Self-validating technique for the measurement of the linewidth enhancement factor in semiconductor lasers

A new method for measuring the linewidth enhancement factor (α-parameter) of semiconductor lasers is proposed and discussed. The method itself provides an estimation of the measurement error, thus self-validating the entire procedure. The α-parameter is obtained from the temporal profile and the instantaneous frequency (chirp) of the pulses generated by gain switching. The time resolved chirp is measured with a polarization based optical differentiator. The accuracy of the obtained values of the α-parameter is estimated from the comparison between the directly measured pulse spectrum and the spectrum reconstructed from the chirp and the temporal profile of the pulse. The method is applied to a VCSEL and to a DFB laser emitting around 1550 nm at different temperatures, obtaining a measurement error lower than ± 8%.

Laser Driven Neutron Sources: Characteristics, Applications and Prospects

The basics of laser driven neutron sources, properties and possible applications are discussed. We describe the laser driven nuclear processes which trigger neutron generation, namely, nuclear reactions induced by laser driven ion beam (ion n), thermonuclear fusion by implosion and photo-induced nuclear (gamma n) reactions. Based on their main properties, i.e. point source (<100 μm) and short durations (< ns), different applications are described, such as radiography, time-resolved spectroscopy and pump-probe experiments. Prospects on the development of laser technology suggest that, as higher intensities and higher repetition rate lasers become available (for example, using DPSSL technology), laser driven methodologies may provide neutron fluxes comparable to that achieved by accelerator driven neutron sources in the near future.

A proposed two-stage two-tether scientific mission at Jupiter

A two-stage mission to place a spacecraft (SC) below the Jovian radiation belts, using a spinning bare tether with plasma contactors at both ends to provide propulsion and power,is proposed. Capture by Lorentz drag on the tether, at the periapsis of a barely hyperbolic equatorial orbit, is followed by a sequence of orbits at near-constant periapsis, drag finally bringing the SC down to a circular orbit below the halo ring. Although increasing both tether heating and bowing, retrograde motion can substantially reduce accumulated dose as compared with prograde motion, at equal tether-to-SC mass ratio. In the second stage,the tether is cut to a segment one order of magnitude smaller, with a single plasma contactor, making the SC to slowly spiral inward over severalmonths while generating large onboard power, which would allow multiple scientific applications, including in situ study of Jovian grains, auroral sounding of upper atmosphere, and space- and time-resolved observations of surface and subsurface.

Non-destructive optical characterization of fruits in the red and near infrared.

Increasing attention is being paid to the possible development of non-invasive tests for the assessment of the quality of Fruits. We propose a novel non-destructive method for the measurement of the internal optical properties of fruits and vegetables by means of lime-resolved reflectance spectroscopy in the visible and NIR range. A Fully automated instrumentation for time-resolved reflectance measurements was developed. It is based on mode-locked laser sources and electronics for time-correlated single photon counting, and provides a time-resolution of 120-160 ps. The system was used to probe the optical properties of several species and varieties of Fruits and vegetables in the red and NIR range (650-1000 nm). In most Fruits, the absorption line shape is dominated by the absorption peak of water, centred around 970 nm. Generally, the absorption spectra also show the spectral features typical of chlorophyll, with maximum at 675 nm. In particular, for what concerns apples, variations in peak intensity are observed depending on the variety, the degree of ripeness as well as the position on the apple. For all the species and varieties considered, the transport scattering coefficient decreases progressively upon increasing the wavelength.

Detection of internal quality in kiwi with a new optical technique (TDRS)

A compact system based on time-resolved diffuse reflectance spectroscopy (TDRS) has been developed to measure internal fruit quality parameters and has been applied to the non-destructive estimation of firmness, sugar content and acidity of kiwifruits. This new optical technique, developed in medical applications and related areas, provides a complete optical characterisation of a diffusive sample as it estimates at the same time and independently the light absorption inside the tissues and the scattering across them. The working principle of the technique is the analysis of the attenuation and broadening of the time-distribution of the remitted light, and the correct interpretation with a proper theoretical model. This main advantage compared to conventional optical techniques (which are only able to register the global attenuation spectrum) added to the compact, portable prototype developed along a three-year work opens the possibilities of this new measurement method in the food industry.

Web based interactive educational software introducing semiconductor laser dynamics: Sound Of Lasers (SOL)

In this work, educational software for intuitive understanding of the basic dynamic processes of semiconductor lasers is presented. The proposed tool is addressed to the students of optical communication courses, encouraging self consolidation of the subjects learned in lectures. The semiconductor laser model is based on the well known rate equations for the carrier density, photon density and optical phase. The direct modulation of the laser is considered with input parameters which can be selected by the user. Different options for the waveform, amplitude and frequency of thpoint. Simulation results are plotted for carrier density and output power versus time. Instantaneous frequency variations of the laser output are numerically shifted to the audible frequency range and sent to the computer loudspeakers. This results in an intuitive description of the “chirp” phenomenon due to amplitude-phase coupling, typical of directly modulated semiconductor lasers. In this way, the student can actually listen to the time resolved spectral content of the laser output. By changing the laser parameters and/or the modulation parameters,consequent variation of the laser output can be appreciated in intuitive manner. The proposed educational tool has been previously implemented by the same authors with locally executable software. In the present manuscript, we extend our previous work to a web based platform, offering improved distribution and allowing its use to the wide audience of the web.

