Ultrasonic air-coupled capacitive arrays

A model is developed which is capable of predicting the pressure field of a rectangular source, as measured by a finite-sized receiver. This novel method treats the problem in a new way, which allows an integration to be performed over the area of the receiver. Previously it has only been possible to model two circular transducers coaxially aligned. The model is used to identify a receiver, which can be used to measure the highly focussed pressure field from a phased array, with only a negligible effect due to the receiver size. Productions from the model are compared to experimental data, and show a good correlation. A parabolic mirror used to focus the field from a circular device in air has been studied, and a model developed to predict the pressure field produced by this device. This is done by an approximation of the mirror surface to a grid of finely spaced points. The model correlates well with measured results. In addition, an image of a defect in a solid sample was produced. Arrays are then used to image solid samples in air. This is done using three techniques. The first is a combined phased source and receiver, which is shown to locate a wire accurately and to measure a step in the surface of a sample. A 2-D array is shown to image a defect in a composite plate, and the potential for a fast through-transmission air-coupled system is indicated. In addition, two post-processing techniques are used on data recorded using an array receiver, to locate an object in air. Of these two techniques, ellipse crossing is shown to have better results for large signal to noise ratios, and SAFT for lower ratios. The combination of theoretical modelling and experimental observations has indicated that the transducers and arrays constructed for use in air are well-understood, and that their characteristics can be predicted

Camera Based Localization for Indoor Optical Wireless Networks

The main focus of this work is to implement device localization in an indoor communication network which employs short range Optical Wireless Communication (OWC) using pencil beams. OWC is becoming increasingly important as a solution to the shortage of available radio spectrum. In order to counter this problem, a radical new approach is proposed by performing wireless communication using optical rather than radio techniques, by deploying optical pencil beam technologies to provide users with access to an indoor optical fiber infrastructure. An architecture based on free-space optics has been adopted. The narrow infrared beam is considered a good solution because of its ability to optimally carry all the information which the optical fiber can transport, in an energy-efficient way. Beam Steered - Infrared Light Communication (BS-ILC) brings the light only where is needed. Multiple beams may independently serve user devices within a room, hence each device can get a non-shared capacity without conflicts with other devices. Infrared light beams, additionally, are allowed to be operated at a higher power than visible light beams, due to a higher eye safety threshold for infrared light. Together with the directivity of a beam, this implies that the received signal-to-noise ratio with BS-ILC can be substantially higher than with Visible Light Communication (VLC), enabling a higher data rate and longer reach at better power efficiency. Current BS-ILC prototypes allow multiple beams with over 100 Gbit/s per beam. This high performance can only be achieved with small footprints, hence the system needs to know the exact location of user devices. In this thesis, an accurate and fast localization/tracking technique using a low-cost camera and simple image processing is presented

Resonant Waveguide Gratings for Color-Selective Diffraction

Resonant waveguide gratings (RWGs), also known as guided mode resonant (GMR) gratings or waveguide-mode resonant gratings, are dielectric structures where these resonant diffractive elements benefit from lateral leaky guided modes and can operate from UV to microwave frequencies, in many different configurations. Some of the guided light is diffracted out of the guide while propagating, coupled back to radiation and interferes with the non-coupled reflected or transmitted waves. This leads to a very high reflection or transmission, giving rise to a Fano or Lorentzian-like lineshape profile at the zeroth order. RWGs are intrinsically very sensitive to angle and wavelength variations, being therefore effective filtering structures, especially for collimated light. Thanks to their high degree of optical tunability (wavelength, phase, polarization, intensity) and the variety of fabrication processes and materials available, RWGs have been implemented in a broad scope of applications in research and industry, such as optical security features, refractive index and fluorescence biosensors, spectrometers and optical couplers. This thesis describes the development and realization of color-selective diffraction devices using RWGs. The properties of paired impedance matched RWGs with finite size and different grating periods, but sharing the same substrate and coated waveguide, are first investigated. In particular, a specific wavelength range is in-coupled inside the waveguide by the first grating from a white incident light beam, and out-coupled from the second grating at a different angle. Periodic arrays of such paired RWGs allow achieving color-selective diffraction. Moreover, specific design methods based on confocal prolate spheroids are derived and used to generate surfaces with different grating periods and orientations, which can filter a specific spectral portion of a point source and to redirect and focus it to another point in space, viz. the observation point. This patterning is particularly beneficial in applications where light re-focusing is required, such as optical security or optical combiners for near-eye displays. Realizations as optical security labels through smartphone-based authentication are presented and discussed. Since the fabrication of such devices is extremely demanding, a fabrication method is developed to reduce the exposure time for the electron beam lithography. This method is beneficial to efficiently fabricate gratings with different periods and oriented at different angles. In particular, a pre-fracturing of the grating lines in one or more smaller stripes, depending on the grating period, is first implemented, followed by the fracturing using a beam step size smaller than the beam diameter. In the last part, optical structures comprising a metallic layer and a dielectric layer on a corrugated glass substrate are described. In essence, the hybridization of plasmon and waveguide modes is studied and used to design a color-selective optical coupler where the hybridized modes are leaking into the substrate at the first diffraction order and are coupled as guided mode. Such coupler may be used as dispersive element when the white light source is divergent allowing, for example, the realization of inexpensive, compact and robust spectrometers

