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International audience; We evaluated whether and how rapidly temperate forest tree species able to stimulate or inhibit nitrification (through biological nitrification inhibition, BNI) also influence denitrifier activity and abundance in soil and identified the main determinants of changes in denitrification. A reciprocal soil core transfer approach was implemented at a long-term experimental site between Douglas fir, a species assumed to stimulate nitrification, and stands of tree species with (spruce and Nordmann fir) or without (Corsican pine and beech) BNI capacity. Sixteen months after soil transfer, potential denitrification decreased in soils transferred from Douglas fir to another tree species and increased in soil cores transferred from any species to Douglas fir. The change in denitrification was not related to the BNI capacity of the tree species 'receiving' the soil cores. Structural equation modelling revealed that in soil cores transferred between Douglas fir and BNI species, changes in denitrification were mainly correlated to nitrifier abundances, i.e. AOA and Nitrobacter, nitrification rates, and soil NO3- availability, whereas in soil cores transferred between Douglas fir and non-BNI species denitrification changes were well correlated to C availability and the abundance of nirK-harbouring denitrifiers. Overall, our results indicate that denitrification rates can change strongly and quickly following soil transfers between tree stands. Particularly, when Douglas fir replaces BNI tree species, this quickly results in both increased nitrification and denitrification rates, thus exacerbating the ensuing risk of increased NO3- leaching and N2O emission from soil.

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.

International audience; Some forest tree species are able to carry out a process known as biological nitrification inhibition, BNI, i.e. they inhibit nitrifiers through the production of specific compounds. We tested the hypothesis that, by restricting N supply to NO2-- and N2O--reducers, BNI would decrease potential N2O production and consumption and in situ N2O emissions. as compared to soils under trees without BNI capacity. Soils were collected from long-term monocultures (>43 ys) of three tree species without BNI capacity (Fagus sylvatica, Pinus nigra and Pseudotsuga menziesii) and two tree species with BNI capacity (Abies nordmanniana and Picea abies). The level of limitation of denitrification by NO3 was high for species with BNI capacity and low for species without BNI capacity, and was correlated with potential nitrification rates and the abundances of genes specifically harboured by ammonia oxidizing archaea and Nitrobacter. However, potential denitrification and actual N2O emissions did not reflect the tree BNI status, and denitrification limitation by soil carbon was higher than limitation by N under three tree species. Structural equation modelling revealed that the ratio between the gene copy abundances of nitritereducers and N2O-reducers was the microbial variable that best explained N2O emissions, along with soil pH and moisture. In addition, the NO3- concentration in the soil solution at 60 cm depth increased with the potential nitrification-to-denitrification ratio, suggesting a higher risk of NO3 leaching under some tree species like Douglas fir.

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.

A two-octave spanning supercontinuum generation in the O-band communication window on an integrated silicon nitride platform is reported. The nitrogen-rich silicon nitride waveguides were fabricated through low temperature processes on an industrial platform (

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.

Silicon photonic devices need to be complemented with other materials for the realization of optically active devices. The unique emission properties of single-walled carbon nanotubes are especially promising for efficient light emitters realization in the telecommunication wavelength ranges and for their integration in silicon photonic devices. However one of the problem of carbon nanotubes is their low quantum efficiency. One of the approach to address this problem is to couple the carbon nanotube emission with micro-resonators in order to the enhance their radiative emission. Here we report on the fabrication and optical characterization of a silicon micro-slot-ring resonator coupled with selected single walled semiconducting carbon nanotubes for high photoluminescence emission in the O band telecommunication wavelength. Hyperspectral photoluminescence maps show spectra with sharp resonances superimposed to the nanotube emission bands which demonstrate the coupling of the nanotubes with the evanescent tails in air of the electric field localized in the photonic modes of the micro-resonator. The enhancement of the photoluminescence is achieved over three different CNTs chirality for more than 30 sharp resonances in a single microring resonator. This work paves the way for the development of integrated light sources based on carbon nanotubes in silicon photonic devices.