Reproducing the Bolund experiment in wind tunnel

The Bolund experiment has been reproduced in a neutral boundary layer wind tunnel (WT) at scale 1:115 for two Reynolds numbers. All the results have been obtained for an incoming flow from the 270o wind direction (transect B in the Bolund experiment jargon). Vertical scans of the velocity field are obtained using non-time resolved two components particle image velocimetry. Time-resolved velocity time series with a three component hot-wire probe have been also measured for transects at 2 and 5 m height and in the vertical transects at met masts M6, M3 and M8 locations. Special attention has been devoted to the detailed characterization of the inflow in order to reduce uncertainties in future comparisons with other physical and numerical simulations. Emphasis is placed on the analysis of spectral functions of the undisturbed flow and those of the flow above the island. The result?s reproducibility and trustworthiness have been addressed through redundancy measurements using particle image velocimetry, two and three components hot-wire anemometry. The bias in the prediction of the mean speed is similar to the one reported during the Bolund experiment by the physical modellers. However, certain reduction of the bias in the estimation of the turbulent kinetic energy is achieved. TheWT results of spectra and cosprectra have revealed a behaviour similar to the full-scale measurements in some relevant locations, showing that WT modelling can contribute to provide valid information about these important structural loading factors.

Wind tunnel analysis of the flow topology over complex topographies

El principal objetivo de este trabajo es aportar conocimiento para contestar la pregunta: ¿hasta que punto los ensayos en túnel aerodinámico pueden contribuir a determinar las características que afectan la respuesta dinámica de los aerogeneradores operando en terreno complejo?. Esta pregunta no es nueva, de hecho, el debate en la comunidad científica comenzó en el primer tercio del siglo pasado y aún está intensamente vivo. El método generalmente aceptado para enfrentar el mencionado problema consiste en analizar un caso de estudio determinado en el cual se aplican tanto ensayos a escala real como análisis computacionales y ensayos en túnel aerodinámico. Esto no es ni fácil ni barato. Esta es la razón por la cual desde el experimento de Askervein en 1988, los modelizadores del flujo atmosférico tuvieron que esperar hasta 2007 a que el experimento de Bolund fuese puesto en marcha con un despliegue de medios técnicos equivalentes (teniendo en cuenta la evolución de las tecnologías de sensores y computación). El problema contempla tantos aspectos que ambas experiencias fueron restringidas a condiciones de atmósfera neutra con efectos de Coriolis despreciables con objeto de reducir la complejidad. Este es el contexto en el que se ha desarrollado la presente tesis doctoral. La topología del flujo sobre la isla de Bolund ha sido estudiada mediante la reproducción del experimento de Bolund en los túneles aerodinámicos A9 y ACLA16 del IDR. Dos modelos de la isla de Bolund fueron fabricados a dos escalas, 1:230 y 1:115. El flujo de entrada en el túnel aerodinámico simulando la capa límite sin perturbar correspondía a régimen de transición (transitionally rough regime) y fue usado como situación de referencia. El modelo a escala 1:230 fue ensayado en el túnel A9 para determinar la presión sobre su superficie. La distribución del coeficiente de presión sobre la isla proporcionó una visualización y estimación de una región de desprendimiento sobre el pequeño acantilado situado al frente de la misma. Las medidas de presión instantánea con suficiente grado de resolución temporal pusieron de manifiesto la no estacionariedad en la región de desprendimiento. El modelo a escala 1:115 fue ensayado utilizando hilo caliente de tres componentes y un sistema de velocimetría por imágenes de partículas de dos componentes. El flujo fue caracterizado por el ratio de aceleración, el incremento normalizado de energía cinética turbulenta y los ángulos de inclinación y desviación horizontal. Los resultados a lo largo de la dirección 270°y alturas de 2 m y 5 m presentaron una gran similitud con los resultados a escala real del experimento de Bolund. Los perfiles verticales en las localizaciones de las torres meteorológicas mostraron un acuerdo significativo con los resultados a escala real. El análisis de los esfuerzos de Reynolds y el análisis espectral en las localizaciones de los mástiles meteorológicos presentaron niveles de acuerdo variados en ciertas posiciones, mientras que en otras presentaron claras diferencias. El mapeo horizontal del flujo, para una dirección de viento de 270°, permitió caracterizar el comportamiento de la burbuja intermitente de recirculación sobre el pequeño acantilado existente al frente de la isla así como de la región de relajación y de la capa de cortadura en la región corriente abajo de Bolund. Se realizaron medidas de velocidad con alta resolución espacial en planos perpendiculares a la dirección del flujo sin perturbar. Estas medidas permitieron detectar y caracterizar una estructura de flujo similar a un torbellino longitudinal con regiones con altos gradientes de velocidad y alta intensidad de turbulencia. Esta estructura de flujo es, sin duda, un reto para los modelos computacionales y puede considerarse un factor de riesgo para la operación de los aerogeneradores. Se obtuvieron y analizaron distribuciones espaciales de los esfuerzos de Reynolds mediante 3CHW y PIV. Este tipo de parámetros no constituyen parte de los resultados habituales en los ensayos en túnel sobre topografías y son muy útiles para los modelizadores que utilizan simulación de grades torbellinos (LES). Se proporciona una interpretación de los resultados obtenidos en el túnel aerodinámico en términos de utilidad para los diseñadores de parques eólicos. La evolución y variación de los parámetros del flujo a lo largo de líneas, planos y superficies han permitido identificar como estas propiedades del flujo podrían afectar la localización de los aerogeneradores y a la clasificación de emplazamientos. Los resultados presentados sugieren, bajo ciertas condiciones, la robustez de los ensayos en túnel para estudiar la topología sobre terreno complejo y su comparabilidad con otras técnicas de simulación, especialmente considerando el nivel de acuerdo del conjunto de resultados presentados con los resultados a escala real. De forma adicional, algunos de los parámetros del flujo obtenidos de las medidas en túnel son difícilmente determinables en ensayos a escala real o por medios computacionales, considerado el estado del arte. Este trabajo fue realizado como parte de las actividades subvencionadas por la Comisión Europea como dentro del proyecto FP7-PEOPLE-ITN-2008WAUDIT (Wind Resource Assessment Audit and Standardization) dentro de la FP7 Marie-Curie Initial Training Network y por el Ministerio Español de Economía y Competitividad dentro del proyecto ENE2012-36473, TURCO (Determinación en túnel aerodinámico de la distribución espacial de parámetros estadísticos de la turbulencia atmosférica sobre topografías complejas) del Plan Nacional de Investigación (Subprograma de investigación fundamental no orientada 2012). El informe se ha organizado en siete capítulos y un conjunto de anexos. En el primer capítulo se introduce el problema. En el capítulo dos se describen los medios experimentales utilizados. Seguidamente, en el capítulo tres, se analizan en detalle las condiciones de referencia del principal túnel aerodinámico utilizado en esta investigación. En el capítulo tres se presentan resultados de ensayos de presión superficial sobre un modelo de la isla. Los principales resultados del experimento de Bolund se reproducen en el capítulo cinco. En el capítulo seis se identifican diferentes estructuras del flujo sobre la isla y, finalmente, en el capitulo siete, se recogen las conclusiones y una propuesta de lineas de trabajo futuras. ABSTRACT The main objective of this work is to contribute to answer the question: to which extend can the wind tunnel testing contribute to determine the flow characteristics that affect the dynamic response of wind turbines operating in highly complex terrains?. This question is not new, indeed, the debate in the scientific community was opened in the first third of the past century and it is still intensely alive. The accepted approach to face this problem consists in analysing a given case study where full-scale tests, computational modelling and wind tunnel testing are applied to the same topography. This is neither easy nor cheap. This is is the reason why since the Askervein experience in 1988, the atmospheric flow modellers community had to wait till 2007 when the Bolund experiment was setup with a deployment of technical means equivalent (considering the evolution of the sensor and computing techniques). The problem is so manifold that both experiences were restricted to neutral conditions without Coriolis effects in order to reduce the complexity. This is the framework in which this PhD has been carried out. The flow topology over the Bolund Island has been studied by replicating the Bolund experiment in the IDR A9 and ACLA16 wind tunnels. Two mock-ups of the Bolund island were manufactured at two scales of 1:230 and 1:115. The in-flow in the empty wind tunnel simulating the incoming atmospheric boundary layer was in the transitionally rough regime and used as a reference case. The 1:230 model was tested in the A9 wind tunnel to measure surface pressure. The mapping of the pressure coefficient across the island gave a visualisation and estimation of a detachment region on the top of the escarpment in front of the island. Time resolved instantaneous pressure measurements illustrated the non-steadiness in the detachment region. The 1:115 model was tested using 3C hot-wires(HW) and 2C Particle Image Velocimetry(PIV). Measurements at met masts M3, M6, M7 and M8 and along Line 270°were taken to replicate the result of the Bolund experiment. The flow was characterised by the speed-up ratio, normalised increment of the turbulent kinetic energy, inclination angle and turning angle. Results along line 270°at heights of 2 m and 5 m compared very well with the full-scale results of the Bolund experiment. Vertical profiles at the met masts showed a significant agreement with the full-scale results. The analysis of the Reynolds stresses and the spectral analysis at the met mast locations gave a varied level of agreement at some locations while clear mismatch at others. The horizontal mapping of the flow field, for a 270°wind direction, allowed to characterise the behaviour of the intermittent recirculation bubble on top of the front escarpment followed by a relaxation region and the presence of a shear layer in the lee side of the island. Further detailed velocity measurements were taken at cross-flow planes over the island to study the flow structures on the island. A longitudinal vortex-like structure with high mean velocity gradients and high turbulent kinetic energy was characterised on the escarpment and evolving downstream. This flow structure is a challenge to the numerical models while posing a threat to wind farm designers when siting wind turbines. Spatial distribution of Reynold stresses were presented from 3C HW and PIV measurements. These values are not common results from usual wind tunnel measurements and very useful for modellers using large eddy simulation (LES). An interpretation of the wind tunnel results in terms of usefulness to wind farm designers is given. Evolution and variation of the flow parameters along measurement lines, planes and surfaces indicated how the flow field could affect wind turbine siting. Different flow properties were presented so compare the level of agreement to full-scale results and how this affected when characterising the site wind classes. The results presented suggest, under certain conditions, the robustness of the wind tunnel testing for studying flow topology over complex terrain and its capability to compare to other modelling techniques especially from the level of agreement between the different data sets presented. Additionally, some flow parameters obtained from wind tunnel measurements would have been quite difficult to be measured at full-scale or by computational means considering the state of the art. This work was carried out as a part of the activities supported by the EC as part of the FP7- PEOPLE-ITN-2008 WAUDIT project (Wind Resource Assessment Audit and Standardization) within the FP7 Marie-Curie Initial Training Network and by the Spanish Ministerio de Economía y Competitividad, within the framework of the ENE2012-36473, TURCO project (Determination of the Spatial Distribution of Statistic Parameters of Flow Turbulence over Complex Topographies in Wind Tunnel) belonging to the Spanish National Program of Research (Subprograma de investigación fundamental no orientada 2012). The report is organised in seven chapters and a collection of annexes. In chapter one, the problem is introduced. In chapter two the experimental setup is described. Following, in chapter three, the inflow conditions of the main wind tunnel used in this piece of research are analysed in detail. In chapter three, preliminary pressure tests results on a model of the island are presented. The main results from the Bolund experiment are replicated in chapter five. In chapter six, an identification of specific flow strutures over the island is presented and, finally, in chapter seven, conclusions and lines for future works related to the presented one are included.

