Fabricació de Lents Zonals de Fresnel per aplicació als raigs X tous

Estudi elaborat a partir d’una estada al Paul Scherrer Institut del Maig a l’Octubre del 2006 amb l’ajuda i supervisió dels Dr. Konstantins Jefimovs i Dr. Christian David. Focalitzar raigs X tous és una necessitat essencial per al microanàlisis, la microscopia, i fer imatges en moltes Instal·lacions de Radiació Sincrotró. Les Lents Zonals de Fresnel (FZP, de la denominació anglesa “Fresnel Zone Plates”) han demostrat donar uns punts focals amb una resolució espacial destacada i una baixa il·luminació de fons. Tanmateix, la fabricació de FZP és complexa i no totalment reproduïble. A més a més, el temps de vida de les FZP és força curt, ja que estant situades sobre membranes de nitrur de silici molt fines i altament absorbents. Per tant, hem fet esforços per implementar FZP de silici, que s’espera que siguin més resistents. L’element està fet d’una oblia de cristall de silici poc absorbent, i no presenta cap interfase entre materials. Així doncs, aquestes lents són especialment adequades per a aguantar les extremes càrregues de radiació de les fonts de raigs X més brillants. Particularment, això és molt important per a les aplicacions a les pròximes generacions de fonts de raigs X, com els Làsers d’Electrons Lliures (FEL, de la denominació anglesa “Free Electron Laser”). El silici també garanteix que no hi hagi cap banda d’absorció en el rang d’energies de la finestra de l’aigua (200-520 eV), fent aquestes lents ideals per a fer imatges de mostres biològiques. En aquest informe, hi ha una descripció detallada de tots els passos involucrats en la fabricació de les Lents Zonals de Fresnel de silici. En resum, les estructures de FZP es modelen sobre una resina utilitzant litografia per feix d’electrons i llavors el patró es transmet al silici mitjançant un gravat d’ions reactius (RIE, de la denominació anglesa ‘Reactive Ion Etching’) utilitzant una fina (20 nm) màscara de Crintermitja. Les membranes de silici es poden aprimar després de la fabricació de les estructures per a garantir una transmissió suficient fins i tot a baixes energies. Aquest informe també inclou l’anàlisi i la discussió d’alguns experiments preliminars per avaluar el rendiment de les Si FZPs fets a la línia de llum PolLux del Swiss Ligth Source amb l’ajuda dels Dr. Jörg Raabe i Dr. George Tzvetkov.

A reflection about the social and technological aspects in flood risk management - the case of the Italian Civil Protection

The right of a person to be protected from natural hazards is a characteristic of the social and economical development of the society. This paper is a contribution to the reflection about the role of Civil Protection organizations in a modern society. The paper is based in the inaugural conference made by the authors on the 9th Plinius Conference on Mediterranean Storms. Two major issues are considered. The first one is sociological; the Civil Protection organizations and the responsible administration of the land use planning should be perceived as reliable as possible, in order to get consensus on the restrictions they pose, temporary or definitely, on the individual free use of the territory as well as in the entire warning system. The second one is technological: in order to be reliable they have to issue timely alert and warning to the population at large, but such alarms should be as "true" as possible. With this aim, the paper summarizes the historical evolution of the risk assessment, starting from the original concept of "hazard", introducing the concepts of "scenario of event" and "scenario of risk" and ending with a discussion about the uncertainties and limits of the most advanced and efficient tools to predict, to forecast and to observe the ground effects affecting people and their properties. The discussion is centred in the case of heavy rains and flood events in the North-West of Mediterranean Region.

