Origin of Second Harmonic Generation from individual Silicon Nanowires

We investigate Second Harmonic Generation from individual silicon nanowires and study the influence of resonant optical modes on the far-field nonlinear emission. We find that the polarization of the Second Harmonic has a size-dependent behavior and explain this phenomenon by a combination of different surface and bulk nonlinear susceptibility contributions. We show that the Second Harmonic Generation has an entirely different origin, depending on whether the incident illumination is polarized parallel or perpendicularly to the nanowire axis. The results open perspectives for further geometry-based studies on the origin of Second Harmonic Generation in nanostructures of high-index centrosymmetric semiconductors.Comment: 7 Pages, 4 Figures + 3 Pages, 6 Figures in Appendi

Vulnérabilité et stratégies de subsistance des microentrepreneurs et de leurs ménages dans les quartiers populaires du centre de Yaoundé, au Cameroun

La promotion de l’entreprenariat et de la microentreprise fait partie des stratégies de réduction de la pauvreté et de développement des quartiers informels largement répandues dans les pays du Sud global. L’urbanisation croissante en Afrique subsaharienne et la capacité d’absorption limitée du secteur formel y rendent la microentreprise aussi attrayante qu’inévitable. La thèse porte sur la vulnérabilité et sur les stratégies de subsistance des microentrepreneurs et de leurs ménages dans douze quartiers populaires du centre de Yaoundé au Cameroun. Elle s’intéresse particulièrement aux motivations qui sous-tendent ces stratégies ainsi qu’aux liens entre microentreprise, logement et crédit au niveau de la gestion du portefeuille d’activités et d’actifs des microentrepreneurs et de leurs ménages. Des entretiens approfondis ont été effectués auprès de 70 microentrepreneurs provenant de 67 ménages différents pour explorer ces questions de manière empirique. Les résultats ont des implications pour la planification urbaine, sociale et économique, de même que pour la réduction de la pauvreté en Afrique et dans les pays du Sud global. Pour la vaste majorité des participants, la microentreprise faisait partie d’une stratégie générale d’adaptation basée sur la diversification où l’objectif de réduction de la vulnérabilité prime sur l’objectif de croissance économique. Ces résultats nous amènent à distinguer entre microentreprises de survie et microentreprises de croissance et à souligner l’importance d’adapter le soutien de manière différente en fonction de l’adéquation entre les motivations, les besoins, les capacités et les occasions d’affaires, de même qu’en fonction des obstacles auxquels les microentrepreneurs et leurs ménages font face. Ces obstacles comprennent la surabondance d’activités de même nature; le manque d’intégration socioéconomique, physique et spatiale; l’accès partiel, déficient ou inexistant aux infrastructures et services collectifs et urbains de même qu’aux services de santé et d’éducation; ainsi que l’absence d’assurance et de protection sociale. La vulnérabilité a aussi un impact substantiel sur les stratégies de subsistance des microentrepreneurs et de leurs ménages. Cette dynamique affecte la croissance et la réduction de la pauvreté, non seulement en raison de l’impact des chocs sur les actifs des ménages, mais aussi en raison des stratégies pour y faire face en internalisant le risque au moyen de stratégies d’assurance (ex ante) et de rétablissement (ex post) qui ont un impact sur l’accumulation et la consolidation d’actifs. La thèse conclut ainsi que la synergie entre réduction de la pauvreté et réduction de la vulnérabilité constitue une condition nécessaire à une graduation socioéconomique durable.Promoting entrepreneurship and microenterprises has been an integral part of widespread strategies to reduce poverty and contribute to the development of informal settlements in the Global South. Increasing urbanization in Sub-Saharan Africa and the limited absorption capacity of the formal sector have made microenterprises both attractive and inevitable. This thesis focuses on the vulnerability and livelihood strategies of microentrepreneurs and their households in 12 informal settlements located at the centre of Yaoundé in Cameroon. It particularly focuses on the motivations that underlie these strategies as well as on the links between microenterprise, housing and credit in relation to how the microentrepreneurs and their households manage their portfolio of assets and activities. In-depth interviews were conducted with 70 microentrepreneurs from 67 different households to explore these questions empirically. The results have implications for urban, social and economic planning as well as for poverty reduction in Africa and in the Global South. For the vast majority of participants, the microenterprise was part of a general adapting strategy based on diversification where the objective of reducing vulnerability superseded the objective of economic growth. These results lead us to distinguish between survival and growth microenterprises, which underscores the importance of adapting support in relation to differing motivations, needs, capacities and opportunities, as well as in relation to the obstacles faced by microentrepreneurs and their households. These obstacles include the overabundance of activities of similar nature; the lack of socioeconomic, physical and spatial integration; the partial, deficient or inexistent access to infrastructures and collective and urban services, including health and education services; and the absence of insurance and social protection. Vulnerability also has a substantial impact on the livelihood strategies of microentrepreneurs and their households. This dynamic affects growth and poverty reduction, not only because of the impact of shocks on household assets, but also because of the strategies to face these that internalize the risk through ex ante insurance strategies and ex post recovery strategies, which impact asset accumulation and consolidation. Thus, we argue that the synergy between poverty reduction and the reduction of vulnerability is a necessary condition to sustainable socioeconomic graduation