Advanced data acquisition system implementation for the ITER Neutron Diagnostic use case using EPICS and FlexRIO technology on a PXIe platform

In the framework of the ITER Control Breakdown Structure (CBS), Plant System Instrumentation & Control (I&C) defines the hardware and software required to control one or more plant systems [1]. For diagnostics, most of the complex Plant System I&C are to be delivered by ITER Domestic Agencies (DAs). As an example for the DAs, ITER Organization (IO) has developed several use cases for diagnostics Plant System I&C that fully comply with guidelines presented in the Plant Control Design Handbook (PCDH) [2]. One such use case is for neutron diagnostics, specifically the Fission Chamber (FC), which is responsible for delivering time-resolved measurements of neutron source strength and fusion power to aid in assessing the functional performance of ITER [3]. ITER will deploy four Fission Chamber units, each consisting of three individual FC detectors. Two of these detectors contain Uranium 235 for Neutron detection, while a third "dummy" detector will provide gamma and noise detection. The neutron flux from each MFC is measured by the three methods: . Counting Mode: measures the number of individual pulses and their location in the record. Pulse parameters (threshold and width) are user configurable. . Campbelling Mode (Mean Square Voltage): measures the RMS deviation in signal amplitude from its average value. .Current Mode: integrates the signal amplitude over the measurement period

