Bending loss improvement and twisting loss studies of flexible multimode polymer waveguides

© 2020 SPIE. Multimode polymer waveguides have attracted considerable interest for use in high-speed on-board communication links as they provide low loss (40 GHz×m) and can be cost-effectively integrated onto standard PCBs. The fabrication of such waveguides on flexible substrates can provide additional advantages: shape flexibility, lightweight and reduced thickness which are particularly important in the aviation and automotive industries. Such flexible and lightweight optical connections will play an important role in next-generation airplanes and driverless cars connecting the multitude of peripheral sensors with the central processing unit at high speed and low latency. However, in such applications, flexible polymer waveguides are required to be bent to meet their stringent space requirements and twisted or stretched when connecting movable parts. Under sharp flexure, the bending or twisting loss dominates the waveguide loss limiting their practical use. In this work therefore, we present a new waveguide design for flexible polymer waveguides with improved bending performance and derive useful layout rules for minimizing twisting losses in such samples. The proposed waveguide structure only requires one additional fabrication step and achieves bending losses below 0.5 dB for a 3 mm bend. In comparison, the conventional waveguide design yields a 2 dB loss under the same bending radius and launch condition. Additionally, useful equations relating the maximum allowed number of twisting turns for low excess loss with sample thickness and width are proposed. Bending and twisting measurements on flexible waveguide samples are presented validating these methods and demonstrating the potential of this technology

Ultra-violet lithography of thick photoresist for the applications in BioMEMS and micro optics

UV lithography of thick photoresist is widely used in microelectromechanical systems (MEMS) and micro-optoelectromechanical systems (MOEMS). SU-8 is a typical negative tone thick photoresist for micro systems, and can be used for both structural material and pattern transfer. This dissertation presents an effort to comprehensively study these important subjects. The first part, and the most fundamental part of this dissertation concentrated on the numerical analysis and experimental study of the wavelength dependent absorbance of SU-8 and the diffraction effects on the sidewall profiles of the microstructures made using UV lithography of SU-8. This study has laid the foundation for all the designs and analysis for the BioMEMS and Micro-optic components and systems using UV lithography of SU-8 in the following chapters of the dissertation. After a full discussion of UV lithography of SU-8, the applications of SU-8 in BioMEMS and micro optics were presented in the following areas: 1) design, analysis, and molding fabrication of biodegradable PLGA microstructures for implanted drug delivery application; 2) design, fabrication, and test of a novel three-dimensional micro mixer/reactor based on arrays of spatially impinging micro-jets; 3) design, analysis, fabrication, and test of a novel new type of truly three-dimensional hydro-focusing unit for flow cytometry applications based on SU-8; 4) Study on a new technology to fabricate out-of-plane pre-aligned microlens and microlens array, and the application of the microlens in a fiber bundle coupler. Finally, a new negative tone thick photoresist based on the composition of EPON resins 165 and 154 were introduced. The synthesis, physical properties, and UV-lithography properties of this new photoresist have been completed. The experimental results have proved it can be a better alternative to SU-8 and can be used in various MEMS and MOEMS applications. Most of the contents have been published or accepted for publications in technical journals or international conferences. Two US patent applications are pending and two more disclosures have been filed for the new technologies presented in this dissertation. There are obviously more work to be done in this promising area and these are presented in the section for future work

All-optical metamaterial modulators : fabrication, simulation and characterization

Artificially structured composite metamaterials consist of sub-wavelength sized structures that exhibit unusual electromagnetic properties not found in nature. Since the first experimental verification in 2000, metamaterials have drawn considerable attention because of their broad range of potential applications. One of the most attractive features of metamaterials is to obtain negative refraction, termed left-handed materials or negative-index metamaterials, over a limited frequency band. Negative-index metamaterials at near infrared wavelength are fabricated with circular, elliptical and rectangular holes penetrating through metal/dielectric/metal films. All three negative-index metamaterial structures exhibit similar figure of merit; however, the transmission is higher for the negative-index metamaterial with rectangular holes as a result of an improved impedance match with the substrate-superstrate (air-glass) combination. In general, the processing procedure to fabricate the fishnet structured negative-index metamaterials is to define the hole-size using a polymetric material, usually by lithographically defining polymer posts, followed by deposition of the constitutive materials and dissolution of the polymer (liftoff processing). This processing (fabrication of posts: multi-layer deposition: liftoff) often gives rise to significant sidewall-angle because materials accumulate on the tops of the posts that define the structure, each successive film deposition has a somewhat larger aperture on the bottom metamaterial film, giving rise to a nonzero sidewall-angle and to optical bianisotropy. Finally, we demonstrate a nanometer-scale, sub-picosecond metamaterial device capable of over terabit/second all-optical communication in the near infrared spectrum. We achieve a 600 fs device response by utilizing a regime of sub-picosecond carrier dynamics in amorphous silicon and ~70% modulation in a path length of only 124 nm by exploiting the strong nonlinearities in metamaterials. We identify a characteristic signature associated with the negative index resonance in the pump-probe signal of a fishnet structure. We achieve much higher switching ratios at the fundamental resonance (~70%) relative to the secondary resonance (~20%) corresponding to the stronger negative index at the fundamental resonance. This device opens the door to other compact, tunable, ultrafast photonic devices and applications

