Portable, Fiber-Based, Diffuse Reflection Spectroscopy (DRS) Systems for Estimating Tissue Optical Properties.

Steady-state diffuse reflection spectroscopy is a well-studied optical technique that can provide a noninvasive and quantitative method for characterizing the absorption and scattering properties of biological tissues. Here, we compare three fiber-based diffuse reflection spectroscopy systems that were assembled to create a light-weight, portable, and robust optical spectrometer that could be easily translated for repeated and reliable use in mobile settings. The three systems were built using a broadband light source and a compact, commercially available spectrograph. We tested two different light sources and two spectrographs (manufactured by two different vendors). The assembled systems were characterized by their signal-to-noise ratios, the source-intensity drifts, and detector linearity. We quantified the performance of these instruments in extracting optical properties from diffuse reflectance spectra in tissue-mimicking liquid phantoms with well-controlled optical absorption and scattering coefficients. We show that all assembled systems were able to extract the optical absorption and scattering properties with errors less than 10%, while providing greater than ten-fold decrease in footprint and cost (relative to a previously well-characterized and widely used commercial system). Finally, we demonstrate the use of these small systems to measure optical biomarkers in vivo in a small-animal model cancer therapy study. We show that optical measurements from the simple portable system provide estimates of tumor oxygen saturation similar to those detected using the commercial system in murine tumor models of head and neck cancer.

Optical Fiber Sensing Based on Reflection Laser Spectroscopy

An overview on high-resolution and fast interrogation of optical-fiber sensors relying on laser reflection spectroscopy is given. Fiber Bragg-gratings (FBGs) and FBG resonators built in fibers of different types are used for strain, temperature and acceleration measurements using heterodyne-detection and optical frequency-locking techniques. Silica fiber-ring cavities are used for chemical sensing based on evanescent-wave spectroscopy. Various arrangements for signal recovery and noise reduction, as an extension of most typical spectroscopic techniques, are illustrated and results on detection performances are presented.

Discrimination of herbal medicines by molecular spectroscopy and chemical pattern recognition

The molecular spectroscopy (including near infrared diffuse reflection spectroscopy, Raman spectroscopy and infrared spectroscopy) with OPUS/Ident software was applied to clustering ginsengs according to species and processing methods. The results demonstrate that molecular spectroscopic analysis could provide a rapid, nondestructive and reliable method for identification of Chinese traditional medicine. It's found that the result of Raman spectroscopic analysis was the best one among these three methods. Comparing with traditional methods, which are laborious and time consuming, the molecular spectroscopic analysis is more effective.

Photodiagnosis of Oral Malignancy using Laser-Induced Fluorescence and Diffuse Reflectance Spectroscopy

Biophotonics Laboratory,Centre for Earth Science Studies

Ag2S sensitized mesoporous Bi2WO6 architectures with enhanced visible light photocatalytic activity and recycling properties

To harvest solar energy more efficiently, novel Ag2S/Bi2WO6 heterojunctions were synthesized by a hydrothermal route. This novel photocatalyst was synthesized by impregnating Ag2S into a Bi2WO6 semiconductor by a hydrothermal route without any surfactants or templates. The as prepared structures were characterized by multiple techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmet-Teller (BET) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectrometry (EDS), UV-vis diffuse reflection spectroscopy (DRS) and photoluminescence (PL). The characterization results suggest mesoporous hierarchical spherical structures with a high surface area and improved photo response in the visible spectrum. Compared to bare Bi2WO6, Ag2S/Bi2WO6 exhibited much higher photocatalytic activity towards the degradation of dye Rhodamine B (RhB). Although silver based catalysts are easily eroded by photogenerated holes, the Ag2S/Bi2WO6 photocatalyst was found to be highly stable in the cyclic experiments. Based on the results of BET, Pl and DRS analysis, two possible reasons have been proposed for the enhanced visible light activity and stability of this novel photocatalyst: (1) broadening of the photoabsorption range and (2) efficient separation of photoinduced charge carriers which does not allow the photoexcited electrons to accumulate on the conduction band of Ag2S and hence prevents the photocorrosion.

