Seminars
Manipulation of Electromagnetic Wave by Metasurface
, C2N-SITE orsay Salle P. Grivet (R-d-c pièce 44),Seminars
During the last few years, vast kinds of metamaterials have been proposed and designed. Most of metamaterials are achieved by arranging sub-wavelength unit cells in a periodic manner filling up a specific three-dimensional space. When simplified to two-dimensional case, the surface version of metamaterials can be considered as metasurfaces. As a two-dimensional version, metasurfaces will obviously take up less space compared with three-dimensional metamaterial. As a result of reduction of transmission path in the materials, metasurfaces also provide an alternative for less-lossy solution. In this talk, our recent work of metasurface in microwave region will be introduced. First, ultra-thin metasurface based on phase discontinuities for circular polarization is introduced, including the theoretical model and application in beaming converging and orbital angular momentum generation. Second, a metasurface-constructed 4-beam antenna is studied based on transformation optics. High-directivity emission of electromagnetic wave is verified by experimental results. Our designs provide a promising approach to miniaturize, planarize and integrate multiple microwave components.
Photonics components for optical communication in chip multicore architectures
, C2N-Site orsay,Seminars
Chip multicore architectures represent the prevailing paradigm in the design of high performance very large scale integration (VLSI) systems. Such computational architectures can take advantage of CMOS silicon photonic integration in order to realize optical interconnection links. Optical interconnections among the different computational cores of an integrated system can provide a huge communication bandwidth and a favourable power budget with respect to the electrical counterpart. The fundamental component, necessary to achieve the signal routing among N transmitters and N receivers, is the switch that acts as the basic building block, which can be replicated several times within the network. In this presentation, an overview of our recent research on photonic components for optical networks on chip will be proposed. In particular, the design of silicon photonic devices, such as active and passive waveguide switches and photonic crystal routers for single layer and multilayer networks, will be presented.
(in french) Développement de méthodes de type élément fini d’ordre élevé, pour la nanophotonique computationnell
None, C2N, Site marcoussis, NoneSeminars
Novel Non-radiative Exciton Harvesting Scheme Yields a 15% Efficiency Improvement in High-Efficiency
, C2N site Marcoussis, salle R. Planel,Seminars
The spectral mismatch between the response of a solar cell and the solar spectrum represents the largest loss contributing factor in all photovoltaic technologies. While sub-bandgap photons cannot be absorbed by the semiconducting material, the excess energy of high energy photons is lost via non radiative relaxation of the carriers in the form of heat. One way to mitigate this limitation is to use luminescence down-shifting (LDS), where high energy photons are absorbed and reemitted at a lower energy by an emitter. High efficiency III-V solar cells typically incorporate an indirect wide band-gap semiconductor as a passivation layer to limit surface recombination at higher photon energies. However, the poor extraction efficiency of the carriers photogenerated in this window layer limits the performance of the devices in the high energy region of the spectrum. To address this problem, in this work, we deposit an epilayer of colloidal CdSxSe1-x/ZnS core/shell quantum dots (QDs) onto InGaP solar cells (fig. 1a), emitting below the AlInP band-gap. In this configuration, while the QDs act as a standard LDS layer, excitons are also funneled from the AlInP window layer to the QD epilayer using near-field Resonance Energy Transfer (RET). The transferred excitons can then radiatively recombine in the QDs and the resulting photons can be transmitted through the window layer to generate extractable carriers in the p-n junction. The overall performance of the solar cells is found to be significantly improved after hybridization, with a 15% relative increase in short- circuit current. The Internal Quantum Efficiency (IQE) of the solar cells after hybridization was strongly enhanced in the UV spectral region, exhibiting an almost two fold increase at 325nm (fig. 1b). RET between the window layer and the QDs epilayer was demonstrated using excitation energy dependent rise-time measurements of the QD luminescence. The contribution of RET to the 1-sun photocurrent of the hybridized cell is estimated to be about 4%, while the direct luminescent down-shifting amounted to 5% of the overall photo-current.