Full complex Fresnel holograms displayed on liquid cristal displays

We propose a method to display full complex Fresnel holograms by adding the information displayed on two analogue ferroelectric liquid crystal spatial light modulators. One of them works in real-only configuration and the other in imaginary-only mode. The Fresnel holograms are computed by backpropagating an object at a selected distance with the Fresnel transform. Then, displaying the real and imaginary parts on each panel, the object is reconstructed at that distance from the modulators by simple propagation of light. We present simulation results taking into account the specifications of the modulators as well as optical results. We have also studied the quality of reconstructions using only real, imaginary, amplitude or phase information. Although the real and imaginary reconstructions look acceptable for certain distances, full complex reconstruction is always better and is required when arbitrary distances are used.

Diffraction theory of Fresnel lenses encoded in low-resolution devices

A mathematical model that describes the behavior of low-resolution Fresnel lenses encoded in any low-resolution device (e.g., a spatial light modulator) is developed. The effects of low-resolution codification, such the appearance of new secondary lenses, are studied for a general case. General expressions for the phase of these lenses are developed, showing that each lens behaves as if it were encoded through all pixels of the low-resolution device. Simple expressions for the light distribution in the focal plane and its dependence on the encoded focal length are developed and commented on in detail. For a given codification device an optimum focal length is found for best lens performance. An optimization method for codification of a single lens with a short focal length is proposed.

Diffraction theory of optimized low-resolution Fresnel encoded lenses

A mathematical model describing the behavior of low-resolution Fresnel encoded lenses (LRFEL's) encoded in any low-resolution device (e.g., a spatial light modulator) has recently been developed. From this model, an LRFEL with a short focal length was optimized by our imposing the maximum intensity of light onto the optical axis. With this model, analytical expressions for the light-amplitude distribution, the diffraction efficiency, and the frequency response of the optimized LRFEL's are derived.

Combinación de pantallas de cristal líquido para la modulación compleja de frentes de onda. Aplicación en reconstrucción de hologramas de Fresnel digitales.

En este trabajo se presentan distintas alternativas para obtener la modulación compleja completa de frentes de onda mediante la suma de la modulación introducida por dos pantallas de cristal líquido. Para los distintos métodos se presentan resultados simulados de reconstrucciones de hologramas de Fresnel digitales.

Fetal testosterone (2D:4D) as predictor of cognitive reflection

The Cognitive Reflection Test (CRT) is a test introduced by S. Frederick (2005) Cognitive reflection and decision making, J Econ Perspect 19(4): 25-42. The task is designed to measure the tendency to override an intuitive response that is incorrect and to engage in further reflection that leads to the correct response. The consistent sex differences in CRT performance may suggest a role for gonadal hormones, particularly testosterone. A now widely studied putative marker for fetal testosterone is the second-to-fourth digit ratio (2D:4D). This paper tests to what extent 2D:4D, as a proxy for prenatal exposure to testosterone, can predict CRT scores in a sample of 623 students. After controlling for sex, we observe that a lower 2D:4D (reflecting a higher exposure to testosterone) is significantly associated with a higher number of correct answers. The result holds for both hands? 2D:4Ds. In addition, the effect appears to be sharper for females than for males. We also control for patience and math proficiency, which are significantly related to performance in the CRT. But the effect of 2D:4D on performance in CRT is not reduced with these controls, implying that these variables are not mediating the relationship between digit ratio and CRT.

Reconstruction of Fresnel Holograms using partial wave front information

n this work we analyze the behavior of complex information in Fresnel domain taking into account the limited capability to display complex transmittance values of current liquid crystal devices, when used as holographic displays. In order to do this analysis we compute the reconstruction of Fresnel holograms at several distances using the different parts of the complex distribution (real and imaginary parts, amplitude and phase) as well as using the full complex information adjusted with a method that combines two configurations of the devices in an adding architecture. The RMS error between the amplitude of these reconstructions and the original amplitude is used to evaluate the quality of the information displayed. The results of the error analysis show different behavior for the reconstructions using the different parts of the complex distribution and using the combined method of two devices. Better reconstructions are obtained when using two devices whose configurations densely cover the complex plane when they are added. Simulated and experimental results are also presented.