Spectroscopic Study on Water Diffusion in Poly(ester urethane) Block Copolymer Matrix

The diffusion of water in a phase-separated biodegradable poly(ester urethane) shape-memory polymer with poly(E-caprolactone) (PCL) as the soft segment was investigated using time-resolved FTIR-ATR. On the basis of the band fitting and water ordering in drawn films, the broad water band in the 3800-2800 cm(-1) region was decomposed into four bands located at 3620, 3510, 3400, and 3260 cm(-1), and the first two components at 3620 and 35 10 cm(-1) were assigned to the vibrations of antisymmetric and symmetric stretching of water hydrogen bonded with the C=O group of the soft segment. The other two were associated with water bonded to the urethane hard segments in the forms of N-H:O-H:O=C bridge hydrogen bond and double hydrogen bonds with two C=O groups, respectively. Furthermore, band fitting and two-dimensional correlation analyses revealed that in the diffusion process, water first diffuses into the continuous soft-rich PCL phase and then into the hard-rich urethane domains, forming double hydrogen bonds with two C=O groups prior to the bridge hydrogen bond in the form of N-H:O-H:O=C.

Electrochemical synthesis of polyaniline nanoparticles

Polyaniline nanoparticles were prepared on a highly oriented pyrolytic graphite (HOPG) surface from dilute polyaniline acidic solution (1 mM aniline + 1 M HClO4) using a pulsed potentiostatic method. Electrochemistry, Fourier transform infrared external reflection spectroscopy (FT-IR-ERS), X-ray photoelectron spectroscopy (XPS) and tapping-mode atomic force microscopy (TMAFM) were: used to characterize the composition and structure of the polyaniline nanoparticles. FT-IR-ERS and XPS results revealed that the polyaniline was in its emeraldine form. TMAFM measurement showed that the electropolymerized polyaniline nanoparticles dispersed on the:HOPG surface with a coverage of about 10(10) cm(-2). These nanoparticles were disk-shaped having a height of 10(-30) Angstrom and an apparent diameter varying from 200 to 600 Angstrom. The particle dimensions increased with the electropolymerization charge (Q) over the interval from 5.7 to 19.3 mu C cm(-2) (C) 2000 Elsevier Science S.A. All rights reserved.

Luminescence properties of the Langmuir-Blodgett film of terbium(III) stearoylanthranilate

Terbium(III) stearoylanthranilate has been prepared as a high property Z-type Langmuir-Blodgett (LB) film on various substrates by a vertical transfer process. The UV-visible absorption spectra and the low angle X-ray diffraction peaks have been collected in order to investigate the molecular arrangement and aggregation in the LB films. The average molecular orientation in multilayer stacking was determined by Attenuated Total Reflection Spectroscopy. The influence of the chemical environment of terbium within the LB films on the luminescence properties has been discussed. (C) 1997 Elsevier Science S.A.

