An experimental study of proton-exchanged lithium niobate optical waveguides

The object of this thesis is to form an understanding of the origin of the problems associated with proton-exchanged waveguides, and to investigate possible solutions. Chapter 1 gives a brief introduction to the properties of lithium niobate, and discusses the methods available for fabricating optical waveguides in the bulk material, with particular emphasis on waveguide fabrication by the proton-exchange process. Some of the devices which have been fabricated by proton-exchange are discussed. The problems associated with proton-exchanged waveguides are reviewed. Chapter 2 deals with the physical and chemical characterisation of proton-exchanged waveguides fabricated using neat benzoic acid melts. The extent of proton-exchange is determined as a function of fabrication time and temperature using optical waveguide prism-coupler measurements, infrared absorption spectroscopy, and atomic absorption spectroscopy. Chapter 3 is concerned with the problem of waveguide mode-index stability. Using a hydrogen isotopic-exchange reaction, the extent of which is obsrved via infrared absorption spectroscopy, information on the (room-temperature) mobility of protons within the guiding layer is obtained for waveguides fabricated using neat benzoic acid melts. The recently reported process of fabricating waveguides in lithium niobate by deuterium-exchange is investigated. The behaviour of proton-exchanged and deuterium-exchanged waveguides with respect to reaction with atmospheric water vapour is investigated, and the optical properties of deuterium-exchanged waveguides are studied. In Chapter 4, a study of annealed and dilute-melt proton-exchanged waveguides is presented. It is shown, using prism-coupler measurements and infrared absorption spectroscopy, that ennealed and dilute-melt waveguides can have very similar optical properties, depending on the amount of annealing and the lithium benzoate mole-fractions used. The extent of proton-exchange is determined with time (between 215oC and 235oC) for dilute-melt waveguides produced using lithium benzoate mole-fractions of up to 1.1%. Isotopic-exchange in annealed and dilute-melt waveguides is also investigated, both at room-temperature and at temperatures commonly used for annealing. A possible explanation for the poor optical properties of (neat-melt) proton-exchanged waveguides is given. Chapter 5 deals with a study of propagation losses (using the two-prism method) and the electro-optic effect in x- and z-cut proton-exchanged waveguides. Measurements of r33 (in proton-exchanged waveguides) and r22 (in titanium-indiffused waveguides) are carried out using an external interferometric method designed by the author. The results of Chapter 4 are used to establish a method by which losses below 0.5dB/cm and a substantially restored electro-optic effect can be achieved (using a combination of dilute-melt fabrication with post-exchange annealing). Prior to the waveguide measurements, the bulk electro-optic effect is investigated for congruent, incongruent, MgO-doped, and annealed (high-temperature) crystals. Finally, in Chapter 6, a summary of the thesis is presented, and suggestions for future work are given

Supercritically-Dried Porous Silicon Powders with Surface Areas Exceeding 1000 M2/G

International audiencePorous silicon micro-particulates have been harvested after electrochemical anodization of lightly-doped p-type silicon wafers in hydrofluoric acid electrolyte containing sulfuric acid as an additive. Post-anodization, significantly higher internal surface areas per unit mass have been realized by utilizing super-critical drying with CO2 solvent instead of air-drying, with up to 1125 m2/g being achieved. Correspondingly higher pore volumes are also evident (>1 cm3/g) and, with average pore diameters ranging between 3–4 nm, a higher micropore content is made accessible. It is proposed that the improvements achieved through super-critical drying indicate that the higher density of micropores expected from the choice of wafer resistivity and electrolyte composition (their presence being confirmed through analysis of the adsorption-desorption isotherms) is facilitated through a higher degree of integrity being maintained within the etched pore structure during electrolyte removal

Porous Silicon Fabrication by Anodisation: Progress towards the Realisation of Layers and Powders with High Surface Area and Micropore Content

International audienceWith a view to producing thick and very high surface area microporous silicon layers (and subsequently powders) by electrochemical anodisation, the incorporation of various types of chemical additives has been investigated, these in combination with hydrofluoric acid electrolyte and high-resistivity p-type parent substrates. Comparison under constant charge conditions shows that anodisation using 50 wt% hydrofluoric acid, or inclusion of the additives hydrochloric acid, sulphuric acid, or ammonium dodecylsulfate with lower concentration hydrofluoric acid, can facilitate powders with internal surface areas of up to 864 m2/g, average pore sizes in the region of 2.8–3.2 nm, and pore volumes in excess of 0.8 cm3/g – all as determined using nitrogen gas adsorption and associated isotherm analysis. Porous silicon powders with appreciable micropore content have thus been achieved, for the first time. Relevant application areas for such material are diverse, and potentially include energetics, impurity gettering, gas sensing microchips, orthopaedic implants, hydrogen storage, and Li-ion battery anodes

Enhanced quantum yield of photoluminescent porous silicon prepared by supercritical drying

