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On-chip avalanche photodetectors (APDs) are attractive for a sensitive detection of high-speed data light signals with low intensities. Silicon-germanium (Si-Ge) APDs are leading candidates to build reliable short-reach optical links. Appeal for Si-Ge APDs stems from larger (lower) gain-bandwidth (excess noise) at reduced voltages, CMOS-compliant production, and monolithic integration compared to their III-V alternatives [1] - [5] . Typically, Si-Ge APDs are metal-semiconductor-metal (MSM) [3] , separate absorption charge multiplication (SACM) [4] , or p-i-n [5] devices - each of them having their own advantages and drawbacks [1] . Although many Si-Ge APDs have appealing performances on their own, they are usually operated with other electronic components such as amplification stages, impeding low power consumption and low-cost detection on Si chips.

We present our latest advances in the development of compact photodetectors based on p-i-n silicon-germanium-silicon hetero-structures. We demonstrate credible high-speed performances, showing that those devices are likely to become key building blocks in next-generation photonic interconnects.

Experimental demonstration of on-chip two-octave supercontinuum generation in the mid-infrared wavelength, ranging from 3 to 13 µm (larger than 2500 cm-1), by using graded-index SiGe waveguides that allow dispersion tailoring and low propagation losses.

Photonics integration in the mid-Infrared (mid-IR) spectral range, and more specifically the fingerprint region between 5 and 20 μm wavelength has garnered a great interest as it provides an immense potential for applications in spectroscopy and sensing. The unique vibrational and rotational resonances of the molecules at these wavelengths can be exploited for non-intrusive, unambiguous detection of the molecular composition of a broad variety of gases, liquids or solids, with a great interest for many high-impact applications. Fourier-transform spectrometers (FTS) are a particularly interesting solution for the on-chip integration due to their superior robustness against fabrication imperfections. However, the performance of current on-chip FTS implementations is limited by tradeoffs between bandwidth and resolution, for a given footprint. In this work we propose and experimentally demonstrate a new FTS approach that gathers the advantages of spatial heterodyning and optical path tuning by thermo-optic effect. The high resolution is provided by spatial multiplexing among different interferometers with increasing imbalance length, while the broadband operation is enabled by fine sampling interval of the optical path delay in each interferometer harnessing the thermo-optic effect. This novel approach overcomes the bandwidth-resolution tradeoff in conventional counterparts. The fabricated device enables a bandwidth as wide as 603 cm-1 (instead of 74 cm-1 with no-thermal tuning) near 7.7 μm wavelength, keeping a resolution better than 15 cm-1 with the same footprint. This device is fabricated in a Ge-rich graded-index SiGe platform with experimentally proven low loss operation up to 8.5 μm wavelength.

International audience; Although many environments like soils are constantly subjected to invasion by alien microbes, invaders usually fail to succeed, succumbing to the robust diversity often found in nature. So far, only successful invasions have been explored, and it remains unknown to what extent an unsuccessful invasion can impact resident communities. Here we hypothesized that unsuccessful invasions can cause impacts to soil functioning by decreasing the diversity and niche breadth of resident bacterial communities, which could cause shifts to community composition and niche structure\textemdashan effect that is likely exacerbated when diversity is compromised. To examine this question, diversity gradients of soil microbial communities were subjected to invasion by the frequent, yet oft-unsuccessful soil invader, Escherichia coli, and evaluated for changes to diversity, bacterial community composition, niche breadth, and niche structure. Contrary to expectations, diversity and niche breadth increased across treatments upon invasion. Community composition and niche structure were also altered, with shifts of niche structure revealing an escape by the resident community away from the invader's resources. Importantly, the extent of the escape varied in response to the community's diversity, where less diverse communities experienced larger shifts. Thus, although transient and unsuccessful, the invader competed for resources with resident species and caused tangible impacts that modified both the diversity and functioning of resident communities, which can likely generate a legacy effect that influences future invasion attempts.

Optical microresonators with high quality factors are key in photonic circuits requiring fine spectral filtering or resonant storage of optical power. Silicon (Si) photonics provides high-performance optoelectronic circuits but yields planar Si microresonators with rather low quality factors ( Q < 10 5 ). On the other hand, bulk resonators achieve exceptionally high quality factors, Q > 10 7 . Si photonic waveguides and bulk resonators have very different sizes and refractive indices that preclude efficient coupling. Here, we show an efficient method to couple bulk resonators and Si waveguides based on subwavelength metamaterial engineering. We demonstrate up to 99% light coupling efficiency for microspheres and microdisks made of silica, lithium niobate, and calcium fluoride, with 0.3 − 3.6 m m diameter. This achievement could enable the heterogeneous integration of bulk resonators and silicon photonic circuits, with potential applications in sensing, communications, and quantum information.

