REALISATION ET ETUDE DE STRUCTURES A MODULATION D'INDICE OPTIQUE EN SILICIUM POREUX

Abstract : This work concerns the study of the vertical and lateral modulation of the optical index in the porous silicon.To realise performing optical structures, we focused our efforts on the optical losses due to the p-type porous silicon samples interfaces. A study on the changes induced by the formation temperature on the interface roughness has been performed to reduce the losses. For a formation temperature of –35°C, and using the most critical formation currents, we obtained a striking improvement, reducing the interface roughness by a factor of 6.This roughness decrease allowed us to realise Bragg mirrors and microcavities of very high optical quality, with reflectivity coefficients as high as 99.5% and cavity mode widths of 5nm. The determination of such high reflection coefficients was realised using very precise measurements by “ring down” spectroscopy, with a precision of 0.01%. Thanks to the characterisation of these structures we were able to evidence a difference in the optical properties between simple layers and underlying layers. The modifications of the refractive index and of the formation velocity have been determined quantitatively by studying the photoluminescence and by simulating the reflection spectra. We also show that dye molecules can be used to efficiently fill the porous silicon cavities,independent on the low or high porosity degree. Moreover, the filling of the pores may be eased by oxidation of the porous silicon layer prior to the filling. Our results show that porous silicon can be a very good matrix for dye molecules, that stay enough dispersed to emit light. The use of dye-filled porous silicon microcavities allow for a sharpening and an amplification of the dye emission. To characterise the lateral refractive index modulation, obtained by photodissolution and photo-electro-formation of the porous silicon layer, we studied the influence of the presence of light on the porosity and on the optical properties of p- and n+-type porous silicon monolayers. Based on this study, we were able to determine the optimal formation parameters for gratings with lateral refractive index modulation. The light diffraction from these samples has also been studied. Keywords: Porous Silicon, optical index modulation, reflectivity, interface roughness, Bragg mirrors, laser dyeCe travail concernait l'étude des modulations verticales et latérales de l'indice optique dans le silicium poreux.Dans le but de réaliser des structures optiques performantes, nous nous sommes intéressés aux pertes dues à la rugosité des interfaces présentes dans les échantillons de silicium poreux de type p. Une étude sur l'influence de la température de formation du silicium poreux a été menée dans le but de réduire ces pertes. Pour la température de formation de -35°C pour les courants les plus critiques, nous avons obtenu une remarquable diminution de la rugosité, qui peut aller jusqu'à un facteur 6.Cette diminution de la rugosité nous a permis de réaliser des miroirs de Bragg et microcavités de très bonne qualité optique avec des coefficients de réflectivité de 99.5% et des largeurs de mode de la cavité de 5 nm. La détermination des grands coefficients de réflectivité est réalisé grâce à des mesures très précises de spectroscopie « ring down » avec une précision de 0.01%. La caractérisation de ces structures nous a permis de mettre en évidence une différence des propriétés optiques des couches simples et des couches enterrées, la variation de l'indice optique et de la vitesse de formation a été quantifiée par une étude de la luminescence et par la simulation des spectres de réflectivité. Nous avons montré que les molécules de colorant peuvent imprégner efficacement les couches de silicium poreux, qu'elles aient une faible ou une grande porosité, et que l'imprégnation peut être favorisée par l'oxydation préalable des couches poreuses. Ces résultats montrent que le silicium poreux peut être une très bonne matrice pour les molécules de colorant qui restent suffisamment dispersées pour pouvoir émettre de la lumière. L'utilisation de structures de type microcavité permet un rétrécissement et une amplification de l'émission du colorant. Pour caractériser la modulation latérale d'indice, obtenue par une photodissolution ou photoélectroformation de la couche de silicium poreux, nous avons étudié l'influence de la lumière sur la porosité et les propriétés optiques des monocouches de silicium poreux de type p et de type n+, ce qui a déterminé les conditions de formation des réseaux à modulation latérale d'indice optique. La diffraction de la lumière des échantillons ainsi obtenus a été étudiée. Mots clés : Silicium poreux, Modulation d'indice optique, Réflectivité, Rugosité d'interface, Miroir de Bragg, Microcavité, Colorant laser

Photovoltaic properties of PSi impregnated with eumelanin

A bulk heterojunction of porous silicon and eumelanin, where the columnar pores of porous silicon are filled with eumelanin, is proposed as a new organic-inorganic hybrid material for photovoltaic applications. The addition of eumelanin, whose absorption in the near infrared region is significantly higher than porous silicon, should greatly enhance the light absorption capabilities of the empty porous silicon matrix, which are very low in the low energy side of the visible spectral range (from about 600 nm downwards). The experimental results show that indeed the photocarrier collection efficiency at longer wavelengths in eumelanin-impregnated samples is clearly higher with respect to empty porous silicon matrices

Nuove tecnologie web per e-learning. Primi risultati della serimentazione in un liceo scientifico

Nuove tecnologie web come i sistemi CMS rendono la realizzazione di piattaforme multimedialidi supporto all'insegnamento un obiettivo fattibile dal punto di vista tecnologico, pur senza grandi investimenti finanziari, di grande interesse dal punto di vista didattico. Si analizzano le caratteristiche generali dei sistemi CMS disponibnili e vengono presentati i primi risultati della sperimentazione di un sistema di e-learning basato su CMS nel biennio di un liceo scientifico

SapRete: saperi in rete per il recupero delle competenze logico matematiche e scientifiche

Questo lavoro descrive e presenta i risultati del progetto dal titolo “SapRete: saperi in rete per il recupero delle competenze logico matematiche e scientifiche”, finanziato dalla Regione Sardegna con l'azione P.O.R. 2000/2006 (Misura 3.6 "Prevenzione della dispersione scolastica e formativa") che ha visto coinvolte tre scuole superiori e due scuole medie di Cagliari e l'Università degli Studi di Cagliari con il Dipartimento di Matematica e Informatica e il Dipartimento di Fisica. Il progetto, della durata di due anni, concluso a giugno 2009, aveva come obiettivo quello di sfruttare le potenzialità e le opportunità offerte dalla rete. In particolare, ci si proponeva di utilizzare un contesto di E- learning, attraverso l’utilizzo della la piattaforma Moodle 1.8 per costruire e sperimentare un repertorio di proposte di azioni didattiche miranti a ridurre il tasso di dispersione scolastica in ingresso nella scuola secondaria di secondo grado. Si volevano inoltre introdurre innovazioni didattiche e di sistema finalizzate all'integrazione nei curricula istituzionali di esperienze di apprendimento logico-matematiche e scientifiche più attraenti e motivanti

Porous silicon-based potentiometric triglycerides biosensor

In this paper we report on the fabrication and characterization of a potentiometric biosensor for the detection of triglycerides, based on lipase impregnated mesoporous silicon matrices. Prototypes, realized on 1x1 cm n+-type silicon wafers, show a very high enzymatic activity. Moreover the properties of these biosensors have been stable in a several months time interval, clearly showing their advantages with respect to traditional triglycerides detection systems

Optical, Electrochemical, and Structural Properties of Er-Doped Porous Silicon

We present here a study of Er doping of n+-type porous silicon. The samples were characterized in situ by their electrochemical behavior and ex situ by optical reflectivity and scanning electron microscopy (SEM). A clear correlation between the optical properties and the Er content of the samples is demonstrated. Refractive index dependence on Er content has also been obtained through simulations of reflectivity spectra in the 350–2500 nm range