Integrated spot size converters for InP based photonic systems

The ever increasing demand for broadband communications requires sophisticated devices. Photonic integrated circuits (PICs) are an approach that fulfills those requirements. PICs enable the integration of different optical modules on a single chip. Low loss fiber coupling and simplified packaging are key issues in keeping the price of PICs at a low level. Integrated spot size converters (SSC) offer an opportunity to accomplish this. Design, fabrication and characterization of SSCs based on an asymmetric twin waveguide (ATG) at a wavelength of 1.55 μm are the main elements of this dissertation. It is theoretically and experimentally shown that a passive ATG facilitates a polarization filter mechanism. A reproducible InP process guideline is developed that achieves vertical waveguides with smooth sidewalls. Birefringence and resonant coupling are used in an ATG to enable a polarization filtering and splitting mechanism. For the first time such a filter is experimentally shown. At a wavelength of 1610 nm a power extinction ratio of (1.6 ± 0.2) dB was measured for the TE- polarization in a single approximately 372 μm long TM- pass polarizer. A TE-pass polarizer with a similar length was demonstrated with a TM/TE-power extinction ratio of (0.7 ± 0.2) dB at 1610 nm. The refractive indices of two different InGaAsP compositions, required for a SSC, are measured by the reflection spectroscopy technique. A SSC layout for dielectric-free fabricated compact photodetectors is adjusted to those index values. The development and the results of the final fabrication procedure for the ATG concept are outlined. The etch rate, sidewall roughness and selectivity of a Cl2/CH4/H2 based inductively coupled plasma (ICP) etch are investigated by a design of experiment approach. The passivation effect of CH4 is illustrated for the first time. Conditions are determined for etching smooth and vertical sidewalls up to a depth of 5 μm.

Contactless measurement of colossal magnetoresistance in La1-xSrxMnO3 using the infrared magnetorefractive effect

The magnetorefractive effect (MRE) has been used for the first time to study the magnetotransport properties of La1-xSrxMnO3 perovskite materials. A direct correlation between the MRE and colossal magnetoresistance was observed. Samples with x = 0-0.3 prepared using the homogeneous coprecipitation and the solid state reaction methods were studied, covering the range of insulating to metallic behaviour. The M RE probed both the magnetically induced modification of the scattering of Drude-like electrons and the magnetic dependence of a stretching vibration mode. (C) 2004 Elsevier B.V. All rights reserved.

Photon Management Structures for Absorption Enhancement in Intermediate Band Solar Cells and Crystalline Silicon Solar Cells

