Conception, fabrication et caractérisation d'un modulateur optique à commande plasmonique sur nitrure de gallium à une longueur d'onde de 1,55 micron

Future optical modulators have to satisfy requirements that current electro-optical modulators fail to do, such as very low command voltage, small size and ability to work in the 0-40GHz range with low losses. It is also necessary to find new ways to modulate light at a faster speed but with low consumption. The work in this thesis addresses these above mentioned problems within the framework of an upstream project which has been financed by the French Ministry of Defence. Its goal is to evaluate the potential gain of plasmonics in the field of semiconductors based optical modulation. We first select a gallium nitride based structure on sapphire whose optical properties exhibit very low propagation losses of 0.6dB/cm. Then we proved the generation of a plasmonic wave for a Au/GaN interface. Optimizations were conducted to get the most effective modulation by varying optical index of the semiconductor. Several devices were fabricated in clean room and were characterized. Results obtained in optical measurements proved the effect of electric field in varying the optical index of GaN until 10-2 for tens of volt; however, RF propagation losses were high, about 16dB/cm for 20GHz. In addition, an electro-absorption structure using multi-quantum wells on InP substrate shows index variation of the order of 2×10-3 for only 2.5V by the prism-coupling technique. This thesis work, which has been the very first one in this domain in this laboratory, will pave the way for future research into new materials for optoelectronic applications and highlight the critical issues of the use of plasmonics for optical modulation in semiconductors.Les futurs modulateurs optiques doivent satisfaire à des exigences auxquelles les modulateurs électro-optiques actuels ne peuvent plus répondre (tension de commande et dimensionnement faible, fonctionnement dans la gamme 0-40GHz à faibles pertes). Il devient alors nécessaire d'envisager de nouveaux moyens de réaliser une modulation rapide à faible consommation. Ce travail s'inscrit au sein d'un projet amont de la DGA, afin d'évaluer le gain potentiel de la plasmonique sur semiconducteurs pour la modulation optique. Nous avons d'abord sélectionné des couches de GaN sur saphir avec d'excellentes propriétés optiques et des pertes de propagation de l'ordre de 0,6dB/cm. Ensuite, nous avons montré la génération d'une résonance plasmonique à l'interface Au/GaN. Un travail d'optimisation a été réalisé en vue de rendre sa modulation efficace par variation de l'indice du GaN. Plusieurs dispositifs de démonstration ont été fabriqués en salle blanche puis caractérisés. Si les résultats optiques obtenus ont montré un effet de variation d'indice nouveau jusqu'à Δn=10-2 pour plusieurs dizaines de volt, les pertes RF de propagation se sont révélées élevées, proches de 16dB/cm à 20GHz. En parallèle, une structure à effet d'électro-absorption utilisant un multipuits quantique sur InP a été conçue et caractérisée par couplage par prisme et a montré des variations d'indice de l'ordre de 2×10-3 à 2,5V. Ces travaux de thèse, précurseurs dans ce domaine au sein du laboratoire, vont permettre d'orienter les recherches futures vers de nouveaux matériaux pour l'optoélectronique, mais aussi de mettre en exergue les points durs de la plasmonique pour la modulation optique sur semiconducteurs

Thermoelectric properties of Ca3Co4O9 thin films grown by pulsed-laser deposition

International audienceThe Ca3Co4O9 (CCO) compound consists in a misfit layered structure formed by superposition of two atomic planes (Ca2CoO3 and CoO2) that makes this oxide a promising material for thermoelectric (TE) application. The thermal and electrical conductivities and the thermoelectric power can be tuned by controlling the oxygen deficiency in CCO films. Pulsed-laser deposition (PLD) performed at low oxygen partial pressure allows to grow oxygen deficient films. In that sense smooth, dense, and homogeneous films, with different stoichiometry and crystalline state were prepared by PLD in order to modify charge carrier concentration and thermal diffusivity of material. Thus, nearly stoichiometric and highly oxygen deficient CCO films were deposited onto sapphire substrate hold at room temperature, 300, 500 and 700°C under 10^-1 and 10^-6 mbar in oxygen partial pressure, respectively. The electrical, thermal and thermoelectric properties are correlated to the structural, and morphological characteristics of thin films by using an original equipment (ZT meter) specifically designed for TE measurements on thin films