Air-slot photonic nanocavities have many relevant advantages for hybrid integration of external light emitters/absorbers in Si-based optoelectronic devices. Still an experimental major drawback is the difficulty of performing near-field polarization mapping of the photonic modes localized in air-regions. Here, we demonstrate that near-field resonant transmission can achieve high-fidelity vectorial imaging of the electric-field distribution of a nanocavity mode in the challenging case of a silicon heterostructure photonic-crystal cavity realized in an air-slot waveguide where the mode is localized both in air and dielectric regions. We discern and control the in-plane independent polarization components of the electric field of the photonic mode by exploiting the Fano imaging technique. The use of metallic coated aperture near-field probes allows to map the two polarizations with a deep sub-wavelength imaging ( $\lambda $ /15) resolution.

International audience; Hetero-structured silicon-germanium-silicon photodetectors operating under low-reverse-voltages with high responsivity, fast response, and low dark-current levels are reported. A bit-error-rate of 10-9 is experimentally achieved for conventional data rates of 10, 20, and 25 Gbps, providing optical power sensitivities of-13.9,-12.7, and-11.3 dBm.

Periodically patterning Si with a pitch small enough to suppress diffraction, we synthesize an effective photonic medium with refractive index between those of Si and the cladding material. This technique releases new degrees of freedom in engineering of light-matter interaction, chromatic dispersion and light propagation in Si photonic waveguides. We report our advances in the development of subwavelength engineered structures for on-chip photonics circuits, including Si membrane waveguides for sensing applications in the near- and mid-infrared, and high pump-rejection filters for quantum photonics.

This work explores the use of Ge-rich graded-index Si

We report an efficient energy-time entangled photon-pair source based on four-wave mixing in a CMOS-compatible silicon photonics ring resonator. Thanks to suitable optimization, the source shows a large spectral brightness of 400 pairs of entangled photons /s/MHz for 500 μW pump power, compatible with standard telecom dense wavelength division multiplexers. We demonstrate high-purity energy-time entanglement, i.e., free of photonic noise, with near perfect raw visibilities (98%) between various channel pairs in the telecom C-band. Such a compact source stands as a path towards more complex quantum photonic circuits dedicated to quantum communication systems.

In this work, we experimentally explore the possibility to circumvent some drawbacks of silicon ring-resonator index sensors [1, 2] by a new approach relying on the detection of the micro-ring resonator spectrum overall envelope. In the purpose of strong light-matter interaction, slot waveguides have been considered to operate in the near infra-red [3]. As a proof-of-concept experimental study, hollow core slot waveguide micro-ring resonators have been filled by a series of commercial index liquids (from Cargille) in various configurations [4].

Polarization management is a key functionality in many photonic applications including telecommunications, polarization diversity circuits and sensing, to name a few. Developing integrated circuits capable of reliably controlling polarization state would result in compact and low cost circuits with improved stability, compared with fiber or bulk optics solutions. Polarization rotators are a key building block of these circuits. Unfortunately, stringent fabrication tolerances make the integration of polarization rotators highly challenging. The main limitation arises from the need to tightly control the profile of the hybrid modes in the rotator waveguide as well as their relative phase shift during propagation. Both values are very sensitive to waveguide geometry variations, seriously hindering their practical application. We have developed a technology independent scheme that enables fabrication error compensation, substantially relaxing device tolerances. In our scheme, three polarization rotation waveguides are interconnected with two tunable phase shifters to correct geometry deviations. Interestingly, these phase shifters also enable dynamic wavelength tuning and output polarization extinction ratio selection. We also showed that, by adding an output phase shifter, we can control the relative phase. Hence, we can yield any desired output state of polarization. We have implemented this scheme in the silicon-on-insulator platform using simple waveguide heaters as tunable phase shifters. We experimentally demonstrated an unprecedented polarization extinction range of 40 dB (±20 dB). Furthermore, the device showed a 98% coverage of the Poincaré sphere with a tunability range covering the complete C-band. These results prove the potential of our scheme to alleviate the, otherwise, very stringent fabrication tolerances, overcoming the major limitation of current integrated polarization managing devices., 2016 Progress in Electromagnetic Research Symposium (PIERS), August 8-11, 2016, Shanghai, China