Propiedades ópticas y estructurales de nanoestructuras de Zn(Cd)(Mg)O

En la presente tesis doctoral se ha realizado un estudio utilizando diferentes técnicas de crecimiento (RPE-MOCVD y spray pyrolysis) y estructuras (nanohilos, pozos y puntos cuánticos y capas) con el objetivo de desarrollar dispositivos que cubran desde el rango visible hasta el ultravioleta. Es por esta razón por la que se han elegido materiales basados en ZnO, debido a la posibilidades que estos ofrecen para variar su bandgap en un amplio rango de energías. Prueba de ello es que en este estudio se ha conseguido cubrir un rango espectral desde 1.86 hasta 4.11 eV, estudiandose además fenómenos físicos como son la difusión e incorporaci ón de la aleación o la adsorción de gases en la super_cie, lo que ha permitido la fabricación de diferentes fotodetectores de gran sensibilidad. Por todo ello, los resultados obtenidos en esta tesis suponen una gran contribución al conocimiento de las propiedades físicas de las aleaciones de Zn(Cd)O y Zn(Mg)O para potenciales aplicaciónes en dispositivos que operen en el rango visible y ultravioleta del espectro, respectivamente. En esta memoria se da en primer lugar una visión de las propiedades de materiales basados en ZnO, entrando en detalle en una de las ventajas que este presenta, la facilidad que tiene este material para formar nanoestructuras. En el capítulo 3 se dan los conceptos teóricos necesarios para comprender las propiedades ópticas de este tipo de materiales, mostrando también los resultados más reseñables obtenidos en ZnO. En los capítulos referentes a los resultados se pueden diferenciar dos grandes bloques. En el primer bloque de resultados se han analizado nanohilos y pozos cuánticos de Zn(Cd)O crecidos por la técnica de RPE-MOCVD (Capítulos 4 y 5). En el segundo se expondrá el estudio realizado sobre capas y puntos cuánticos de Zn(Mg)O crecidos por la técnica spray pyrolysis como se describe en mayor detalle a continuación. Nanohilos y pozos cuánticos de Zn(Cd)O crecidos por RPE-MOCVD Teóricamente aleando el ZnO con CdO es posible disminuir el valor del band- gap desde 3.37 eV hasta 0.95 eV, cubriendo por completo el espectro visible. El desarrollo del ternario Zn(Cd)O permitiría la fabricación de heteroestructuras y pozos cuánticos, muy importantes en el desarrollo de dispositivos optoelectrónicos que cubran la parte visible del espectro. Sin embargo, la diferencia de estructura cristalina entre estos dos materiales junto a la baja solubilidad del Cd y su alta presión de vapor, di_culta la obtención de material de alta calidad cristalina con alto contenido en Cd. En esta tesis doctoral se ha realizado una completa caracterización óptica y estructural de nanohilos de Zn(Cd)O credidos por la técnica de RPE-MOCVD. Estos nanohilos tinene unas longitudes comprendidas entre 1 y 3 _m y diámetros entre 100 y 200 nm. La concentración máxima introducida de Cd en estas estructuras ha sido de hasta un 54% manteniendo la estructura wurtzita del ZnO, siendo este el mayor contenido de Cd introducido hasta la fecha en nanostructuras basada en ZnO. Este hecho se traduce en una variación de la energía de emisión entre 3.31 y 1.86 eV con el aumento en Cd. El uso de diferentes técnicas de alta resoluci ón de caracterización estructural ha permitido demostrar la presencia de una sola fase estructural wurtzita sin observarse ningún indicio de separación de fases ni acumulación de Cd a lo largo del nanohilo para todos los contenidos de Cd. Con el propósito de fabricar dispositivos en nanohilos individuales, parte de esta tesis doctoral ha estado dedicada a estudiar el impacto que el recocido térmico tiene en las propiedades ópticas y eléctricas de nanohilos de Zn(Cd)O. El recocido térmico es un proceso clave en la optimización de dispositivos, ya sea para la obtenci ón de contactos óhmicos, reducción de defectos o difusión de dopantes por ejemplo. En este estudio se ha observado una mejora muy signi_cativa de las propiedades de emisión de los nanohilos cuando estos eran recocidos a temperaturas mayores que la de crecimiento (300 oC). En las muestras con Cd se ha observado además que el recocido también produce un desplazamiento de la emisión hacia mayores energías debido a una reducción homogénea del contenido de Cd. Medidas de fotoluminiscencia con resolución temporal muestran el impacto que tiene la localización del excitón en las _uctuaciones de potencial, debidas a una distribución estadística del Cd, en la dinámica de los portadores. Comparando el tiempo de vida de los portadores entre los nanohilos recocidos y sin recocer se ha observado un aumento de este parámetro en las estructuras recocidas. Este aumento es fundamentalmente debido a una reducción de centros de recombinación no radiativa asociados a defectos presentes a lo largo del nanohilo. Además, se ha estudiado la evolución de los tiempos de vida de los portadores en función de la temperatura, registrándose una menor estabilidad con la temperatura de los tiempos de vida en las muestras recocidas. Este resultado sugiere que el recocido térmico consigue reducir parte del desorden de la aleación en la estructura. Tras haber caracterizados los nanohilos se desarrollaron una serie de procesa dos para la fabricación de dispositivos basados en nanohilos individuales. Se fabricaron en concreto fotodetectores sensibles al UV, en los que se observó también la alta sensibilidad que muestran a la adsorción de gases en la super_cie, incrementada por la gran relación super_cie/volúmen característica de las nanoestructuras. Estos procesos de adsorción observados tienen un impacto directo sobre las propiedades ópticas y electricas de los dispositivos como se ha demostrado. Por ello que en esta tesis se hayan estudiado en detalle este tipo de procesos, ideando maneras para tener un mayor control sobre ellos. Finalmente se crecieron estructuras de pozos cuántico de ZnCdO/ZnO en nanohilos con contenidos de Cd nominales de 54 %. Las medidas ópticas realizadas mostraron como al aumentar la anchura del pozo de 0.7 a 10 nm, la emisión relacionada con el pozo se desplazaba entre 3.30 y 1.97 eV. Este gran desplazamiento representa el mayor obtenido hasta la fecha en pozos cuánticos de ZnCdO/ZnO. Sin embargo, al caracterizar estructuralmente estas muestras se observó la presencia de procesos de difusión de Cd entre el pozo y la barrera. Como se ha podido medir, este tipo de procesos reducen sustancialmente la concentración de Cd en el pozo al difundirse parte a la barrera. cambiando completamente la estructura de bandas nominal de estas estructuras. Este estudio demuestra la importancia del impacto de los procesos de difusión en la interpretación de los efectos de con_namiento cuántico para este tipo de estructuras. Capas y puntos cuánticos de Zn(Mg)O crecidos por spray pyrolysis La técnica de spray pyrolysis, debido a su simplicidad, bajo coste y capacidad de crecer sobre grandes áreas conservando una alta calidad cristalina presenta un gran interés en la comunidad cientí_ca para el potencial desarrollo de dispositivos comerciales. En esta tesis se ha estudiado las propiedades ópticas y eléctricas de capas y puntos cuánticos de Zn(Mg)O crecidos por esta técnica. Al contrario que pasa con el Cd, al introducir Mg en la estructura wurtzita de ZnO se consigue aumentar el bandgap del semiconductor. Sin embargo, al igual que pasa con el CdO, la diferencia de estructura cristalina entre el ZnO y el MgO limita la cantidad de Mg que se puede incorporar, haciendo que para una cierta concentración de Mg aparezcan el fenómeno de separación de fases. En esta tesis se ha conseguido incorporar hasta un contenido de Mg del 35% en la estructura wurtzita del ZnO utilizando la técnica de spray pyrolysis, resultado que representa la mayor concentración de Mg publicada hasta la fecha. Este hecho ha posibilitado variar la energía del borde de absorción desde 3.30 a 4.11 eV. En estas capas se ha realizado una completa caracterización óptica observándose una diferencia entre las energías del borde de absorción y del máximo de emisión creciente con el contenido en Mg. Esta diferencia, conocida como desplazamiento de Stokes, es debida en parte a la presencia de _uctuaciones de potencial producidas por un desorden estadístico de la aleación. Se han fabricado fotodetectores MSM de alta calidad utilizando las capas de Zn(Mg)O previamente caracterizadas, observándose un desplazamiento del borde de absorción con el aumento en Mg desde 3.32 a 4.02 eV. Estos dispositivos muestran altos valores de responsividad (10-103 A/W) y altos contrastes entre la responsividad bajo iluminación y oscuridad (10-107). Estos resultados