Development of geoelectrical monitoring of the critical zone processes on microfluidic chips

International audienceWe miniaturize the low-frequency (<1kHz) geoelectrical acquisition using advanced micro-fabrication technologies to investigate coupled processes in the critical zone (CZ). With this innovation in the experimental acquisition, we focus on the development of the complex electrical conductivity monitoring with the spectral induced polarization (SIP) method. The interpretation of the SIP signal is based on the development of petrophysical models that relate the complex electrical conductivity to structural, hydrodynamical, and geochemical properties or distributions. State-of-the-art petrophysical models, however, suffer from a limited range of validity and presume too many microscopic mechanisms to define macroscale parameters. Thus, direct observations of the underlying processes coupled with geoelectrical monitoring are keys to deconvolute the signature of the biochemical-physical mechanisms and, then, developing more reliable models. Microfluidic experiments enable direct visualization of flows, reactions, and transport at the pore-scale thanks to transparent micromodels coupled with optical microscopy and high-resolution imaging techniques. Micromodels are a two-dimensional representation of the porous medium, ranging in complexity from single channels to replicas of natural rocks. Cutting-edge micromodels use reactive minerals to investigate the water-mineral interactions involved in the CZ. In this work, we propose a new kind of micromodels equipped with four aligned electrodes within the channel for SIP monitoring of calcite dissolution, a key multiphase process of the CZ involved in karstification. We highlight the strong correlation between SIP response and dissolution through electrical signal examination and image analysis. In particular, degassed CO2 bubbles generated by the dissolution play a major role. Our technological advancement brings a deeper understanding of the physical interpretation of the complex electrical conductivity and will provide a further understanding of the CZ dynamic processes through SIP observation

Microplasma arrays operating in DC fabricated by microelectronic technologies

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MEMS based nanofluidics device for the study of the nonlinear dynamics associate with the geochemical processes

International audienceTo understand the evolution of man-impacted hydrosystems with the Earth Sciences, MEMS based nanofluidics devices were prepared. This paper presents the detailed different fabrication steps used for the preparation of successful nanofluidics devices

Original bench design to characterize simultaneously thermal and thermoelectrical properties of MesoPorous Silicon and Graphenized MesoPorous Silicon

International audienceAs derived from silicon substrate, meso-porous Si could be a competitive candidate for many thermoelectric applications for micro-systems namely due to the huge decrease of its intrinsic thermal conductivity. As reported in the literature, this can offer a higher figure of merit as well as a more significant efficiency for energy harvesting at the microscale level at room temperature. Also, to consider the electronic transport that is lowered by the fabrication process, a graphenisation step is investigated both on the structural and thermoelectric behavior of the mesoporous complex Si matrix.Porous silicon membrane is obtained by electrochemical etching process from an electrolytic solution of hydrofluoric acid and ethanol. Depending on numerous parameters (porosity, pore size distribution, porous thickness...) the Seebeck coefficient has been investigated accordingly to key process parameters and correlated to the membrane morphology and structural characteristics. Using a new home-made thermoelectric device specifically designed for porous membranes will make possible to evaluate the Seebeck coefficient in the range of 10 to 70 °C. Systematic investigation of graphenized and non-graphenized membranes is conducted to determine the contribution of the graphene in the enhancement of the thermoelectric properties. In parallel, electrical measurements (Van Der Pauw and Hall effect techniques) and thermal characterization (Pulsed photothermal and pulsed electrothermal methods) are also achieved for the estimation of the ZT coefficient (thermoelectric performance of a material and depends on the σ, electrical and k, thermal conductivity and S, the Seebeck coefficient) and the power factor (given by σS2)

Micro-spiked mesoporous silicon formed by UV picosecond laser irradiation

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