Land-use practices aiming at increasing agro-ecosystem sustainability, e.g. no-till systems and use of temporary grasslands, have been developed in cropping areas, but their environmental benefits could be counterbalanced by increased N2O emissions produced, in particular during denitrification. Modelling denitrification in this context is thus of major importance. However, to what extent can changes in denitrification be predicted by representing the denitrifying community as a black box, i.e. without an adequate representation of the biological characteristics (abundance and composition) of this community, remains unclear. We analysed the effect of changes in land uses on denitrifiers for two different agricultural systems: (i) crop/grassland conversion and (ii) cessation/application of tillage. We surveyed potential denitrification (PD), the abundance and genetic structure of denitrifiers (nitrite reducers), and soil environmental conditions. N2O emissions were also measured during periods of several days on control plots. Time-integrated N2O emissions and PD were well correlated among all control plots. Changes in PD were partly due to changes in denitrifier abundance but were not related to changes in the structure of the denitrifier community. Using multiple regression analysis, we showed that changes in PD were more related to changes in soil environmental conditions than in denitrifier abundance. Soil organic carbon explained 81% of the variance observed for PD at the crop/temporary grassland site, whereas soil organic carbon, water-filled pore space and nitrate explained 92% of PD variance at the till/no-till site, without any residual effect of denitrifier abundance. Soil environmental conditions influenced PD by modifying the specific activity of denitrifiers, and to a lesser extent by promoting a build-up of denitrifiers. Our results show that an accurate simulation of carbon, oxygen and nitrate availability to denitrifiers is more important than an accurate simulation of denitrifier abundance and community structure to adequately understand and predict changes in PD in response to land-use changes.

Despite their role in soil functioning, the ecology of nitrite-oxidizing bacteria, NOB, and their response to disturbances such as those generated by agricultural practices are scarcely known. Over the course of 17 months, we surveyed the potential nitrite oxidation, PNO, the abundance of the Nitrobacter- and Nitrospira-like NOB (by quantitative PCR) and the community structure of the Nitrobacter-like NOB (by PCR-DGGE and cloning-sequencing targeting the nxrA gene) in soils for four treatments: after establishment of tillage on a previously no-tillage system, after cessation of tillage on a previously tillage system, and on control tillage and no-tillage systems. Key soil variables (moisture, organic carbon content and gross mineralization--i.e. ammonification--measured by the 15N dilution technique) were also surveyed. PNO was always higher for the no-tillage than tillage treatments. Establishment of tillage led to a strong and rapid decrease in PNO whereas cessation of tillage did not change PNO even after 17 months. PNO was strongly and positively correlated to the abundance of Nitrobacter-like NOB and was also strongly related to gross mineralization, a proxy of N-availability; in contrast, PNO was weakly and negatively correlated to the abundance of Nitrospira-like NOB. Selection of a dominant population was observed under no-tillage, and PNO was loosely correlated to the community structure of Nitrobacter-like NOB. Our results demonstrate that Nitrobacter-like NOB are the key functional players within the NOB community in soils with high N availability and high activity level, and that changes in PNO are due to shifts between Nitrospira-like and Nitrobacter-like NOB and to a weaker extent by shifts of populations within Nitrobacter-like NOB.

The evolution of silicon optical modulators is recalled, from the first effect demonstrations to the characterization of high-performance devices integrated in optical waveguides. Among possibilities to achieve optical modulation in silicon-based materials, the carrier depletion effect has demonstrated good capacities. Carrier depletion in Si and SiGe/Si structures has been theoretically and experimentally investigated. Large phase modulation efficiency, low optical loss, and large cutoff frequency are obtained by considering simultaneously optical and electrical structure performances. Integrated Mach-Zehnder interferometers and resonators are compared to convert phase modulation into intensity modulation. Finally, recent results on high-speed and low-loss silicon optical modulator using an asymmetric Mach-Zehnder interferometer are presented. It is based on a p-doped slit embedded in the intrinsic region of a lateral pin diode integrated in a silicon-on-insulator waveguide. This design allows a good overlap between the optical mode and carrier density variations. An insertion loss of 5 dB has been measured with a -3 dB bandwidth of 15 GHz.

In a field experiment we have examined the effect of long-term grassland management regimes (viz., intensive versus extensive) and dominant plant species (viz., Arrhenatherum elatius, Holcus lanatus and Dactylis glomerata) on soil organic carbon (SOC) build up, soil microbial communities using biomarker phospholipid fatty acids (PLFA), and the relationship between SOC and PLFAs of major groups of microorganisms (viz., bacteria, fungi, and actinomycetes). The results have revealed that changes in SOC were not significantly affected by the intensity of management or by the plant species composition or by their interaction. The amount of PLFA of each microbial group was affected weakly by management regime and plant species, but the canonical variance analysis (CVA), based on individual PLFA values, demonstrated significant (P < 0.05) effects of management regime and plant species on the composition of microbial community. Positive and significant (P < 0.01) relationships were observed between PLFA of bacteria (R2 = 0.47), fungi (R2 = 0.33), actinomycetes (R2 = 0.71) and total microbial PLFA (R2 = 0.53) and SOC content.