El objetivo de la tesis es investigar los beneficios que el atrapamiento de la luz mediante fenómenos difractivos puede suponer para las células solares de silicio cristalino y las de banda intermedia. Ambos tipos de células adolecen de una insuficiente absorción de fotones en alguna región del espectro solar. Las células solares de banda intermedia son teóricamente capaces de alcanzar eficiencias mucho mayores que los dispositivos convencionales (con una sola banda energética prohibida), pero los prototipos actuales se resienten de una absorción muy débil de los fotones con energías menores que la banda prohibida. Del mismo modo, las células solares de silicio cristalino absorben débilmente en el infrarrojo cercano debido al carácter indirecto de su banda prohibida. Se ha prestado mucha atención a este problema durante las últimas décadas, de modo que todas las células solares de silicio cristalino comerciales incorporan alguna forma de atrapamiento de luz. Por razones de economía, en la industria se persigue el uso de obleas cada vez más delgadas, con lo que el atrapamiento de la luz adquiere más importancia. Por tanto aumenta el interés en las estructuras difractivas, ya que podrían suponer una mejora sobre el estado del arte. Se comienza desarrollando un método de cálculo con el que simular células solares equipadas con redes de difracción. En este método, la red de difracción se analiza en el ámbito de la óptica física, mediante análisis riguroso con ondas acopladas (rigorous coupled wave analysis), y el sustrato de la célula solar, ópticamente grueso, se analiza en los términos de la óptica geométrica. El método se ha implementado en ordenador y se ha visto que es eficiente y da resultados en buen acuerdo con métodos diferentes descritos por otros autores. Utilizando el formalismo matricial así derivado, se calcula el límite teórico superior para el aumento de la absorción en células solares mediante el uso de redes de difracción. Este límite se compara con el llamado límite lambertiano del atrapamiento de la luz y con el límite absoluto en sustratos gruesos. Se encuentra que las redes biperiódicas (con geometría hexagonal o rectangular) pueden producir un atrapamiento mucho mejor que las redes uniperiódicas. El límite superior depende mucho del periodo de la red. Para periodos grandes, las redes son en teoría capaces de alcanzar el máximo atrapamiento, pero sólo si las eficiencias de difracción tienen una forma peculiar que parece inalcanzable con las herramientas actuales de diseño. Para periodos similares a la longitud de onda de la luz incidente, las redes de difracción pueden proporcionar atrapamiento por debajo del máximo teórico pero por encima del límite Lambertiano, sin imponer requisitos irrealizables a la forma de las eficiencias de difracción y en un margen de longitudes de onda razonablemente amplio. El método de cálculo desarrollado se usa también para diseñar y optimizar redes de difracción para el atrapamiento de la luz en células solares. La red propuesta consiste en un red hexagonal de pozos cilíndricos excavados en la cara posterior del sustrato absorbente de la célula solar. La red se encapsula en una capa dieléctrica y se cubre con un espejo posterior. Se simula esta estructura para una célula solar de silicio y para una de banda intermedia y puntos cuánticos. Numéricamente, se determinan los valores óptimos del periodo de la red y de la profundidad y las dimensiones laterales de los pozos para ambos tipos de células. Los valores se explican utilizando conceptos físicos sencillos, lo que nos permite extraer conclusiones generales que se pueden aplicar a células de otras tecnologías. Las texturas con redes de difracción se fabrican en sustratos de silicio cristalino mediante litografía por nanoimpresión y ataque con iones reactivos. De los cálculos precedentes, se conoce el periodo óptimo de la red que se toma como una constante de diseño. Los sustratos se procesan para obtener estructuras precursoras de células solares sobre las que se realizan medidas ópticas. Las medidas de reflexión en función de la longitud de onda confirman que las redes cuadradas biperiódicas consiguen mejor atrapamiento que las uniperiódicas. Las estructuras fabricadas se simulan con la herramienta de cálculo descrita en los párrafos precedentes y se obtiene un buen acuerdo entre la medida y los resultados de la simulación. Ésta revela que una fracción significativa de los fotones incidentes son absorbidos en el reflector posterior de aluminio, y por tanto desaprovechados, y que este efecto empeora por la rugosidad del espejo. Se desarrolla un método alternativo para crear la capa dieléctrica que consigue que el reflector se deposite sobre una superficie plana, encontrándose que en las muestras preparadas de esta manera la absorción parásita en el espejo es menor. La siguiente tarea descrita en la tesis es el estudio de la absorción de fotones en puntos cuánticos semiconductores. Con la aproximación de masa efectiva, se calculan los niveles de energía de los estados confinados en puntos cuánticos de InAs/GaAs. Se emplea un método de una y de cuatro bandas para el cálculo de la función de onda de electrones y huecos, respectivamente; en el último caso se utiliza un hamiltoniano empírico. La regla de oro de Fermi permite obtener la intensidad de las transiciones ópticas entre los estados confinados. Se investiga el efecto de las dimensiones del punto cuántico en los niveles de energía y la intensidad de las transiciones y se obtiene que, al disminuir la anchura del punto cuántico