son en parte debidos a la presencia de mecanismos de ganancia y una reducción de la corriente de oscuridad en las muestras con alto contenido de Mg. Utilizando esta misma técnica de crecimiento se han crecido puntos de Zn(Mg)O con concentraciones nominales de Mg entre 0 y 100 %, con dimensiones medias entre 4 y 6 nm. Las medidas estructurales realizadas muestran que hasta un valor de Mg de 45 %, los puntos están compuestos por una única fase estructural, wurtzita. A partir de esa concentración de Mg aparece una fase cúbica en los puntos, coexistiendo con la fase hexagonal hasta una concentración nominales del 85 %. Para concentraciones mayores de Mg, los puntos muestran una única fase estructural cúbica. Medidas de absorción realizadas en estos puntos de Zn(Mg)O muestran un desplazamiento del borde de absorción entre 3.33 y 3.55 eV cuando la concentraci ón de Mg en los puntos aumenta hasta el 40 %. Este desplazamiento observado es debido solamente a la fase wurtzita del Zn(Mg)O donde se incorpora el Mg. ABSTRACT This PhD theis presents a study using di_erent growth techniques (RPEMOCVD and spray pyrolysis) and structures (nanowires, quantum dots and wells and layers) in order to develop devices that extend from the visible to the ultraviolet range. For this reason ZnO based materials have been choosen, because they o_er the possibility to tunne the bandgap in this energy range. Proof of this is that this study has managed to cover a spectral range from 1.86 to 4.11 eV, also being studied physical phenomena such as di_usion and incorporation of alloy or adsorption of gases on the surface, allowing the develop di_erent highly sensitive photodetectors. Therefore, the results obtained in this thesis are a great contribution two large blockso the knowledge of the physical properties of alloys Zn(Cd)O and Zn(Mg)O for potential applications in devices that operate in the visible and ultraviolet range, respectively. In the _rst chapter, the general properties of ZnO-based materials are presented, showing the facilities that these kind of materials o_er to obtain di_erent nanoestructures. In Chapter 3, optical theoretical concepts are given to understand the optical properties of these materials, also showing the most signi_cant results of ZnO. In the chapters related with the results, two blocks could be distinguish. In the _rst one, Zn(Cd)O nanowires and quantum wells grown by RPE-MOCVD have been analyzed (Chapters 4 and 5). The second block of results shows the study performed in Zn(Mg)O _lms and quantum dots grown by spray pyrolysis. Zn(Cd)O nanowires and quantum wells grown by RPE-MOCVD In summary, the results of the PhD thesis are a great contribution to the knowledge of the physical properties of Zn(Cd)O and Zn(Mg)O alloys and their application for high performance devices operating in the visible and UV ranges, respectively. The performance of the device is still limited due to alloy solubility and p-doping stability, which opens a door for future research in this _eld. Theoretically, annealing ZnO with CdO allows to reduce the bandgap from 3.37 to 0.95 eV, covering the whole visible spectrum. The development of ZnCdO alloys allows the fabrication of heterostructures and quantum wells, necessary for the development of high performance optoelectronic devices. However, the di_erent crystal structures between CdO and ZnO and the low solubility of Cd and its high vapor pressure, hinders the growth of ZnCdO alloys with high Cd contents. In this PhD thesis Zn(Cd)O nanowires have been optically and structurally characterized, obtaining a maximum Cd content of 54% while maintaining their wurtzite structure. This Cd content, which allows lowering the bandgap down to 1.86 eV, is the highest concentration ever reported in nanostructures based on ZnO. The combination of optical and structural characterization techniques used during this thesis has allowed the demonstration of the presence of a single wurtzite structure, without observing any indication of phase separation or Cd accumulation along the nanowire. Annealing processes are essential in the fabrication of optoelectronic devices. For this reason, a complete study of the annealing e_ects in the optical and electrical properties of Zn(Cd)O nanowires has been performed. In the _rst place, annealing nanowires at higher temperatures than their growth temperature (300 oC) allows a signi_cant improvement of their emission properties. However, in the samples that contain Cd a shift in the emission towards higher energies has been observed due to a homogeneous reduction of the Cd content in the nanowires. Time resolved photoluminescence measurements show the impact of the exciton localization in the potential _uctuations due to a statistical alloy disorder. An increase in the carrier lifetime has been obtained for the annealed nanowires. This increase is mainly due to the reduction of non-radiative recombination centers associated with the defects present in the material. Furthermore, temperature dependent time resolved photoluminescence measurements suggest a reduction of the alloy disorder in the annealed samples. In this thesis, single nanowire photodetectors with a high responsivity in the UV range have been demonstrated. Due to the high surface/volume ratio, these structures are very sensitive to gas adsorption at the surface, which largely de_nes the optical and electrical properties of the material and, therefore, of the device. With the aim of obtaining time stable devices, the dynamic adsorption-desorption processes have been studied, developing di_erent approaches that allow a higher control over them. Finally, ZnCdO/ZnO quantum wells have been grown with a nominal Cd concentration of 54% inside the well. The performed optical measurements show that increasing the well width from 0.7 to 10 nm, shifts the emission related with the well from 3.30 to 1.97 eV. This result represents the highest shift reported in the literature. However, a detailed structural characterization shows the presence of di_usion phenomena which substantially reduce the concentration of Cd in the well, while increasing it in the barrier. This type of phenomena should be considered when ac curately interpretating the quantum con_nement e_ects in Zn(Cd)O/ZnO quantum wells. Theoretically, annealing ZnO with CdO allows to decrease the bandgap from 3.37 to 0.95 eV, covering the whole visible spectrum. Zn(Mg)O _lms and quantum dots grown by spray pyrolysis Due to its simplicity, low-cost and capacity to grow over large areas conserving a high crystal quality, spray pyrolysis technique presents a great interest in the scienti_c community for developing comercial devices. In this thesis, a complete study of the optical and structural properties of Zn(Mg)O _lms and quantum dots grown by spray pyrolysis has been performed. Contrary to Zn(Cd)O alloys, when introducing Mg in the ZnO wurtzite structure an increase in the bandgap in obtained. Once again, the di_erence in the crystal structure of ZnO and MgO limits the amount of Mg that can be introduced before phase separation appears. In this PhD thesis, a maximum Mg content of 35% has been incorporated in the wurtzite structure using spray pyrolysis. This variation in the Mg content translates into an increase of the absorption edge from 3.30 to 4.11 eV. Up to this date, this result represents the highest Mg content introduced by spray pyrolysis in a ZnO wurzite structure reported in the literature. The comparison of the emission and absorption spectra shows the presence of an increasing Stokes shift with Mg content. This phenomenon is partialy related with the presence of potential _uctuations due to an statistic alloy disorder. MSM photodetectors have been processed on previously characterized Zn(Mg)O _lms. These devices have shown a shift in the absorption edge from 3.32 to 4.02 eV with the increase in Mg content, high responsivity values (10-103 A/W) and high contrast ratios between illuminated and dark responsivities (10-107). These values are explained by the presence of a gain mechanism and a reduction of dark current in the ZnMgO samples. Zn(Mg)O quantum dots have also been grown using spray pyrolysis with Mg concentrations between 0 and 100% and with average widths ranging 4 to 6 nm. Structural measurements show that at a Mg concentration of 45% the cubic phase appears, coexisting with the hexagonal phase up to an 85% concentration of Mg content. From 85% onwards the quantum dots show only the cubic phase. Absorption measurements performed in these structures reveal a shift in the absorption edge from 3.33 to 3.55 eV when the Mg content increases up to 40 %.