International audience; This paper reports on recent results on the integration of carbon nanotubes on the silicon photonic platform. Light coupling from carbon nanotubes in a fully integrated silicon resonator will be presented.

International audience; Ge/SiGe Quantum well structures have a strong potential to revolutionize silicon photonics. This paper reviews recent works including high speed modulator and photodetector, QW engineering to tune the wavelength and waveguide integration. OCIS codes: (130.3120) Integrated optics devices; (230.4205) Multiple quantum well (MQW) modulators; (130.0250) Optoelectronics.

This paper reports modeling and simulation of low-loss coupling between polarization-insensitive silicon-on-insulator (SOI) microwaveguides and single-mode optical fiber, using a two-dimensional taper. The proposed structure is a reduction of both sizes of a square-strip waveguide from a 350/spl times/350-nm/sup 2/ cross section to about 155/spl times/155-nm/sup 2/ cross section. At the output facet, the guided mode is spread out and has a non-Gaussian profile, but low-loss coupling can be achieved by optimizing the overlap integral between the submicrometer waveguide mode and the optical fiber one. The calculated coupling losses are lower than 0.2 dB, the alignment tolerance is larger than 4 /spl mu/m at /spl plusmn/1 dB for the coupling losses, and the total length is about 10 /spl mu/m.

It has been suggested that grazing could stimulate the aboveground dry mass and/or nitrogen yields available to grazers (i.e., the grazing optimization hypothesis, GOH), but the actual importance of this effect is still controversial. The GOH has only been tested for a few grasslands and one savanna ecosystem, but not for the moistest and warmest grasslands and savannas of the world. The objectives of this study were to test the GOH in the humid savanna of Lamto (West Africa) by analyzing the growth of grasses in response to a field trial with three levels of clipping × two levels of fertilization. We quantified the effects of clipping and fertilization on the dry matter and nitrogen yields to grazers (i.e., mass or nitrogen amount in clipped-off tissues during the experiment) and on the remaining yield (i.e., mass or nitrogen amount in residual phytomass at the end of the experiment) over a three-month period. Total phytomass yield, the sum of yield to grazers, and the remaining yield was maintained under moderate clipping frequency and fertilization as compared to control conditions. Both clipping frequencies decreased the remaining phytomass yield as compared to control plots. Clipping frequency significantly increased nitrogen concentrations in the total yield, in the remaining yield, and in the yield to grazers. Total nitrogen yield and nitrogen yield to grazers were 65% and 91% higher on the plots experiencing moderate clipping frequency with fertilization as compared to control plots. The study shows that grazers in this humid savanna system could potentially modify ecosystem processes in such a way as to partly alleviate nutritional deficiencies, but only in the presence of increased nitrogen availability.

Summary 1The rate of CO2 emission by two major termite species (Ancistrotermes cavithorax and Odontotermes n. pauperans) was studied in a West African savanna (Lamto, Cote d’Ivoire). First, in three major savanna types (grassy, shrubby and woody savannas), CO2 emission from the soil surface was measured using a closed container system. Control soil without termite fungus-comb chambers, and soil of eroded termite mound with or without Odontotermes fungus-comb chambers was sampled in each savanna type. Second, the mass-specific respiration rate of the different components of termite fungus-comb chambers (i.e. workers, soldiers, fungus comb and chamber walls) was measured under laboratory conditions. CO2 emission by termites at the landscape-scale was computed from both field biomass data and laboratory measurements. 2Whatever the savanna type, CO2 emission from the soil surface was not different between control soil and soil of eroded termite mound without termite fungus-comb chambers, but was significantly higher in areas with fungus-comb chambers than in areas without fungus-comb chambers (10–19 µmol CO2 m−2 s−1vs 5–10 µmol CO2 m−2 s−1). 3The mass-specific respiration rates were higher for individuals of O. pauperans than for individuals of A. cavithorax. Total respiration rate from an individual fungus-comb chamber was around 56 and 143 µmol CO2 h−1 for Ancistrotermes and Odontotermes, respectively. 4Despite a low mass-specific respiration rate, fungus comb accounted for 51% of the total respiration flux from whole chambers in Odontotermes and for 82% in Ancistrotermes. The laboratory-derived respiration rate from individual Odontotermes chambers was consistent with the field estimates. 5At the landscape-scale, the CO2 emission due to A. cavithorax and O. pauperans was 0·022 and 0·050 µmol CO2 m−2 s−1, respectively. This total (27·2 g C m−2 years−1) represented 4·9% of the total above-ground net primary production in this ecosystem and 11·3% of the carbon not mineralized by annual fires.