The effect of supercritical drying (SCD) on the preparation of porous silicon (pSi) powders has been investigated in terms of photoluminescence (PL) efficiency. Since the pSi contains closely spaced and possibly interconnected Si nanocrystals (32% at room temperature) has been achieved, prompting the need for further detailed studies to establish the dominant causes of such an improvement

Biogenic Nanostructured Porous Silicon as a Carrier for Stabilization and Delivery of Natural Therapeutic Species

Nanostructured mesoporous silicon (pSi) derived from the silicon-accumulator plant Tabasheer (<i>Bambuseae</i>) is demonstrated to serve as a potential carrier matrix for carrying and stabilizing naturally active, but otherwise metastable, therapeutic agents. Particularly, in this study, garlic oil containing phytochemicals (namely, allicin) that are capable of inhibiting <i>Staphylococcus aureus</i> (<i>S. aureus</i>) bacterial growth were incorporated into Tabasheer-derived porous silicon. Thermogravimetric analysis (TGA) indicated that relatively high amounts of the extract (53.1 ± 2.2 wt %) loaded into pSi are possible by simple infiltration. Furthermore, by assessing the antibacterial activity of the samples using a combination technique of agar disk diffusion and turbidity assays against <i>S. aureus</i>, we report that biogenic porous silicon can be utilized to stabilize and enhance the therapeutic effects of garlic oil for up to 4 weeks when the samples were stored under refrigerated conditions (4 °C) and 1 week at room temperature (25 °C). Critically, under ultraviolet (UV) light (365 nm) irradiation for 24 h intervals, plant-derived pSi is shown to have superior performance in protecting garlic extracts over porous silica (pSiO₂) derived from the same plant feedstock or extract-only controls. The mechanism for this effect has also been investigated

Single Plant Derived Nanotechnology for Synergistic Antibacterial Therapies

<div><p>Multiple new approaches to tackle multidrug resistant infections are urgently needed and under evaluation. One nanotechnology-based approach to delivering new relevant therapeutics involves silicon accumulator plants serving as a viable silicon source in green routes for the fabrication of the nanoscale drug delivery carrier porous silicon (pSi). If the selected plant leaf components contain medicinally-active species as well, then a single substance can provide not only the nanoscale high surface area drug delivery carrier, but the drug itself. With this idea in mind, porous silicon was fabricated from joints of the silicon accumulator plant <i>Bambuseae</i> (Tabasheer) and loaded with an antibacterial extract originating from leaves of the same type of plant (<i>Bambuseae arundinacea</i>). Preparation of porous silicon from Tabasheer includes extraction of biogenic silica from the ground plant by calcination, followed by reduction with magnesium in the presence of sodium chloride, thereby acting as a thermal moderator that helps to retain the mesoporous structure of the feedstock. The purified product was characterized by a combination of scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), and low temperature nitrogen gas adsorption measurements. Antimicrobial activity and minimum inhibitory concentration of a leaf extract of <i>Bambuseae arundinacea</i> was tested against the bacteria <i>Escherichia Coli</i> (<i>E</i>. <i>Coli</i>) and <i>Staphylococcus aureus</i> (<i>S</i>. <i>Aureus</i>), along with the fungus <i>Candida albicans</i> (<i>C</i>. <i>Albicans</i>). A <i>S</i>. <i>aureus</i> active ethanolic leaf extract was loaded into the above Tabasheer-derived porous silicon. Initial studies indicate sustained <i>in vitro</i> antibacterial activity of the extract-loaded plant derived pSi (25 wt %, TGA), as measured by disk diffusion inhibitory zone assays. Subsequent chromatographic separation of this extract revealed that the active antimicrobial species present include stigmasterol and 2,6-dimethoxy-p-benzoquinone.</p></div

Influence of Surface Chemistry on the Release of an Antibacterial Drug from Nanostructured Porous Silicon

Nanostructured mesoporous silicon possesses important properties advantageous to drug loading and delivery. For controlled release of the antibacterial drug triclosan, and its associated activity versus <i>Staphylococcus aureus</i>, previous studies investigated the influence of porosity of the silicon matrix. In this work, we focus on the complementary issue of the influence of surface chemistry on such properties, with particular regard to drug loading and release kinetics that can be ideally adjusted by surface modification. Comparison between drug release from as-anodized, hydride-terminated hydrophobic porous silicon and the oxidized hydrophilic counterpart is complicated due to the rapid bioresorption of the former; hence, a hydrophobic interface with long-term biostability is desired, such as can be provided by a relatively long chain octyl moiety. To minimize possible thermal degradation of the surfaces or drug activity during loading of molten drug species, a solution loading method has been investigated. Such studies demonstrate that the ability of porous silicon to act as an effective carrier for sustained delivery of antibacterial agents can be sensitively altered by surface functionalization