trans/cis (Z/E) photoisomerization of the chromophore of photoactive yellow protein is not a prerequisite for the initiation of the photocycle of this photoreceptor protein

The chromophore of photoactive yellow protein (PYP) (i.e., 4-hydroxycinnamic acid) has been replaced by an analogue with a triple bond, rather than a double bond (by using 4-hydroxyphenylpropiolic acid in the reconstitution, yielding hybrid I) and by a “locked” chromophore (through reconstitution with 7-hydroxycoumarin-3-carboxylic acid, in which a covalent bridge is present across the vinyl bond, resulting in hybrid II). These hybrids absorb maximally at 464 and 443 nm, respectively, which indicates that in both hybrids the deprotonated chromophore does fit into the chromophore-binding pocket. Because the triple bond cannot undergo cis/trans (or E/Z) photoisomerization and because of the presence of the lock across the vinyl double bond in hybrid II, it was predicted that these two hybrids would not be able to photocycle. Surprisingly, both are able. We have demonstrated this ability by making use of transient absorption, low-temperature absorption, and Fourier-transform infrared (FTIR) spectroscopy. Both hybrids, upon photoexcitation, display authentic photocycle signals in terms of a red-shifted intermediate; hybrid I, in addition, goes through a blue-shifted-like intermediate state, with very slow kinetics. We interpret these results as further evidence that rotation of the carbonyl group of the thioester-linked chromophore of PYP, proposed in a previous FTIR study and visualized in recent time-resolved x-ray diffraction experiments, is of critical importance for photoactivation of PYP.