The temperature dependence of C 3 photosynthesis is known to vary with growth environment and with species. In an attempt to quantify this variability, a commonly used biochemically based photosynthesis model was parameterized from 19 gas exchange studies on tree and crop species. The parameter values obtained described the shape and amplitude of the temperature responses of the maximum rate of Rubisco activity ( V cmax ) and the potential rate of electron transport ( J max ). Original data sets were used for this review, as it is shown that derived values of V cmax and its temperature response depend strongly on assumptions made in derivation. Values of J max and V cmax at 25 ° C varied considerably among species but were strongly correlated, with an average J max : V cmax ratio of 1·67. Two species grown in cold climates, however, had lower ratios. In all studies, the J max : V cmax ratio declined strongly with measurement temperature. The relative temperature responses of J max and V cmax were relatively constant among tree species. Activation energies averaged 50 kJ mol − 1 for J max and 65 kJ mol − − − 1 for V cmax , and for most species temperature optima averaged 33 ° ° ° C for J max and 40 ° C for V cmax . However, the cold climate tree species had low temperature optima for both J max ( 19 ° C) and V cmax (29 ° ° ° C), suggesting acclimation of both processes to growth temperature. Crop species had somewhat different temperature responses, with higher activation energies for both J max and V cmax , implying narrower peaks in the temperature response for these species. The results thus suggest that both growth environment and

A comb slot photonic crystal waveguide geometry allowing flexible dispersion engineering of wide slot photonic crystal waveguides is proposed. Additionally, a figure of merit combining confinement within the slot and high group index is introduced to quantify the electromagnetic density within slow wave slot waveguides, and is applied to the proposed waveguide geometries. Group indices exceeding 130 and 90 over bandwidths of 1.5 and 2 nm are achieved for slots wider than 300 nm. The figure of merit exhibits sevenfold and fortyfold enhancements in comparison with classical slot waveguides with slot widths around 50 and 100 nm, respectively.

The silicon photonic wire evanescent field (PWEF) sensor platform offers the advantages of small sensor size, high levels of function integration, and low cost manufacturing that comes with the use of established semiconductor fabrication processes. The technology should be fully compatible with existing infrastructure in molecular analysis and research and the manufactured sensor array chip cost should be low enough that the chips can be considered disposable. Furthermore, since many applications require simultaneous monitoring of many different simultaneous binding reactions, the possibility of integrating tens or even hundreds of independent molecular sensors on a single disposable sensor chip is very compelling. We present an overview of our multiplexed photonic wire sensor chip and a reader instrument that allows up to 128 independent binding reactions to be monitored simultaneously. A complete photonic wire molecular biosensor microarray chip architecture and supporting instrumentation is discussed. This microarray system is used to demonstrate a multiplexed assay for serotyping E. coli bacteria based on polyclonal antibody probe molecules. A coherent detection scheme that enables direct read-out of the optical phase and an order of magnitude enhancement of sensitivity compared to conventional detection is also discussed. Finally, we present advances in Fourier-transform interferometer arrays for spectroscopic sensing. A planar waveguide Fourier-transform spectrometer with densely arrayed Mach-Zehnder interferometers is presented. Subwavelength gratings are used to produce an optical path difference without waveguide bends. The fabricated device comprises of an array of 32 Mach-Zehnder interferometers, which produce a spatial interferogram without any moving parts, yielding a spectral resolution of 50 pm and a free-spectral range of 0.78 nm. As a result of similar propagation losses in subwavelength grating waveguides and conventional strip waveguides, loss imbalance is minimized and high interferometric extinction ratio of -25 to -30 dB is obtained. Furthermore, phase and amplitude errors arising from normal fabrication variation are compensated by the spectral retrieval process using calibration measurements., 2014 16th International Conference on Transparent Optical Networks (ICTON), 6-10 July 2014, Graz, Austria

