-complete optical transparency. We numerically and experimentally investigate several light trapping electrode structures. First, we study the spectral and angular optical transmission of embedded interdigitated metallic electrodes with inclined wire surfaces and demonstrate efficient broadband angle-insensitive polarization-independent light trapping. Proof-of-principle experiments are carried out, demonstrating several of the features observed in our numerical studies. Second, a novel type of grating-based light trapping transparent electrode is discussed. In this approach, diffraction from metal wires covered with nanoscale silicon gratings is used to achieve total internal reflection. We show that careful grating optimization achieves strong suppression of specular reflection, enabling a more than fivefold reduction of shadowing losses. The realization of a high light-trapping efficiency in a coplanar structure makes the design a promising candidate for integration in real-world optoelectronic devices. Finally, the transmission of high-index metasurfaces is investigated. Such structures may enable efficient light redirection around metallic contacts, if reflection losses by the metasurface can be suppressed. We demonstrate that the traditional anti-reflection coating approach fails for such structures, and present an improved design approach that reduces reflection losses over a broad range of structural parameters

Identification and Development of Novel Optics for Concentrator Photovoltaic Applications

Concentrating photovoltaic (CPV) systems are a key step in expanding the use of solar energy. Solar cells can operate at increased efficiencies under higher solar concentration and replacing solar cells with optical devices to capture light is an effective method of decreasing the cost of a system without compromising the amount of solar energy absorbed. CPV systems are however still in a stage of development where new designs, methods and materials are still being created in order to reach a low levelled cost of energy comparable to standard silicon based photovoltaic (PV) systems. This work outlines the different types of concentration photovoltaic systems, their various design advantages and limitations, and noticeable trends. Comparisons on materials, optical efficiency and optical tolerance (acceptance angle) are made in the literature review as well as during theoretical and experimental investigations. The subject of surface structure and its implications on concentrator optics has been discussed in detail while highlighting the need for enhanced considerations towards material and hence the surface quality of optics. All of the findings presented contribute to the development of higher performance CPV technologies. Specifically high and ultrahigh concentrator designs and the accompanied need for high accuracy high quality optics has been supported. A simulation method has been presented which gives attention to surface scattering which can decrease the optical efficiency by 10-40% (absolute value) depending on the material and manufacturing method. New plastic optics and support structures have been proposed and experimentally tested including the use of a conjugate refractive-reflective homogeniser (CRRH). The CRRH uses a reflective outer casing to capture any light rays which have failed total internal reflection (TIR) due to non-ideal surface topography. The CRRH was theoretically simulated and found to improve the optical efficiency of a cassegrain concentrator by a maximum of 7.75%. A prototype was built and tested where the power output increase when utilising the CRRH was a promising 4.5%. The 3D printed support structure incorporated for the CRRH however melted under focused light, which reached temperatures of 226.3°C, when tested at the Indian Institute of Technology Madras in Chennai India. The need for further research into prototyping methods and materials for novel optics was also demonstrated as well as the advantages of broadening CPV technology into the fields of biomimicry. The cabbage white butterfly was proven to concentrate light onto its thorax using its highly reflective and lightweight wings in a basking V-shape not unlike V-trough concentrators. These wings were measured to have a unique structure consisting of ellipsoidal pterin beads aligned in ladder like structures on each wing scale which itself is then tiled in a roof like pattern on the wing. Such structures of a reflective material may be the answer to lightweight materials capable of increasing the power to weight ratio of CPV technology greatly. Experimental testing of the large cabbage white wings with a silicon solar cell confirmed a 17x greater power to weight ratio in comparison to the same set up with reflective film instead of the wings. An ultrahigh design was proposed taking into account manufacturing considerations and material options. The geometrical design was of 5800x of which an optical efficiency of either ~75% with state of the art optics should produce and effective concentration of ~4300x. Relatively standard quality optics on the other hand should give an optical efficiency of ~55% and concentration ratio ~3000x. A prototype of the system is hypothesised to fall between these two predictions. Ultrahigh designs can be realised if the design process is as comprehensive as possible, considering materials, surface structure, component combinations, anti-reflective coatings, manufacturing processes and alignment methods. Most of which have been addressed in this work and the accompanied articles. Higher concentration designs have been shown to have greater advantages in terms of the environmental impact, efficiency and cost effectiveness. But these benefits can only be realised if designs take into account the aforementioned factors. Most importantly surface structure plays a big role in the performance of ultrahigh concentrator photovoltaics. One of the breakthroughs for solar concentrator technology was the discovery of PMMA and its application for Fresnel lenses. It is hence not an unusual notion that further breakthroughs in the optics for concentrator photovoltaic applications will be largely due to the development of new materials for its purpose. In order to make the necessary leaps in solar concentrator optics to efficient cost effective PV technologies, future novel designs should consider not only novel geometries but also the effect of different materials and surface structures. There is still a vast potential for what materials and hence surface structures could be utilised for solar concentrator designs especially if inspiration is taken from biological structures already proven to manipulate light

LASER Tech Briefs, Spring 1994

Topics in this Laser Tech Brief include: Electronic Components and Circuits. Electronic Systems, Physical Sciences, Materials, Mechanics, Fabrication Technology, and books and reports