Synthesis and Characterization of Multi-Component Enrichment Polymer Layers for Chemical Sensor Applications

This dissertation presents the building and study of a universal enrichment polymer layer system (EPLS). Thin polymer films have been utilized as enrichment layers for evanescent waveguide chemical sensors. The chemical nature of the polymer provides affinity which promotes the analyte to be absorbed. Having one highly sensitive polymer layer is suitable for a single target volatile organic compound (VOC). Here, the development of multi-layered and multi-component thin polymer films has been done to allow for more diverse affinity. Several parameters were identified to make the EPLSs suitable as enrichment layers for chemical sensor devices. The evanescent sensor devices used chalcogenide (ChG) glass, which is an infrared (IR) transparent material, and the principle of attenuated total reflection (ATR). This allowed the use of mid-IR spectroscopy to identify the absorbance of the absorbed VOCs in the polymer films. Changes of the absorbance due to influences of the EPLS were observed. These changes have not been reported before by researchers but can potentially be used to aid in fast and accurate identification of chemical compounds. The thicknesses of the total EPLS were kept to ≤ 30 nm so the evanescent wave would not be completely absorbed by the EPLS and absorbed VOC. Poly(glycidyl methacrylate) (PGMA) was a binding polymer for all EPLSs. As such, a single component PGMA film was tested to understand how the polymer influences the EPLS. Sensitivity to VOC concentration was conducted by mixture analysis and dilution by nitrogen gas in dynamic flow conditions. Comparison of each EPLS is done as well as to determine wavelengths of interest. Polymers were applied to ChG microdisk and amorphous silicon microring resonators and were found to increase sensitivity versus no polymer film at all. Two distinct layered enrichment nanoscale systems were synthesized and characterized - a six layer system and a five layer system. The polymer layered systems were characterized by atomic force microscopy, ellipsometry, and IR spectroscopy. Polymers utilized were PGMA, poly(acrylic acid), 60% epoxidized poly(butadiene), and poly(4-vinyl pyridine). In-situ ellipsometry was done to determine the swelling fraction of the film. In-situ attenuated total reflection (ATR) FT-IR spectroscopy was used to identify absorbance differences. Each EPLS proved to promote unique interactions which brought about differences in VOC absorbance in the mid-IR region

Femtosecond Laser Micromachining of Advanced Fiber Optic Sensors and Devices

Research and development in photonic micro/nano structures functioned as sensors and devices have experienced significant growth in recent years, fueled by their broad applications in the fields of physical, chemical and biological quantities. Compared with conventional sensors with bulky assemblies, recent process in femtosecond (fs) laser three-dimensional (3D) micro- and even nano-scale micromachining technique has been proven an effective and flexible way for one-step fabrication of assembly-free micro devices and structures in various transparent materials, such as fused silica and single crystal sapphire materials. When used for fabrication, fs laser has many unique characteristics, such as negligible cracks, minimal heat-affected-zone, low recast, high precision, and the capability of embedded 3D fabrication, compared with conventional long pulse lasers. The merits of this advanced manufacturing technique enable the unique opportunity to fabricate integrated sensors with improved robustness, enriched functionality, enhanced intelligence, and unprecedented performance. Recently, fiber optic sensors have been widely used for energy, defense, environmental, biomedical and industry sensing applications. In addition to the well-known advantages of miniaturized in size, high sensitivity, simple to fabricate, immunity to electromagnetic interference (EMI) and resistance to corrosion, all-optical fiber sensors are becoming more and more desirable when designed with characteristics of assembly free and operation in the reflection configuration. In particular, all-optical fiber sensor is a good candidate to address the monitoring needs within extreme environment conditions, such as high temperature, high pressure, toxic/corrosive/erosive atmosphere, and large strain/stress. In addition, assembly-free, advanced fiber optic sensors and devices are also needed in optofluidic systems for chemical/biomedical sensing applications and polarization manipulation in optical systems. Different fs laser micromachining techniques were investigated for different purposes, such as fs laser direct ablating, fs laser irradiation with chemical etching (FLICE) and laser induced stresses. A series of high performance assembly-free, all-optical fiber sensor probes operated in a reflection configuration were proposed and fabricated. Meanwhile, several significant sensing measurements (e.g., high temperature, high pressure, refractive index variation, and molecule identification) of the proposed sensors were demonstrated in this dissertation as well. In addition to the probe based fiber optic sensors, stress induced birefringence was also created in the commercial optical fibers using fs laser induced stresses technique, resulting in several advanced polarization dependent devices, including a fiber inline quarter waveplate and a fiber inline polarizer based on the long period fiber grating (LPFG) structure