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.

Uniform grating couplers based on versatile L-shaped waveguides are experimentally demonstrated, with coupling loss of -2.7 dB and back-reflections of -20 dB. Apodized couplers with subwavelength-grating metamaterials predict improved fiber-chip coupling down to -0.46 dB within device layouts compatible with lithographic technologies available in nanophotonic foundries., 2019 IEEE 16th International Conference on Group IV Photonics (GFP), August 28-30, 2019, Singapore, Singapore

International audience; Mid-infrared racetrack resonators are demonstrated working at 8µm wavelength. The devices are based on a graded SiGe platform providing low propagation loss on a large wavelength range in the mid-IR. Different resonators designs have been fabricated, with varying gap distances in the directional coupler. Q factors of more than 3000 have been experimentally demonstrated. These results pave the way towards compact mid-IR sensors or efficient active devices.

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.

International audience; In this paper, the recent progress on a new Ge-rich SiGe platform for mid-IR integrated photonics is presented. Low-loss spiral waveguides working over a broadband wavelength range are discussed, followed by a sensing proof-of-concept using a standalone photoresist with a known spectral absorption pattern. In addition, the development of new mid-IR interferometric devices for wavelength filtering and enhancement of the light-matter interaction are presented. Finally, efficient designs to exploit the third-order nonlinearities in these Ge-rich SiGe waveguides at mid-IR wavelengths are shown. The demonstration of these key building blocks will pave the way towards the implementation of new mid-IR photonic integrated systems with multiple functionalities.

Mid-infrared (mid-IR) silicon photonics is becoming a prominent research with remarkable potential in several applications such as in early medical diagnosis, safe communications, imaging, food safety and many more. In the quest for the best material platform to develop new photonic systems, Si and Ge depart with a notable advantage over other materials due to the high processing maturity accomplished during the last part of the 20th century through the deployment of the CMOS technology. From an optical viewpoint, combining Si with Ge to obtain SiGe alloys with controlled stoichiometry is also of interest for the photonic community since permits to increase the effective refractive index and the nonlinear parameter, providing a fascinating playground to exploit nonlinear effects. Furthermore, using Ge-rich SiGe gives access to a range of deep mid-IR wavelengths otherwise inaccessible (λ ~2-20 μm). In this paper, we explore for the first time the limits of this approach by measuring the spectral loss characteristic over a broadband wavelength range spanning from λ = 5.5 μm to 8.5 μm. Three different SiGe waveguide platforms are compared, each one showing higher compactness than the preceding through the engineering of the vertical Ge profile, giving rise to different confinement characteristics to the propagating modes. A flat propagation loss characteristic of 2-3 dB/cm over the entire wavelength span is demonstrated in Ge-rich graded-index SiGe waveguides of only 6 μm thick. Also, the role of the overlap fraction of the confined optical mode with the Si-rich area at the bottom side of the epitaxial SiGe waveguide is put in perspective, revealing a lossy characteristic compared to the other designs were the optical mode is located in the Ge-rich area at the top of the waveguide uniquely. These Ge-rich graded-index SiGe waveguides may pave the way towards a new generation of photonic integrated circuits operating at deep mid-IR wavelengths.

International audience; Mid-infrared (mid-IR) silicon photonics is expected to lead key advances in different areas including spectroscopy, remote sensing, nonlinear optics or free-space communications, among others. Still, the inherent limitations of the silicon-on-insulator (SOI) technology, namely the early mid-IR absorption of silicon oxide and silicon at λ~3.6 µm and at λ ~8.5 µm respectively, remain the main stumbling blocks that prevent this platform to fully exploit the mid-IR spectrum (λ ~2-20 µm). Here, we propose using a compact Ge-rich graded-index Si1-xGex platform to overcome this constraint. A flat propagation loss characteristic as low as 2-3 dB/cm over a wavelength span from λ = 5.5 µm to 8.5 µm is demonstrated in Ge-rich Si1-xGex waveguides of only 6 µm thick. The comparison of three different waveguides design with different vertical index profiles demonstrates the benefit of reducing the fraction of the guided mode that overlaps with the Si substrate to obtain such flat low loss behavior. Such Ge-rich Si1-xGex platforms may open the route towards the implementation of mid-IR photonic integrated circuits with low-loss beyond the Si multi-phonon absorption band onset, hence truly exploiting the full Ge transparency window up to λ ~15 µm.