Adaptive optics for laser processing

The overall aim of the work presented in this thesis is to develop an adaptive optics (AO) technique for application to laser-based manufacturing processes. The Gaussian beam shape typically coming from a laser is not always ideal for laser machining. Wavefront modulators, such as deformable mirrors (DM) and liquid crystal spatial light modulators (SLM), enable the generation of a variety of beam shapes and furthermore offer the ability to alter the beam shape during the actual process. The benefits of modifying the Gaussian beam shape by means of a deformable mirror towards a square flat top profile for nanosecond laser marking and towards a ring shape intensity distribution for millisecond laser drilling are presented. Limitations of the beam shaping capabilities of DM are discussed. The application of a spatial light modulator to nanosecond laser micromachining is demonstrated for the first time. Heat sinking is introduced to increase the power handling capabilities. Controllable complex beam shapes can be generated with sufficient intensity for direct laser marking. Conventional SLM devices suffer from flickering and hence a process synchronisation is introduced to compensate for its impact on the laser machining result. For alternative SLM devices this novel technique can be beneficial when fast changes of the beam shape during the laser machining are required. The dynamic nature of SLMs is utilised to improve the marking quality by reducing the inherent speckle distribution of the generated beam shape. In addition, adaptive feedback on the intensity distribution can further improve the quality of the laser machining. In general, beam shaping by means of AO devices enables an increased flexibility and an improved process control, and thus has a significant potential to be used in laser materials processing

Unconventional Approaches to Structured Semiconductors

The function of semiconductor devices is intrinsically tied to their structure. While there are already myriad techniques in use today to fabricate an extreme diversity of devices, new processes are regularly developed to make the production of previously unrealizable structures, and consequently devices, possible. This dissertation deals with several unconventional approaches to generating ordered semiconductor structures. One chapter discusses a novel technique to measure the various forces that impede the alignment of randomly dispersed microstructures. The technique made it possible to both determine the magnitude of the interactions that the particles must overcome in order to be organized into a useful structure and assess the functional form of the forces that the microstructure is experiencing, thereby giving insight into the physical origin of said forces. The following chapter deal with the spontaneous structure formation seen in photoelectrodeposited semiconductor films. One chapter investigates how the natural tendency of these films to form oriented, high aspect ratio structures can be coupled to the geometry of the substrate on which they are grown. This work demonstrates that extremely straight, high aspect ratio structures can be grown over macroscopic areas by making simple modifications of the substrate. The final chapter characterizes the iridescence that these films exhibit. A simple physical explanation for the origin of the coloration is posited and verified. Then the information gleaned about the optical response of these films is used to generate vibrant, colorful patterns on electrode using consumer electronics.</p