Nitric oxide an pH measurement with AlGaN/GaN based ISFETs

This thesis deals with the optimization of aluminum-gallium nitride/gallium nitride (AlGaN/GaN) ion sensitive field effect transistors (ISFETs), including the material parameters associated with fabrication, and the implementation of these optimized sensors for the detection of nitric oxide (NO), specifically aimed at biological detection. As the sensors will be used in fluidic environments, requirements regarding the chemical and mechanical stability of passivation can be quite demanding. It was demonstrated that polyimide exhibits the best passivation properties for these transistors in comparison to the well-known ‘hard passivation’ materials Si3N4 or SiO2. In order to employ polyimide as the insulation, a unique ECR (Electron Cyclotron Resonance) plasma process was developed to enable patterning while protecting the active sensor area of each of the AlGaN/GaN devices. This active area is the so-called two-dimensional electron gas (2DEG), which is spontaneously formed between AlGaN and GaN. The ECR plasma step delivers the essential anisotropic polyimide etching to insulate each ISFET with no measureable damage to the 2DEG. Furthermore, it was demonstrated that a contamination free surface was attained through the use of this fabrication process, providing good device functionality from the initial measurement-state of the ISFET, without the need of the additional cleaning procedures. A number of new technological processes were developed involving AlGaN/GaN ISFET gate area functionalization to enable NO measurement. A complete analysis of the sensor performance based on these functionalization methods showed tungsten trioxide and graphene functionalization techniques to be the most useful and compatible. These experiments also verify NO sensitivity in the presence of known interfering substances. Additionally, the possibility to make simultaneous pH and NO measurements was demonstrated via a suitable reduction of pH sensitivity of the functionalized transistors. Preliminary biocompatibility tests were demonstrated using L929 (mouse fibroblast) cells. Finally, a miniaturized AlGaN/GaN ISFET array was developed. A sensor size reduction and pitch size of 10 µm x 10 µm and 100 µm x 100 µm, respectively, was employed to improve precision for in vitro cell culture or tissue related experiments. With both the large-scale devices, as well as those miniaturized for the ISFET array, sensitivities of up to 57.0 mV/pH (values extremely near the theoretical Nernstian limit of 58.2 mV/pH at 20 °C) could be achieved. By combining the sensors with this achieved pH sensitivity and the NO sensors in the small-scale ISFET arrays, future work could enable simultaneous NO and pH measurement on a single chip across a local gradient in physiological applications.Diese Arbeit befasst sich mit der Optimierung von Aluminium-Gallium-Nitrid/Gallium-Nitrid (AlGaN/GaN) -Ionen-sensitiven-Feldeffekttransistoren (ISFETs), einschließlich der zur Prozessierung notwendigen Materialparameter, so wie die Implementierung dieser optimierten Sensoren zur Detektion von Stichstoffmonoxid (NO), im Speziellen für biologische Anwendungen. Durch den angestrebten Einsatz der Transistoren in Flüssigkeiten werden an die chemische und mechanische Stabilität der Passivierung hohe Anforderungen gestellt. Im Vergleich mit den bekannten 'harten' Passivierungsmaterialien wie Si3N4 oder SiO2 konnte gezeigt werden, dass Polyimid die besten Isolationseigenschaften aufweist. Um Polyimid als Passivierung einzusetzen, musste aber ein neuartiger ECR (Electron Cyclotron Resonance) Plasmaprozess entwickelt werden, der einerseits die AlGaN/GaN-Elemente strukturiert und gleichzeitig den aktiven Sensorbereich schützt. Dabei handelt es sich um das sogenannte zweidimensionale Elektronengas (2DEG), das sich spontan zwischen der AlGaN- und GaN-Schicht ausbildet. Der ECR Plasmaschritt ermöglicht das notwendige anisotrope Ätzen zur Isolierung der ISFETs gegeneinander ohne eine messbare Degeneration des 2DEG. Dieser Prozess hinterlässt eine kontaminationsfreie Oberfläche und somit sofort messbare ISFETs, was vorher benötigte Reinigungsschritte überflüssig macht. Um die Detektion von NO zu erlauben, wurde eine Reihe neuer technologischer Prozesse entwickelt, wie etwa die entsprechende Gate-Funktionalisierung der AlGaN/GaN-ISFETs. Wolframtrioxid und Graphen stellten sich bei der vollständigen Analyse des Sensorverhaltens als die Besten der untersuchten Funktionalisierungen heraus. Beim Nachweis der NO-Sensitivität gegenüber bekannten störenden Substanzen, konnte über die Verringerung der pH-Sensitivität des funktionalisierten Transistors, eine gleichzeitige Messung des pH-Wertes und NO durchgeführt werden. Mit Hilfe von L929-Zellen (Maus-Fibroblasten) wurden darüber hinaus die ersten Tests zur Biokompatibilität des Systems durchgeführt. Um die Genauigkeit für in vitro Zellkulturen oder Gewebe-basierte Experimente zu erhöhen, wurde ein miniaturisiertes AlGaN/GaN-ISFET-Array entwickelt, mit einer Miniaturisierung und einem Pitch von 10 mm x 10 mm bzw. 100 mm x 100 mm. Mit einzelnen Sensoren wie auch den miniaturisierten Arrays kann eine Sensitivität von bis zu 57.0 mV/pH (nahe am theoretischen Nernst'schen Verhalten mit 58.2 mV/pH bei 20 °C) erreicht werden. Die Kombination von miniaturisierten Arrays und der Verringerung der pH-Sensitivität könnte in zukünftigen Arbeiten eine simultane NO- sowie pH-Messung auf einem Chip über einen lokalen Gradienten physiologischer Anwendungen ermöglichen