respecto a su valor en los prototipos actuales, se puede conseguir una transición más intensa entre el nivel intermedio fundamental y la banda de conducción. Tomando como datos de partida los niveles de energía y las intensidades de las transiciones calculados como se ha explicado, se desarrolla un modelo de equilibrio o balance detallado realista para células solares de puntos cuánticos. Con el modelo se calculan las diferentes corrientes debidas a transiciones ópticas entre los numerosos niveles intermedios y las bandas de conducción y de valencia bajo ciertas condiciones. Se distingue de modelos de equilibrio detallado previos, usados para calcular límites de eficiencia, en que se adoptan suposiciones realistas sobre la absorción de fotones para cada transición. Con este modelo se reproducen datos publicados de eficiencias cuánticas experimentales a diferentes temperaturas con un acuerdo muy bueno. Se muestra que el conocido fenómeno del escape térmico de los puntos cuánticos es de naturaleza fotónica; se debe a los fotones térmicos, que inducen transiciones entre los estados excitados que se encuentran escalonados en energía entre el estado intermedio fundamental y la banda de conducción. En el capítulo final, este modelo realista de equilibrio detallado se combina con el método de simulación de redes de difracción para predecir el efecto que tendría incorporar una red de difracción en una célula solar de banda intermedia y puntos cuánticos. Se ha de optimizar cuidadosamente el periodo de la red para equilibrar el aumento de las diferentes transiciones intermedias, que tienen lugar en serie. Debido a que la absorción en los puntos cuánticos es extremadamente débil, se deduce que el atrapamiento de la luz, por sí solo, no es suficiente para conseguir corrientes apreciables a partir de fotones con energía menor que la banda prohibida en las células con puntos cuánticos. Se requiere una combinación del atrapamiento de la luz con un incremento de la densidad de puntos cuánticos. En el límite radiativo y sin atrapamiento de la luz, se necesitaría que el número de puntos cuánticos de una célula solar se multiplicara por 1000 para superar la eficiencia de una célula de referencia con una sola banda prohibida. En cambio, una célula con red de difracción precisaría un incremento del número de puntos en un factor 10 a 100, dependiendo del nivel de la absorción parásita en el reflector posterior. Abstract The purpose of this thesis is to investigate the benefits that diffractive light trapping can offer to quantum dot intermediate band solar cells and crystalline silicon solar cells. Both solar cell technologies suffer from incomplete photon absorption in some part of the solar spectrum. Quantum dot intermediate band solar cells are theoretically capable of achieving much higher efficiencies than conventional single-gap devices. Present prototypes suffer from extremely weak absorption of subbandgap photons in the quantum dots. This problem has received little attention so far, yet it is a serious barrier to the technology approaching its theoretical efficiency limit. Crystalline silicon solar cells absorb weakly in the near infrared due to their indirect bandgap. This problem has received much attention over recent decades, and all commercial crystalline silicon solar cells employ some form of light trapping. With the industry moving toward thinner and thinner wafers, light trapping is becoming of greater importance and diffractive structures may offer an improvement over the state-of-the-art. We begin by constructing a computational method with which to simulate solar cells equipped with diffraction grating textures. The method employs a wave-optical treatment of the diffraction grating, via rigorous coupled wave analysis, with a geometric-optical treatment of the thick solar cell bulk. These are combined using a steady-state matrix formalism. The method has been implemented computationally, and is found to be efficient and to give results in good agreement with alternative methods from other authors. The theoretical upper limit to absorption enhancement in solar cells using diffractions gratings is calculated using the matrix formalism derived in the previous task. This limit is compared to the so-called Lambertian limit for light trapping with isotropic scatterers, and to the absolute upper limit to light trapping in bulk absorbers. It is found that bi-periodic gratings (square or hexagonal geometry) are capable of offering much better light trapping than uni-periodic line gratings. The upper limit depends strongly on the grating period. For large periods, diffraction gratings are theoretically able to offer light trapping at the absolute upper limit, but only if the scattering efficiencies have a particular form, which is deemed to be beyond present design capabilities. For periods similar to the incident wavelength, diffraction gratings can offer light trapping below the absolute limit but above the Lambertian limit without placing unrealistic demands on the exact form of the scattering efficiencies. This is possible for a reasonably broad wavelength range. The computational method is used to design and optimise diffraction gratings for light trapping in solar cells. The proposed diffraction grating consists of a hexagonal lattice of cylindrical wells etched into the rear of the bulk solar cell absorber. This is encapsulated in a dielectric buffer layer, and capped with a rear reflector. Simulations are made of this grating profile applied to a crystalline silicon solar cell and to