Comprehensive studies on the processes involved in photosynthetic acclimation after a sudden change in light regime are scarce, particularly for trees. We tested (i) the ability of photosynthetic acclimation in the foliage of walnut trees growing outdoors after low-to-high and high-to-low light transfers made early or late in the vegetation cycle, and (ii) the relative importance of changes in total leaf nitrogen versus changes in the partitioning of leaf nitrogen between the different photosynthetic functions during a 2 month period after transfer. Changes in maximum carboxylation rate, light-saturated electron transport rate, respiration rate, total leaf nitrogen, ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco) and total chlorophylls were surveyed before and after the change in light regime. Respiration rate acclimated fully within 1 week of transfer, and full acclimation was observed 1 month after transfer for the amount of Rubisco. In contrast, total nitrogen and photosynthetic capacity acclimated only partially during the 2 mouth period. Changes in photosynthetic capacity i were driven by changes in both total leaf nitrogen and leaf nitrogen partitioning. The extent of acclimation also depended strongly on leaf age at the time of the change in light regime.

The spatial variations in the stable carbon isotope composition ( δ 13 C) of air and leaves (total matter and soluble sugars) were quantified within the crown of a well-watered, 20year-old walnut tree growing in a low-density orchard. The observed leaf carbon isotope discrimination ( Δ ) was compared with that computed by a three-dimensional model simulating the intracanopy distribution of irradiance, transpiration and photosynthesis (previously parameterized and tested for the same tree canopy) coupled to a biophysically based model of carbon isotope discrimination. The importance of discrimination associated with CO 2 gradients encountered from the substomatal sites to the carboxylation sites was evaluated. We also assessed by simulation the effect of current irradiance on leaf gas exchange and the effect of long-term acclimation of photosynthetic capacity and stomatal and internal conductances to light regime on intracanopy gradients in Δ . The main conclusions of this study are: (i) leaf Δ Δ can exhibit important variations (5 and 8‰ in total leaf material and soluble sugars, respectively) along light gradients within the foliage of an isolated tree; (ii) internal conductance must be taken into account to adequately predict leaf Δ , and (iii) the spatial variations in Δ Δ and water-use efficiency resulted from the short-term response of leaf gas exchange to variations in local irradiance and, to a much lesser extent, from the long-term acclimation of leaf characteristics to the local light regime.

The model RATP (radiation absorption, transpiration and photosynthesis) is presented. The model was designed to simulate the spatial distribution of radiation and leaf-gas exchanges within vegetation canopies as a function of canopy structure, canopy microclimate within the canopy and physical and physiological leaf properties. The model uses a three-dimensional (3D) representation of the canopy (i.e. an array of 3D cells, each characterized by a leaf area density). Radiation transfer is computed by a turbid medium analogy, transpiration by the leaf energy budget approach, and photosynthesis by the Farquhar model, each applied for sunlit and shaded leaves at the individual 3D cell-scale. The model typically operates at a 20‐30 min time step. The RATP model was applied to an isolated, 20-yearold walnut tree grown in the field. The spatial distribution of wind speed, stomatal response to environmental variables, and light acclimation of leaf photosynthetic properties were taken into account. Model outputs were compared with data acquired in the field. The model was shown to simulate satisfactorily the intracrown distribution of radiation regime, transpiration and photosynthetic rates, at shoot or branch scales.

In situ measurements and model simulations were used to analyse the spatial variations of wind speed (U) and leaf boundary layer conductance (g H ), and their effects on leaf transpiration and photosynthesis within the crown of a 20-year-old walnut tree (Juglans regia L.). Wind speed attenuation within the tree crown was strongly correlated to the cumulative leaf area along the wind path into the crown, as deduced from the 3D distribution of the tree foliage, obtained by the combined use of digitizing and allometric relationships. Simulations by the RATP model, i.e., a 3D numerical model of radiation transfer, leaf transpiration and photosynthesis (Sinoquet et al., 1999), were used (i) to study the spatial variations of transpiration, photosynthesis and leaf-to-atmosphere coupling within the tree crown; and (ii) to analyse coupling at tree level, in order to evaluate the sensitivity of total transpiration to climate variations. The main results of the simulations were: (i) spatial variations of stomatal conductance were mainly responsible for the important spatial variation of the leaf-to-air decoupling factor ( ) within the tree crown; (ii) high coupling was observed at tree level, so that whole tree transpiration was primarily controlled by air VPD and secondarily by radiation; (iii) wind velocity and direction influenced only weakly the local transpiration, and had no effect on photosynthetic rates. ©1999 Elsevier Science B.V. All rights reserved.