Vat photopolymerisation 3D printing of controlled drug delivery devices

Pharmaceutical three-dimensional (3D) printing has led to a paradigm shift in the way medicines are designed and manufactured, moving away from the traditional ‘one-size-fits-all’ approaches and advancing towards personalised medicines. Among different 3D printing techniques, vat photopolymerisation 3D printing affords superior printing resolution, which in turn enables fabrication of micro-structures and smooth finishes. This thesis aims to investigate different vat photopolymerisation 3D printing techniques for the fabrication of personalised drug delivery devices for different routes of administration. Stereolithography (SLA) and digital light processing (DLP) 3D printing was used to manufacture devices with flexible materials for localised delivery of a single drug in the bladder and at the anterior segment of the eye. In vitro release studies demonstrated drug releases from these devices were sustained over weeks. Subsequently, to investigate the feasibility of loading more than one drug in a single dosage form, clinically relevant multi-layer antihypertensive polypills were fabricated using SLA 3D printing. A drug-photopolymer interaction was observed from these polypills, and Michael’s addition reaction was confirmed to have occurred. Despite these studies demonstrating the viable use of vat photopolymerization 3D printing for fabricating drug delivery devices, the bulky nature of current printers could be a barrier to clinical integration. As such, a smartphone-enabled DLP 3D printing system was developed to fabricate personalised oral dosage forms and patient-specific drug delivery devices. The portability of this printer could secure exciting opportunities for manufacturing personalised medicines at point-of-care settings. Overall, this thesis showed the potential of vat photopolymerisation 3D printing in preparing different patient-centric drug delivery devices with tuneable and sustained release profiles as well as advancing traditional treatments towards digital healthcare

Research and Technology Objectives and Plans (RTOP), summary

A compilation of summary portions of each of the Research and Technology Operating Plans (RTOPS) used for management review and control of research currently in progress throughout NASA is presented. Subject, technical monitor, responsible NASA organization, and RTOP number indexes are included