a quantum dot intermediate band solar cell. The grating period, well depth, and lateral well dimensions are optimised numerically for both solar cell types. This yields the optimum parameters to be used in fabrication of grating equipped solar cells. The optimum parameters are explained using simple physical concepts, allowing us to make more general statements that can be applied to other solar cell technologies. Diffraction grating textures are fabricated on crystalline silicon substrates using nano-imprint lithography and reactive ion etching. The optimum grating period from the previous task has been used as a design parameter. The substrates have been processed into solar cell precursors for optical measurements. Reflection spectroscopy measurements confirm that bi-periodic square gratings offer better absorption enhancement than uni-periodic line gratings. The fabricated structures have been simulated with the previously developed computation tool, with good agreement between measurement and simulation results. The simulations reveal that a significant amount of the incident photons are absorbed parasitically in the rear reflector, and that this is exacerbated by the non-planarity of the rear reflector. An alternative method of depositing the dielectric buffer layer was developed, which leaves a planar surface onto which the reflector is deposited. It was found that samples prepared in this way suffered less from parasitic reflector absorption. The next task described in the thesis is the study of photon absorption in semiconductor quantum dots. The bound-state energy levels of in InAs/GaAs quantum dots is calculated using the effective mass approximation. A one- and four- band method is applied to the calculation of electron and hole wavefunctions respectively, with an empirical Hamiltonian being employed in the latter case. The strength of optical transitions between the bound states is calculated using the Fermi golden rule. The effect of the quantum dot dimensions on the energy levels and transition strengths is investigated. It is found that a strong direct transition between the ground intermediate state and the conduction band can be promoted by decreasing the quantum dot width from its value in present prototypes. This has the added benefit of reducing the ladder of excited states between the ground state and the conduction band, which may help to reduce thermal escape of electrons from quantum dots: an undesirable phenomenon from the point of view of the open circuit voltage of an intermediate band solar cell. A realistic detailed balance model is developed for quantum dot solar cells, which uses as input the energy levels and transition strengths calculated in the previous task. The model calculates the transition currents between the many intermediate levels and the valence and conduction bands under a given set of conditions. It is distinct from previous idealised detailed balance models, which are used to calculate limiting efficiencies, since it makes realistic assumptions about photon absorption by each transition. The model is used to reproduce published experimental quantum efficiency results at different temperatures, with quite good agreement. The much-studied phenomenon of thermal escape from quantum dots is found to be photonic; it is due to thermal photons, which induce transitions between the ladder of excited states between the ground intermediate state and the conduction band. In the final chapter, the realistic detailed balance model is combined with the diffraction grating simulation method to predict the effect of incorporating a diffraction grating into a quantum dot intermediate band solar cell. Careful optimisation of the grating period is made to balance the enhancement given to the different intermediate transitions, which occur in series. Due to the extremely weak absorption in the quantum dots, it is found that light trapping alone is not sufficient to achieve high subbandgap currents in quantum dot solar cells. Instead, a combination of light trapping and increased quantum dot density is required. Within the radiative limit, a quantum dot solar cell with no light trapping requires a 1000 fold increase in the number of quantum dots to supersede the efficiency of a single-gap reference cell. A quantum dot solar cell equipped with a diffraction grating requires between a 10 and 100 fold increase in the number of quantum dots, depending on the level of parasitic absorption in the rear reflector.

Total internal reflection Raman spectroscopy of poly(alpha-olefin) oils in a lubricated contact

The rheology of a poly(alpha-olefin) base oil (PAO) in a sliding point contact has been investigated by total internal reflection (TIR) Raman spectroscopy. TIR Raman has the sensitivity to analyse nanometer-thick lubricant films in a tribological contact. The Raman signal generated from the sliding contact was used to determine the lubricant film thickness. The experimentally obtained film thicknesses were compared with theoretical calculations and a transition from Newtonian to non-Newtonian behaviour was observed at high shear rates. The Raman spectra showed no significant changes in the conformation of the PAO chains under the applied conditions of pressure and shear, but the polarisation dependence of the spectra revealed a preferred orientation of the hydrocarbon side chains in the shear-thinned region. Monolayers formed by a boundary lubricant, arachidic acid, dissolved in the PAO could be detected on the surfaces in the elastohydrodynamic regime.