MEMS tunable infrared metamaterial and mechanical sensors

Sub-wavelength resonant structures open the path for fine controlling the near-field at the nanoscale dimension. They constitute into macroscopic “metamaterials” with macroscale properties such as transmission, reflection, and absorption being tailored to exhibit a particular electromagnetic response. The properties of the resonators are often fixed at the time of fabrication wherein the tunability is demanding to overcome fabrication tolerances and afford fast signal processing. Hybridizing dynamic components such as optically active medium into the device makes tunable devices. Microelectromechanical systems (MEMS) compatible integrated circuit fabrication process is a promising platform that can be merged with photonics or novel 2D materials. The prospect of enormous freedom in integrating nanophotonics, MEMS actuators and sensors, and microelectronics into a single platform has driven the rapid development of MEMS-based sensing devices. This thesis describes the design and development of four tunable plasmonic structures based on active media or MEMS, two graphene-based MEMS sensors and a novel tape-based cost-effective nanotransfer printing techniques. First of all, we present two tunable plasmonic devices with the use of two active medium, which are electrically controlled liquid crystals and temperature-responsive hydrogels, respectively. By incorporating a nematic liquid crystal layer into quasi-3D mushroom plasmonic nanostructures and thanks to the unique coupling between surface plasmon polariton and Rayleigh anomaly, we have achieved the electrical tuning of the properties of plasmonic crystal at a low operating electric field. We also present another tunable plasmonic device with the capability to sense environmental temperature variations. The device is bowtie nanoantenna arrays coated with a submicron-thick, thermos-responsive hydrogel. The favorable scaling of plasmonic dimers at the nanometer scale and ionic diffusion at the submicron scale is leveraged to achieve strong optical resonance and rapid hydrogel response, respectively. Secondly, we present two MEMS -based tunable near-to-mid infrared metamaterials on a silicon-on-insulator wafer via electrically and thermally actuating the freestanding nanocantilevers. The two devices are developed on the basis of the same fabrication process and are easy-to-implement. The electrostatically driven metamaterial affords ultrahigh mechanical modulation (several tens of MHz) of an optical signal while the thermo-mechanically tunable metamaterial provides up to 90% optical signal modulation at a wavelength of 3.6 õm. Next, we present MEMS graphene-based pressure and gas flow sensors realized by transferring a large area and few-layered graphene onto a suspended silicon nitride thin membrane perforated with micro-through-holes. Due to the increased strain in the through-holes, the pressure sensor exhibits a very high sensitivty outperformed than most existing MEMS-based pressure sensors using graphene, silicon, and carbon nanotubes. An air flow sensor is also demonstrated via patterning graphene sheets with flow-through microholes. The flow rate of the air is measured by converting the mechanically deflection of the membrane into the electrical readout due to the graphene piezeroresistors. Finally, we present a tape-based multifunctional nanotransfer printing process based on a simple stick-and-peel procedure. It affords fast production of large-area metallic and dielectric nanophotonic sensing devices and metamaterials using Scotch tape

Implantable Low-Noise Fiberless Optoelectrodes for Optogenetic Control of Distinct Neural Populations

The mammalian brain is often compared to an electrical circuit, and its dynamics and function are governed by communication across different types neurons. To treat neurological disorders like Alzheimer’s and Parkinson’s, which are characterized by inhibition or amplification of neural activity in a particular region or lack of communication between different regions of the brain, there is a need to understand troubleshoot neural networks at cellular or local circuit level. In this work, we introduce a novel implantable optoelectrode that can manipulate more than one neuron type at a single site, independently and simultaneously. By delivering multi-color light using a scalable optical waveguide mixer, we demonstrate manipulation of multiple neuron types at precise spatial locations in vivo for the first time. We report design, micro-fabrication and optoelectronic packaging of a fiber-less, multicolor optoelectrode. The compact optoelectrode design consists of a 7 μm x 30 μm dielectric optical waveguide mixer and eight electrical recording sites monolithically integrated on each shank of a 22 μm-thick four-shank silicon neural probe. The waveguide mixers are coupled to eight side-emitting injection laser diodes (ILDs) via gradient-index (GRIN) lenses assembled on the probe backend. GRIN-based optoelectrode enables efficient optical coupling with large alignment tolerance to provide wide optical power range (10 to 3000 mW/mm2 irradiance) at stimulation ports. It also keeps thermal dissipation and electromagnetic interference generated by light sources sufficiently far from the sensitive neural signals, allowing thermal and electrical noise management on a multilayer printed circuit board. We demonstrated device verification and validation in CA1 pyramidal layer of mice hippocampus in both anesthetized and awake animals. The packaged devices were used to manipulate variety of multi-opsin preparations in vivo expressing different combinations of Channelrhodopsin-2, Archaerhodopsin and ChrimsonR in pyramidal and parvalbumin interneuron cells. We show effective stimulation, inhibition and recording of neural spikes at precise spatial locations with less than 100 μV stimulation-locked transients on the recording channels, demonstrating novel use of this technology in the functional dissection of neural circuits.PHDBiomedical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/137171/1/kkomal_1.pd