Recent works towards the development of Ge-rich SiGe photonic integrated circuits will be presented, such as the demonstration of low-loss waveguides and ultra-wideband Mach Zehnder interferometer from 5.5 to 8.6 μm wavelength, as well as the first steps towards the realization of efficient wideband optical sources.

Temporary (ley) grassland introduced into cropping cycles has been advocated as being beneficial for the delivery of ecosystem services by agricultural soils. The management of these temporary grasslands has unknown effects on soil organic matter (SOM) concentrations and biogeochemical properties of the cropland soils following the grassland phase. Here, we investigated the legacy effect of differently managed temporary grasslands, i.e. change of soil properties lasting beyond three years of crop. We assessed soil organic carbon (SOC) quantity and SOM biogeochemical signature (composition of soil neutral carbohydrates and lignin), as well as microbial activities (potential C and N mineralization and denitrification). We used a long-term field experiment on Cambisol with temperate climate in western France, where temporary grassland management practices differed in terms of duration (3 or 6 years) and presence or absence of N fertilisation. Topsoil (10 cm) samples were collected after a 3-yr crop rotation (maize, wheat, barley). Our results showed that N fertilisation during the grassland phase was necessary to maintain soil C and N concentrations beyond three years of crop. Temporary grassland management may affect microbial activities as indicated by contrasting polysaccharide and lignin composition. It had however, no effect on potential CO2 and N2O emissions during laboratory incubations. The biogeochemical signature of SOM was close to continuous grassland only in treatments with 6 yrs of fertilized temporary grassland. We thus, conclude that the legacy effects of a grassland phase on SOC quantity and properties of SOM depend on its management.

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

International audience; This work explores the use of Ge-rich graded-index Si 1-x Ge x rib waveguides as building blocks to develop integrated nonlinear optical devices for broadband operation in the mid-IR. The vertical Ge gradient concentration in the waveguide core renders unique properties to the guided optical mode, providing tight mode confinement over a broadband mid-IR wavelength range from λ = 3 µm to 8 µm. Additionally, the gradual vertical confinement pulls the optical mode upwards in the waveguide core, overlapping with the Ge-rich area where the nonlinear refractive index is larger. Moreover, the Ge-rich graded-index Si 1-x Ge x waveguides allow efficient tailoring of the chromatic dispersion curves, achieving flat anomalous dispersion for the quasi-TM optical mode with D ≤ 14 ps/nm/km over a ~ 1.4 octave span while retaining an optimum third-order nonlinear parameter, γ eff. These results confirm the potential of Ge-rich graded-index Si 1-x Ge x waveguides as an attractive platform to develop mid-IR nonlinear approaches requiring broadband dispersion engineering.

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.

International audience; 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. Additionally, the resonator has been engineered so as to generate a frequency comb structure 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.