Termitaria are major sites of functional heterogeneity in tropical ecosystems, through their strong influence on soil characteristics, in particular soil physico-chemical properties and water status. These factors have important consequences on nutrient availability for plants, plant spatial distribution, and vegetation dynamics. However, comprehensive information about the influence of termite-rehandled soil on soil water regime is lacking. In a humid shrubby savanna, we characterized the spatial variations in soil texture, soil structure and maximum soil water content available for plants (AWC max) induced by a large termite mound, at three deepths (0–0.10, 0.20–0.30 and 0.50–0.60 m). In addition, during a three month period at the end of the rainy season, soil water potential was surveyed by matrix sensors located on the termite mound and in the surrounding soil at the same depths and for the 80–90 cm layer. Concurrently, the leaf shedding patterns of two coexisting deciduous shrub species exhibiting contrasted soil water uptake patterns were compared for individuals located on termite mounds and in undisturbed control areas. For all the soil layers studied, clay and silt contents were higher for the mound soil. Total soil clods porosity was higher on the mound than in control areas, particularly in the 0.20-0.60 m layer, and mound soil exhibited a high shrinking/swelling capacity. AWCmax of the 0-0.60 m soil layer was substantially higher on the termite mound (112 mm) than in the surroundings (84 mm). Furthermore, during the beginning of the dry season, soil water potential measured in situ for the 0.20-0.90 m soil layer was higher on the mound than in the control soil. In contrast, soil water potential of the 0-0.10 m soil layer was similar on the mound and in the control soil. In the middle of the dry season, the leaf shedding pattern of Crossopteryx febrifuga shrubs (which have limited access to soil layers below 0.60 m) located on mounds was less pronounced than that of individuals located on control soil. In contrast, the leaf shedding pattern of the shrub Cussonia barteri (which has a good access to deep soil layers) was not influenced by the termite mound. We conclude that in this savanna ecosystem, termite mounds appear as peculiar sites which exhibit improved soil water availability for plants in upper soil layers, and significantly influence aspects of plant function. Implications of these results for understanding and modelling savanna function and dynamics, and particularly competitive interactions between plant species, are discussed.

Semiconducting SWNT extraction was studied using an ultra-centrifugation method assisted by a conjugated polymer. It was demonstrated that the emission intensity can be highly improved and that in some conditions emission from one single nanotube chirality can be achieved. This optimized material was integrated on several photonic structures. The ability of nanotubes to emit light when drop-casted on a silicon waveguide was first demonstrated and its thermal stability was further investigated. We concluded over interesting integration ability on Silicon-On-Insulator (SOI) substrate, with coupling efficiency up to the order of 10%. The integration of SWNT on Bragg mirrors based cavity was also investigated. This is the first milestone towards a carbon nanotube based fully integrated LASER.

This chapter describes how the semiconducting-single-wall nanotube (SWNT) extraction affects their luminescence properties and how such properties can be exploited to fabricate optical sources emitting in near-infrared wavelengths ranging from 1 to 2 μm. The main applications of carbon nanotube (CNT) lasers are aimed at optical interconnects and biophotonics. We review recent achievements in obtaining optical gain in thin films doped with CNTs. Techniques for determining optical gain in CNTs are considered with particular focus on the variable strip length (VSL) method and choices to integrate nanotubes in the optical waveguide are discussed.

In this paper, we first review and compare the two main techniques allowing to purify and extract selectively semiconducting single-walled carbon nanotubes (s-SWNT)). These purification steps are essential to obtain an optical-quality material. Such material is then suitable for optical applications and photonic devices. We present two major advances in carbon nanotubes optics and photonics: on one hand, the demonstration of optical gain in s-swnt by two independent methods, with modal gain as high as 160 ±10 cm -1 at 1.3 μm and on the other hand, the integration of SWNT in silicon-on-insulator (SOI) platform as a potential material for integrated photonic devices. Overall coupling efficiency could be estimated up to 10 -1 .

Leaf and grass canopy photosynthetic rates were measured in a West African humid savanna during several stages of the early growing season. The results obtained on the dominant grass speciesHyparrhenia diplandraand data published previously show that C4savanna grasses exhibit a remarkably high leaf photosynthetic capacity despite their low nitrogen content. A variation of leaf photosynthetic capacity in relation to leaf rank on stems is observed which is interpreted by ageing and shading effects within the canopy. Seasonal variations of the canopy CO2assimilation rate is explained in relation to variations of leaf area index and canopy nitrogen content. Despite low nitrogen content or low leaf area index, maximum canopy net photosynthesis was high (24 μmol CO2m-3s-1for LAI = 1.5). The high photosynthetic nitrogen use efficiency exhibited by leaves of humid savanna grass species is a major attribute explaining high photosynthetic rates of the grass canopy in this environment. This result sustains the emerging opinion that tropical savannas could be highly productive despite the generally low nutrient status they experience.