Hydrodynamics of Nematic Liquid Crystals for Diffractive Optical Elements

Nematic Liquid Crystals (NLCs) are used widely as adaptive optical materials in devices such as lenses, beam steerers and displays. Usually in NLC Diffractive Optical Elements (DOEs), limitations exist in one or more essential parameters, such as diffraction angle, diffraction intensity, aperture size or adaptive behaviour. This thesis will investigate an unusual method to create NLC DOEs, through inducing hydrodynamic flow within the materials. Here, several techniques are used to induce periodic flow patterns within the materials, which result in periodic changes to the device’s optical properties. These periodic structures are evaluated for potential as adaptive optical components. Fraunhofer diffraction theory is introduced as a means to evaluate the potential of various DOEs theoretically. Details of a computer programme developed during the project is presented, which allows calculation of Fraunhofer diffraction patterns. This programme is used to provide quantified analysis of losses in diffraction efficiency caused by imperfect or non-optimized DOEs. The application of these results may be used in aiding DOE device design, which will be discussed. The first method used to create hydrodynamic domains uses a low frequency electric field applied across the NLC. This induces periodic ion flow within the material, leading to the NLCs adopting a state of electrohydrodynamic instability (EHDI). Of several EHDI modes identified, the 1D Normal Roll (NR) mode was most promising as a DOE. The periods of these gratings are strongly dependent upon device spacing (d). In all calamitic materials, the grating period continuously varied from d to as electric field frequency was increased. This lead to a simultaneous decrease in diffraction efficiency. Elastic constant dependency on EHDI is also investigated, where a material of low k33 is created using a bent dimeric mixture. This displays a desirable property of lower grating period by a factor of around 1.5. The second method of creating hydrodynamic patterns in NLCs uses bulk and surface acoustic waves. Acoustic wave transmission in bulk NLCs is discussed. Techniques of measuring the speed of sound (vs) in fluids are given, which are used to obtain a value for vs in the NLC mixture E7 of 1720±70ms-1 at ambient temperatures. NLC structural changes under acoustic fields are examined. These investigations are used to create a novel device where the surface acoustic wavelength was varied using a chirped electrode structure. This created a hydrodynamic grating of continuously variable pitch from 100 μm to 450 μm using frequency modulation The findings and performance of the hydrodynamic gratings investigated are evaluated in the context of currently available DOE technologies. Possible further device improvements and theoretical limits using the results from Fraunhofer diffraction modelling are discusse

Optical MEMS

Optical microelectromechanical systems (MEMS), microoptoelectromechanical systems (MOEMS), or optical microsystems are devices or systems that interact with light through actuation or sensing at a micro- or millimeter scale. Optical MEMS have had enormous commercial success in projectors, displays, and fiberoptic communications. The best-known example is Texas Instruments’ digital micromirror devices (DMDs). The development of optical MEMS was impeded seriously by the Telecom Bubble in 2000. Fortunately, DMDs grew their market size even in that economy downturn. Meanwhile, in the last one and half decade, the optical MEMS market has been slowly but steadily recovering. During this time, the major technological change was the shift of thin-film polysilicon microstructures to single-crystal–silicon microsructures. Especially in the last few years, cloud data centers are demanding large-port optical cross connects (OXCs) and autonomous driving looks for miniature LiDAR, and virtual reality/augmented reality (VR/AR) demands tiny optical scanners. This is a new wave of opportunities for optical MEMS. Furthermore, several research institutes around the world have been developing MOEMS devices for extreme applications (very fine tailoring of light beam in terms of phase, intensity, or wavelength) and/or extreme environments (vacuum, cryogenic temperatures) for many years. Accordingly, this Special Issue seeks to showcase research papers, short communications, and review articles that focus on (1) novel design, fabrication, control, and modeling of optical MEMS devices based on all kinds of actuation/sensing mechanisms; and (2) new developments of applying optical MEMS devices of any kind in consumer electronics, optical communications, industry, biology, medicine, agriculture, physics, astronomy, space, or defense

Holography

Holography - Basic Principles and Contemporary Applications is a collection of fifteen chapters, describing the basic principles of holography and some recent innovative developments in the field. The book is divided into three sections. The first, Understanding Holography, presents the principles of hologram recording illustrated with practical examples. A comprehensive review of diffraction in volume gratings and holograms is also presented. The second section, Contemporary Holographic Applications, is concerned with advanced applications of holography including sensors, holographic gratings, white-light viewable holographic stereograms. The third section of the book Digital Holography is devoted to digital hologram coding and digital holographic microscopy