International audience; Optical interconnects are expected to overcome the copper link bottlenecks. Amongthe different developed technologies, silicon photonics becomes one of the preferred solutions. Indeed,due to its compatibility with the CMOS platform, silicon photonics is considered as aviable solution for the development of the future generation photonic interconnects. However,the on-chip integration of all photonic and optoelectronic building blocks (sources, modulatorsand detectors. . . ) on the silicon platform is very complex and is not cost-effective due to thevarious materials used: Ge for detection, Si for waveguiding and modulators and III-V for lasing.We propose here to use carbon nanotubes (CNTs) integrated into silicon photonics for the developmentof all optoelectronic devices. Indeed, CNTs are nanomaterials of particular interest inphotonics thanks to their fundamental optical properties including near-IR luminescence, Starkeffect, Kerr effect and absorption. In consequence, CNTs have the ability to emit, modulateand detect light in the wavelength range of the silicon transparency. Furthermore, they are alsoconsidered as an electronic compatible material thanks to the recent advances in nanoelectronicsbased on CNTs.Here, we report on the study of the light emission coupling from CNTs into optical resonatorsimplemented on the silicon-on-insulator (SOI) platform including ring resonators, nano-beamcavities, micro-disks. . . . A theoretical and experimental analysis of the light interaction of CNTswith micro-ring resonators based on slot waveguides and slot photonic crystal heterostructurecavities will be presented.

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

International audience; Silicon is the mainstream material in the electronic industry and it is rapidly expanding its dominance into the field of photonics. Indeed, silicon photonics has been the subject of intense research activities to pave the way for next generation of energy-efficient high-speed computing, information processing and communications systems. The trend is to use optics in intimate proximity to the electronic circuit, which implies a high level of optoelectronic integration. Over the last decade, the field of silicon photonics has advanced at a remarkable pace. Recent advances in integrated germanium receiver will be presented including the developments of a new Ge-based detector integration and avalanche Ge structures.

We report our recent advances in development of subwavelength engineered dielectric metamaterial structures for integrated photonics., 18th International Conference on Transparent Optical Networks (ICTON), July 10 -14, 2016, Trento, Italy

Agricultural lands are subject to changes in use for a variety of environmental and economic reasons. Among these changes, shifts between crops and grasslands are particularly common. Understanding the effects of these changes on soil functioning is crucial to anticipating crop productivity and environmental impacts and yet has rarely been studied. The purpose of this study was to investigate the in situ temporal dynamics of soil N cycling and C pools after grassland/cropland conversions and risks of N losses from such systems. The experiment was conducted in western France at a long-term experimental site with a temperate oceanic climate. We followed soil N cycling and C pools for four experimental treatments over 36 months: (i) continuous grassland, (ii) continuous rotation of annual crops, (iii) conversion from grassland to a rotation of annual crops, and (iv) conversion from a rotation of annual crops to grassland. Gross N mineralization, potential N immobilization and potential nitrification were estimated using the 15 N dilution/enrichment technique and FLUAZ model, based on samples from the 0–10 cm soil layer (0, 0.75, 3, 12, 24 and 36 months after change in land use) and 10–30 cm soil layer (after 24 and 36 months) in all 4 treatments. Nitrogen pools (NH 4 + , NO 3 − , and organic N), total carbon (C), dissolved organic C, microbial biomass C, and C mineralization were also determined on all soil samples. The continuous grassland soil had rates of N mineralization and N immobilization twice as high as a rotation of annual crops, as well as a slightly lower rate of nitrification and a higher organic C content. Ploughing the grassland led to a rapid shift of soil organic matter pools, N fluxes and microbial activities towards characteristics of the cropland. These effects were attributed to cultivation mixing the soil layers. In contrast, the restoration of grassland on soil previously cropped with annual species did not significantly change soil N or C characteristics within 24 months although changes began to appear after 36 months. This time lag corresponded to the time needed for the complete establishment of root systems for the grasses. Therefore we found a strong asymmetry in the patterns of change in gross N fluxes and C pools between the two conversions. Overall, we found that responses in N fluxes were strongly correlated to soil C pools, except for nitrification. The ratio of potential nitrification to gross ammonium immobilization, which indicates the risk of N losses in agricultural systems, was greater in annual crop rotation than grassland. This indicates that adding grasslands into a rotation of annual crops does not necessarily reduce the risk of N loss within 2 years but does reduce the risk after 3 years.