A 90 µm long 23 GHz Ge/SiGe modulator using the QCSE in waveguide configuration was demonstrated with an ER higher than 10 dB for a wide spectral range. Energy consumption was only 108 fJ/bit.

Room temperature direct gap electroluminescence (EL) from Ge quantum well waveguides is reported. The excitonic transition and dependence of EL on injection currents and temperature are observed. EL is shown to have a TE polarization.

A 23 GHz Ge/SiGe multiple quantum well electro-absorption waveguide modulator is demonstrated with 10 dB extinction ratio (ER). 9 dB ER is achieved with 1V swing with energy consumption limited to 108 fJ per bit.

Semiconducting single wall carbon nanotubes (s-SWNTs) have generated a growing interest for several years due to their extraordinary optical properties. A strong enhancement of the photoluminescence properties has been obtained thanks to the extraction of s-SWNTs. These advances led to the first demonstration of optical gain in carbon nanotubes and are a precursor to obtain nanotube-based laser. Finally, we will present the integration of s-SWNT in silicon photonic structures, and experimentally demonstrate light emission in silicon waveguides. These results constitute a significant milestone towards the development of carbon nanotube based laser sources in silicon.

Silicon photonics has generated a growing interest in the last years. An overview of the field will be presented, highlighting the main results obtained about optical modulators and germanium photodetectors integrated in silicon on insulator waveguides operating around 1.55 µm.

An overview of integrated silicon-based optical modulators and germanium on silicon photodetectors for optical interconnects in CMOS circuits and high frequency integrated transceivers are presented.

A 3 dB bandwidth of 35 GHz at 1.31 /spl mu/m and 1.55 /spl mu/m wavelengths is reported for inter-digited germanium on silicon-on-insulator metal-semiconductor-metal photodetector with finger spacing of 500 nm.

Photosynthetic light acclimation of leaves can result from (i) changes in mass-based leaf nitrogen concentration, Nm, (ii) changes in leaf mass:area ratio, Ma, and (iii) partitioning of total leaf nitrogen among different pools of the photosynthetic machinery. We studied variations in Nm and Ma within the crowns of two peach (Prunus persica L. Batsch) trees grown in an orchard in Portugal, and one peach tree grown in an orchard in France. Each crown was digitized and a 3-D radiation transfer model was used to quantify the intra-crown variations in time-integrated leaf irradiance,PARi. Nitrogen concentration, leaf mass:area ratio, chlorophyll concentration, and photosynthetic capacity were also measured on leaves sampled on five additional peach trees in the orchard in Portugal. The data were used to compute the coefficients of leaf nitrogen partitioning among carboxylation, bioenergetics, and light harvesting pools. Leaf mass:area ratio and area-based leaf nitrogen concentration, Na, were nonlinearly related toPARi, and photosynthetic capacity was linearly related to Na. Photosynthetic light acclimation resulted mainly from changes in Ma and leaf nitrogen partitioning, and to a lesser extent from changes in Nm. This behavior contrasts with photosynthetic light acclimation observed in other tree species like walnut (Juglans regia L.) in which acclimation results primarily from changes in Ma.

We demonstrate high-speed silicon modulators based on carrier depletion in interleaved pn junctions fabricated on 300 mm-SOI wafers using CMOS foundry facilities. 950 µm-long Mach Zehnder (MZ) and ring resonator (RR) modulator with a 100 µm radius, were designed, fabricated and characterized. 40 Gbit/s data transmission has been demonstrated for both devices. The MZ modulator exhibited a high extinction ratio of 7.9 dB with only 4 dB on-chip losses at the operating point.

Single crystal silicon nanowires with a width down to 25?nm and a length-to-width (L/w) aspect ratio up to 177 were fabricated by surface micromachining of thin SOI wafers. It is demonstrated that these top-down fabricated clamped-clamped nanowires are laterally buckled when L/w is larger than about 44. This is attributed to an unexpected high compressive residual pre-stress estimated to be in the??270 to??335 MPa range from a simple nonlinear post-buckling model. The origin of this stress is investigated by considering several axial stress generation mechanisms in the silicon nanowires such as thermomechanical stresses, surface layers and deformation stresses of dies induced by patterning or die attachment. It is shown that none of these mechanisms can generate the observed initial compressive stress and high temperature steps during SOI wafers fabrication and/or thinning are likely to be the main cause of high compressive stress generation. Possible occurrence of a large stress in top-down fabricated Si nanowires is often ignored and must be considered in future works notably for electrical and thermal transport investigations.