Effect of high current density on the admittance response of interface states in ultrathin MIS tunnel junctions.

International audienceThe effect of a high current density on the measured admittance of ultrathin Metal-Insulator-Semiconductor (MIS) tunnel junctions is investigated to obtain a reliable energy distribution of the density, D-S(E), of defects localized at the semiconductor interface. The behavior of admittance Y(V, T, omega) and current density J(V, T) characteristics is illustrated by rectifying Hg//C12H25-Si junctions incorporating n-alkyl molecular layers (1.45 nm thick) covalently bonded to n-type Si(111). Modeling the forward bias admittance of a nonequilibrium tunnel junction reveals several regimes which can be observed either in C(omega approximate to 0) vs. (J) plots of the low frequency capacitance over six decades in current or in M ''(omega) plots of the electrical modulus over eight decades in frequency. At low current density, the response of interface states above mid-gap is unaffected and a good agreement is found between the interface states densities derived from the modeling of device response time tau(R)(V) and from the low-high frequency capacitance method valid for thick MIS devices; the low defect density near mid-gap (D-S 1 mA cm(-2)), the admittance depends strongly on both the density of localized states and the dc current density, so that the excess capacitance method overestimates D-S. For very high current densities > 10 mA cm(-2)), the observation of a linear C(omega approximate to 0) vs. (J) dependence could indicate some Fermi level pinning in a high interface density of states located near the Si conduction band. The temperature-independent excess capacitance C(omega approximate to 0) - C(1 MHz) observed at very small J, not predicted by the admittance model, is attributed to some dipolar relaxation in the molecular junction

Barrier height distribution and dipolar relaxation in metal-insulator-semiconductor junctions with molecular insulator: Ageing effects

International audienceElectrical transport through molecular monolayers being very sensitive to disorder effects, admittance and current density characteristics of Hg // C12H25 – n Si junctions incorporating covalently bonded n-alkyl molecular layers, were investigated at low temperature (150-300 K), in the as-grafted state and after ageing at the ambient. This comparison reveals local oxidation effects both at the submicron scale in the effective barrier height distribution and at the molecular scale in the dipolar relaxation. In the bias range dominated by thermionic emission and modified by the tunnel barrier attenuation, , where is the thickness of the molecular tunnel barrier and is the inverse attenuation length at zero applied bias, some excess current is attributed to a distribution of low barrier height patches. Complementary methods are used to analyze the current density J(V, T) characteristics of Metal-Insulator-Semiconductor tunnel diodes. Assuming a Gaussian distribution of barrier heights centered at provides an analytical expression of the effective barrier height, ; this allows fitting of the distribution standard deviation and tunnel parameter over a wide temperature range. In a more realistic modeling including the voltage dependence of barrier height and circular patch area, the so-called “pinch-off” effect is described by a distribution of parameter which combines interface potential modulation and patch area variations. An arbitrary distribution of values, fitted to low-temperature J(V) data, is equally well described by Gaussian or exponential functions. Ageing in air also increases the interface oxidation of Si substrate and affects the density of localized states near mid gap, which typically rises to the high 1011 eV-1.cm-2 range, as compared with < 1011 eV-1.cm-2 in the as-grafted state. The bias-independent relaxation observed near 1 kHz at low temperature may be attributed either to dipoles in the alkyl chain induced by the strong permanent dipoles of interface silicon oxide or to a local relaxation of water molecules trapped at the OML / silicon interface. The respective roles of SiO2 formation and water physisorption on the decrease of patch barrier height are also discussed