International audience; Silicon photonics, due to its compatibility with the CMOS platform and unprecedented integration capability, has become the preferred solution for the implementation of next generation optical interconnects. However, current Si photonics require on-chip integration of several materials, including III-V for lasing, doped silicon for modulation and Ge for detection. The very different requirements of these materials result in complex fabrication processes that offset the cost-effectiveness of the Si photonics approach. We are developing an alternative route towards the integration of optoelectronic devices in Si photonic, relying on the use of single wall carbon nanotubes (SWNTs). SWNTs can be considered as a Si compatible material able to emit, modulate and detect near-infrared light. Hence, they hold a unique potential to implement all active devices in the Si photonics platform. In addition, solution processed SWNTs can be integrated on Si using spin-coating techniques, obviating the need of complex epitaxial growth or chip bonding approaches. Here, we report on our recent progress in the coupling of SWNTs light emission into optical resonators implemented on the silicon-on-insulator (SOI) platform.

International audience; Hybrid systems based on carbon nanotubes emitting in the telecom wavelength range and Si-photonic platforms are promising candidates for developing integrated photonic circuits. Here, we consider semiconducting single walled carbon nanotubes (s-SWNTs) emitting around 1300 nm or 1550 nm wavelength. The nanotubes are deposited on quartz substrate for mapping their photoluminescence in hyperspectral near-field microscopy. This method allows for a sub-wavelength resolution in detecting the spatial distribution of the emission of single s-SWNTs at room temperature. Optical signature delocalized over several micrometers is observed, thus denoting the high quality of the produced carbon nanotubes on a wide range of tube diameters. Noteworthy, the presence of both nanotube bundles and distinct s-SWNT chiralities is uncovered.

International audience; Recent advances in the theoretical and experimental studies of Ge/SiGe quantum wells structures will be presented with a focus on optical telecommunication applications. Firstly, high-speed stand-alone Ge/SiGe QW electro-absorption modulators and photodetectors will be reported. It will be followed by the presentation of different methods for engineering Ge/SiGe QW to tune the operating wavelength to 1.3 µm. Then the demonstration of a strong electro-refraction Ge/SiGe QW will be reported. This effect could be used to achieve optical modulation but requires the embedding of the QWs in a Mach-Zehnder interferometer. Finally it will be shown that these active devices can be combined with advanced passive structures using Ge-rich SiGe virtual substrate on graded buffer as a waveguide.

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.

Polarization controllers are key elements in many fields of optics, including coherent communications, optical imaging, and quantum applications. Here we present a technology-independent polarization controller scheme based on electrically tunable phase shifters and polarization rotators with largely relaxed fabrication tolerances. Using this scheme, we experimentally demonstrate a fully integrated polarization controller in the silicon-on-insulator platform that is tunable over the complete C-band and achieves a polarization extinction range of 40 dB (±20 dB ). These results constitute, to the best of our knowledge, the highest polarization extinction range achieved in a fully integrated device, and overcome the existing limitation in the trade-off between integration and performance in polarization management circuits.

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; Optical antennas based on noble metal nanoparticles can increase the photoluminescence of quantum dots,but the exact strength of this enhancement depends on the brightness (i.e., the intrinsic quantum yield ηi) ofthe emitters. Here we perform temperature-dependent measurements on a system of PbS colloidal quantumdots coupled with Au ring arrays that bring quantitative insight into this phenomenon. We show that althoughthe boost in photoluminescence is lower at cryogenic temperatures where the nanocrystals become very bright emitters, the spectral signature of this enhancement is remarkably independent of ηi . These observations remain true even at wavelengths where the losses by absorption in the metal nanoparticles considerably increase due to the excitation of localized plasmon resonances, in contradiction with standard theory that treats the emitters as a collection of two-level systems. We propose a mechanism in which the quantum dots are modeled as multilevel and inhomogeneously broadened emitters to account for these findings.

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.