Structural changes in hemoglobin during adsorption to solid surfaces: Effects of pH, ionic strength, and ligand binding

We have studied the adsorption of two structurally similar forms of hemoglobin (met-Hb and HbCO) to a hydrophobic self-assembled methyl-terminated thiol monolayer on a gold surface, by using a Quartz Crystal Microbalance (QCM) technique. This technique allows time-resolved simultaneous measurements of changes in frequency (f) (c.f. mass) and energy dissipation (D) (c.f. rigidity/viscoelastic properties) of the QCM during the adsorption process, which makes it possible to investigate the viscoelastic properties of the different protein layers during the adsorption process. Below the isoelectric points of both met-Hb and HbCO, the ΔD vs. Δf graphs displayed two phases with significantly different slopes, which indicates two states of the adsorbed proteins with different visco-elastic properties. The slope of the first phase was smaller than that of the second phase, which indicates that the first phase was associated with binding of a more rigidly attached, presumably denatured protein layer, whereas the second phase was associated with formation of a second layer of more loosely bound proteins. This second layer desorbed, e.g., upon reduction of Fe3+ of adsorbed met-Hb and subsequent binding of carbon monoxide (CO) forming HbCO. Thus, the results suggest that the adsorbed proteins in the second layer were in a native-like state. This information could only be obtained from simultaneous, time-resolved measurements of changes in both D and f, demonstrating that the QCM technique provides unique information about the mechanisms of protein adsorption to solid surfaces.

Mobilization of the A-kinase N-myristate through an isoform-specific intermolecular switch

Although the catalytic (C) subunit of cAMP-dependent protein kinase is N-myristylated, it is a soluble protein, and no physiological role has been identified for its myristyl moiety. To determine whether the interaction of the two regulatory (R) subunit isoforms (RI and RII) with the N-myristylated C subunit affects its ability to target membranes, the effect of N-myristylation and the RI and RII subunit isoforms on C subunit binding to phosphatidylcholine/phosphatidylserine liposomes was examined. Only the combination of N-myristylation and RII subunit interaction produced a dramatic increase in the rate of liposomal binding. To assess whether the RII subunit also increased the conformational flexibility of the C subunit N terminus, the effect of N-myristylation and the RI and RII subunits on the rotational freedom of the C subunit N terminus was measured. Specifically, fluorescein maleimide was conjugated to Cys-16 in the N-terminal domain of a K16C mutant of the C subunit, and the time-resolved emission anisotropy was determined. The interaction of the RII subunit, but not the RI subunit, significantly increased the backbone flexibility around the site of mutation and labeling, strongly suggesting that RII subunit binding to the myristylated C subunit induced a unique conformation of the C subunit that is associated with an increase in both the N-terminal flexibility and the exposure of the N-myristate. RII subunit thus appears to serve as an intermolecular switch that disrupts of the link between the N-terminal and core catalytic domains of the C subunit to expose the N-myristate and poise the holoenzyme for interaction with membranes.

Chromophore structural changes in rhodopsin from nanoseconds to microseconds following pigment photolysis

Rhodopsin is a prototypical G protein-coupled receptor that is activated by photoisomerization of its 11-cis-retinal chromophore. Mutant forms of rhodopsin were prepared in which the carboxylic acid counterion was moved relative to the positively charged chromophore Schiff base. Nanosecond time-resolved laser photolysis measurements of wild-type recombinant rhodopsin and two mutant pigments then were used to determine reaction schemes and spectra of their early photolysis intermediates. These results, together with linear dichroism data, yielded detailed structural information concerning chromophore movements during the first microsecond after photolysis. These chromophore structural changes provide a basis for understanding the relative movement of rhodopsin’s transmembrane helices 3 and 6 required for activation of rhodopsin. Thus, early structural changes following isomerization of retinal are linked to the activation of this G protein-coupled receptor. Such rapid structural changes lie at the heart of the pharmacologically important signal transduction mechanisms in a large variety of receptors, which use extrinsic activators, but are impossible to study in receptors using diffusible agonist ligands.

NMR of laser-polarized xenon in human blood

By means of optical pumping with laser light it is possible to enhance the nuclear spin polarization of gaseous xenon by four to five orders of magnitude. The enhanced polarization has allowed advances in nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI), including polarization transfer to molecules and imaging of lungs and other void spaces. A critical issue for such applications is the delivery of xenon to the sample while maintaining the polarization. Described herein is an efficient method for the introduction of laser-polarized xenon into systems of biological and medical interest for the purpose of obtaining highly enhanced NMR/MRI signals. Using this method, we have made the first observation of the time-resolved process of xenon penetrating the red blood cells in fresh human blood—the xenon residence time constant in the red blood cells was measured to be 20.4 ± 2 ms. The potential of certain biologically compatible solvents for delivery of laser-polarized xenon to tissues for NMR/MRI is discussed in light of their respective relaxation and partitioning properties.