Recent advances in the field of “nanoporous magnetic materials”
, C2N-SITE orsay Salle P. Grivet (R-d-c pièce 44),Seminars
Nanoporous materials have been conventionally used in areas like catalysis, gas-sensing or electrochemical energy storage, where a high surface area-to-volume ratio promotes an enhanced performance. Recently, new progress has been made in the synthetic pathways to produce magnetic nanoporous oxides and alloys with novel functionalities. Nanoporous oxide-diluted magnetic semiconductors (e.g., In2O3 or SnO2 doped with transition metals) have been obtained via nanocasting, using mesoporous SiO2 as parent templates [1,2]. The resulting materials show a ferromagnetic-like response at room temperature while keeping a high degree of porosity. In turn, highly ordered antiferromagnetic mesoporous frameworks can be used as hosts to accommodate ferrimagnetic phases within their gyroidal 3D porous network, hence rendering composite materials with high coercivity and pronounced shifts in the hysteresis loops. Finally, nanoporous metallic materials can be electrically actuated (i.e., using a constant electric field) to tune their coercivity. Eventually, a reduction of coercivity is obtained upon application of voltage, hence lowering the energy power consumption when these materials would be incorporated in real applications. Besides our recent results, the most significant challenges related to the near-future implementation of new applications (spintronics, water remediation, electrocatalysis, etc. [3,4]) based on nanoporous magnetic materials will be also highlighted. References 1. E. Pellicer, et al. Adv. Funct. Mater. 23 (2013) 900. 2. E. Pellicer et al., J. Phys.Chem. C 117 (2013) 17084. 3. J. Zhang et al., Nanoscale 6 (2014) 12490. 4. M. Guerrero et al., ACS Appl. Mater. Interf. 6 (2014) 13994.
Reality and constraints of silicon photonic robust architectures towards Industrialization
, C2N-SITE orsay Salle P. Grivet (R-d-c pièce 44),Seminars
There is a recurring statement about silicon photonics which points out that one major breakthrough of that technology is the CMOS foundry processes compatibility. The objective of this seminar is to understand better the rationale about CMOS foundry compatibility, how silicon photonics can benefit from it and the associated constraints of that strategy. It occurs that for decades, the core of industrial semiconductor manufacturing has been driven by silicon processing. Consequently, vast fabrication skills and material expertise were acquired during that period. Still now, silicon remains a solid candidate for contemporary Moore nodes. Moreover, few derivative integrated technologies have emerged from silicon processing, typically: bipolar electronics, MEMS and imaging devices. Nowadays, silicon state-of-the-art manufacturing occurs on 300mm wafer substrates using DUV-193nm immersion lithography to process transistors for the 14nm technological node. Thus, both high volume and high precision processing can be achieved with the same technology. Therefore, even if such dimensions and volumes are not yet targeted for silicon photonics, it makes no doubt that silicon CMOS foundry knowhow is giving a huge start pulse to silicon photonics as it has been the case for other derivative technologies.
Anthropogenic climate change and the global carbon cycle
, C2N, Site marcoussis,Seminars
For the first time in human's history, last year global average of CO2 in the atmosphere has exceeded 400 parts per million. And this increase in atmospheric carbon dioxide is responsible for most of the warming we have experienced in the past 50 years. In this seminar, I will start by assessing the recent evolution of anthropogenic carbon emissions, and show how this additional carbon is partitioned between the atmosphere, causing global warming, the ocean and the land biosphere. I will explain the mechanisms by which these carbon sinks, ocean and land biosphere, keep up by absorbing half of our carbon emissions. I will also show that these sinks are vulnerable, especially because of global warming, and that a reduced efficiency of the sinks may lead to a positive carbon-climate feedback in the coming decades.
Anthropogenic climate change and the global carbon cycle
, C2N, bat D1, site de Marcoussis,Seminars
Anthropogenic climate change and the global carbon cycle
Graphene in the mid-infrared: pn junctions for high responsivity photodetectors, acoustic Dirac plas
, C2N site Marcoussis, salle R. Planel,Seminars
Graphene in the mid-infrared: pn junctions for high responsivity photodetectors, acoustic Dirac plas
Graphene in the mid-infrared: pn junctions for high responsivity photodetectors, acoustic Dirac plasmons with few nanometer confinement
, C2N, bat D1, site de Marcoussis,Seminars
"Graphene in the mid-infrared: pn junctions for high responsivity photodetectors, acoustic Dirac plasmons with few nanometer confinement" Mid-infrared technology (2-20μm wavelength) could benefit from various graphene properties. In particular, photodetectors based on pn junctions exhibit fast and efficient response in the visible and terahertz ranges, but in the intermediate wavelength range, this response is limited by the graphene small absorption and device design [1,2]. Light absorption can then be strongly enhanced by exciting plasmons in nanostructured graphene[3], but until now, experimental absorption peaks are weaker and broader than they could theoretically be. This presentation will combine some of our recent studies regarding 1) the development and optimization of photodetectors based either on the photothermoelectric effect and (polymer) transparent gates[4], or on the photogating of graphene by pyro-resistive substrates[5,6]; and 2) a new method to couple light to graphene plasmons by placing a metallic rod array at a nanometer scale distance to the graphene, which efficiently excites acoustic plasmon modes which are vertically confined down to 2 nm [7], while preserving the graphene quality.