New insights into radiative transfer within sea ice derived from autonomous optical propagation measurements

The radiative transfer of shortwave solar radiation through the sea ice cover of the polar oceans is a crucial aspect of energy partitioning at the atmosphere–ice–ocean interface. A detailed understanding of how sunlight is reflected and transmitted by the sea ice cover is needed for an accurate representation of critical processes in climate and ecosystem models, such as the ice–albedo feedback. Due to the challenges associated with ice internal measurements, most information about radiative transfer in sea ice has been gained by optical measurements above and below the sea ice. To improve our understanding of radiative transfer processes within the ice itself, we developed a new kind of instrument equipped with a number of multispectral light sensors that can be frozen into the ice. A first prototype consisting of a 2.3 m long chain of 48 sideward planar irradiance sensors with a vertical spacing of 0.05 m was deployed at the geographic North Pole in late August 2018, providing autonomous, vertically resolved light measurements within the ice cover during the autumn season. Here we present the first results of this instrument, discuss the advantages and application of the prototype, and provide first new insights into the spatiotemporal aspect of radiative transfer within the sea ice itself. In particular, we investigate how measured attenuation coefficients relate to the optical properties of the ice pack and show that sideward planar irradiance measurements are equivalent to measurements of total scalar irradiance

Thermal treatments for biaxially textured Cu-Ni alloys for YBCO coated conductors

We have studied the thermal treatments necessary to texture Cu-Ni alloys in view of applications as substrate for the YBCO coated conductors. Several alloy compositions chosen in order to avoid magnetism of the tapes have been elaborated by the rolling (RABiTS) method. Analyses of the texture as a function of annealing temperature have been done by X-ray and neutron diffractions, pole figures by EBSD and X-rays. The results show that the annealing temperature is dependant of the alloy composition and must be determined precisely for each composition. Good biaxial textures have been obtained on these cheap alloys

Polarization state of the optical near-field

The polarization state of the optical electromagnetic field lying several nanometers above complex dielectric structures reveals the intricate light-matter interaction that occurs in this near-field zone. This information can only be extracted from an analysis of the polarization state of the detected light in the near-field. These polarization states can be calculated by different numerical methods well-suited to near--field optics. In this paper, we apply two different techniques (Localized Green Function Method and Differential Theory of Gratings) to separate each polarisation component associated with both electric and magnetic optical near-fields produced by nanometer sized objects. The analysis is carried out in two stages: in the first stage, we use a simple dipolar model to achieve insight into the physical origin of the near-field polarization state. In the second stage, we calculate accurate numerical field maps, simulating experimental near-field light detection, to supplement the data produced by analytical models. We conclude this study by demonstrating the role played by the near-field polarization in the formation of the local density of states.Comment: 9 pages, 11 figures, accepted for publication in Phys. Rev.

Spectroscopic Ellipsometry Study of Spark Plasma Sintered Nano Silver

International audienceSpark Plasma Sintered (SPS) silver layers were studied by Spectroscopic Ellipsometry (SE) using an M-2000V spectroscopic ellipsometer from J. A. Woollam Co., Inc. which operate in rotating compensator mod

Monitoring water accumulation in a glacier using time lapse magnetic resonance surveys

International audienceSince the catastrophic subglacial lake outburst flood in 1892, the risk of a new event in the glacier of Tête Rousse, in the Alps (close to the Mont Blanc) has been thoroughly studied until now (Vincent et al., 2010, 2012). In the last 5 years, the combination of several geophysical technics has provided valuable input for the glaciologists to better understand the structure and the evolution of sub-glacial liquid water (Garambois et al, 2015). Ground penetrating radar which has proven for long to be a very efficient tool in glacial environment has been used here, providing fine imaging of internal structures, bed rock depth estimate, crevasses and the top of the main cavity. In addition, Magnetic resonance has been performed in 2009, confirming the existence of the liquid water volume, and applied in 2010 along a tight array of loops to provide a 3 D image and an estimate of the total water volume. Indeed, this latter parameter is of major importance to evaluate the level of risk