Two-photon injection of polaritons in semiconductor microstructures.

We experimentally demonstrate that two-photon pumping of "dark" excitons in quantum wells embedded in semiconductor microcavities can result in exciton-polariton injection and photon lasing. In the case of a semiconductor micropillar pumped at half of the exciton frequency, we observe a clear threshold behavior, characteristic of the vertical cavity surface emitting laser transition. These results are interpreted in terms of stimulated emission of terahertz photons, which allows for conversion of "dark" excitons into exciton-polaritons.

Electrically injected photon-pair source at room temperature.

One of the main challenges for future quantum information technologies is the miniaturization and integration of high performance components in a single chip. In this context, electrically driven sources of nonclassical states of light have a clear advantage over optically driven ones. Here we demonstrate the first electrically driven semiconductor source of photon pairs working at room temperature and telecom wavelengths. The device is based on type-II intracavity spontaneous parametric down-conversion in an AlGaAs laser diode and generates pairs at 1.57 µm. Time-correlation measurements of the emitted pairs give an internal generation efficiency of 7×10(-11) pairs/injected electron. The capability of our platform to support the generation, manipulation, and detection of photons opens the way to the demonstration of massively parallel systems for complex quantum operations.

Polariton condensation in photonic molecules.

We report on polariton condensation in photonic molecules formed by two coupled micropillars. We show that the condensation process is strongly affected by the interaction with the cloud of uncondensed excitons and thus strongly depends on the exact localization of these excitons within the molecule. Under symmetric excitation conditions, condensation is triggered on both binding and antibinding polariton states of the molecule. On the opposite, when the excitonic cloud is injected in one of the two pillars, condensation on a metastable state is observed and a total transfer of the condensate into one of the micropillars can be achieved. Our results highlight the crucial role played by relaxation kinetics in the condensation process.

Subpicosecond pulse generation from a 1.56 µm mode-locked VECSEL.

Near-transform-limited subpicosecond pulses at 1.56 µm were generated from an optically pumped InP-based vertical-external-cavity surface-emitting laser (VECSEL) passively mode-locked at 2 GHz repetition rate with a fast InGaAsNSb/GaAs semiconductor saturable absorber mirror (SESAM). The SESAM microcavity resonance was adjusted via a selective etching of phase layers specifically designed to control the magnitude of both the modulation depth and the intracavity group delay dispersion of the SESAM. Using the same VECSEL chip, we observed that the mode-locked pulse duration could be reduced from several picoseconds to less than 1 ps with a detuned resonant SESAM.

Plasmonic properties of silver nanostructures coated with an amorphous silicon-carbon alloy and their applications for sensitive sensing of DNA hybridization.

The use of an amorphous silicon-carbon alloy overcoating on silver nanostructures in a localized surface plasmon resonance (LSPR) sensing platform allows for decreasing the detection limit by an order of magnitude as compared to sensors based on gold nanostructures deposited on glass. In addition, silver based multilayer structures show a distinct plasmonic behaviour as compared to gold based nanostructures, which provides the sensor with an increased short-range sensitivity and a decreased long-range sensitivity.

Template assisted highly ordered novel self assembly of micro-reservoirs and its replication.

A novel method is developed for template assisted fabrication of a regular assembly of microcavity arrays. Simple micropatterns on PDMS mold are used to create complex geometries via solvent vapor back pressure in a biodegradable polymer. Cavities are in turn replicated in complimentary PDMS mushroom like microstructures.

Selective adhesion of Bacillus cereus spores on heterogeneously wetted silicon nanowires.

The article reports on the selective adhesion of Bacillus cereus spores on patterned and heterogeneously wetted superhydrophobic silicon nanowires surfaces. Superhydrophilic patterns on superhydrophobic silicon nanowire (SiNW) surfaces were prepared by a standard optical lithography technique. Exposure of the patterned surface to a suspension of B. cereus spores in water led to their specific adsorption in superhydrophobic areas. Comparable results were obtained on a patterned hydrophobic/hydrophilic flat silicon (Si) surface even though a higher concentration of spores was observed on the hydrophobic areas, as compared to the superhydrophobic regions of the SiNW substrate. The surfaces were characterized using scanning electron microscopy (SEM), fluorescence spectroscopy, and contact angle measurements.

Cell adhesion properties on chemically micropatterned boron-doped diamond surfaces.

The adhesion properties of living cells were investigated on a range of chemically modified boron-doped diamond (BDD) surfaces. We studied the influence of oxidized, H-, amine- (NH(2)-), methyl- (CH(3)-), trifluoromethyl- (CF(3)-) and vinyl- (CH(2)═CH-) terminated BDD surfaces on human osteosarcoma U2OS and mouse fibroblast L929 cells behavior. Cell-surface interactions were analyzed by fluorescence microscopy in terms of cell attachment, spreading and proliferation. U2OS cells poorly adhered on hydrophobic surfaces and their growth was blocked. In contrast, L929 cells were mainly influenced by the presence of perfluoroalkyl chains in regard to their morphology. The results were subsequently applied to selectively micropattern U2OS cells on dual hydrophobic/hydrophilic surfaces prepared by a UV/ozone lithographic approach. U2OS cells colonized preferentially hydrophilic (oxide-terminated) motifs, forming confluent arrays with distinguishable edges separating the alkyl regions.

Surface plasmon resonance on gold and silver films coated with thin layers of amorphous silicon-carbon alloys.

The paper reports on a novel surface plasmon resonance (SPR) substrate architecture based on the coating of a gold (Au) or silver (Ag) substrate with 5 nm thin amorphous silicon-carbon alloy films. Ag/a-Si(1-x)C(x):H and Au/a-Si(1-x)C(x):H multilayers are found to provide a significant advantage in terms of sensitivity over both Ag and Au for SPR refractive index sensing. The possibility for the subsequent linking of stable organic monolayers through Si-C bonds is demonstrated. In a proof-of-principle experiment that this structure can be used for real-time biosensing experiments, amine terminated biotin was covalently linked to the acid-terminated SPR surface and the specific streptavidin-biotin interaction recorded.

Investigation of the electronic transport in GaN nanowires containing GaN/AlN quantum discs.

We report the investigation of electronic transport in GaN nanowires containing GaN/AlN quantum discs (QDiscs). The nanowires were grown by plasma-assisted molecular beam epitaxy and contacted by electron-beam lithography. Three nanowire samples containing QDiscs are analyzed and compared to a reference binary n-i-n GaN nanowire sample. The current-voltage measurements on single nanowires show that if the QDiscs are covered with a lateral GaN shell, the current mainly flows through the shell close to the lateral surface and the wire conductivity is extremely sensitive to the environmental conditions. On the contrary, if no GaN shell is present, the current flows through the QDisc region and a reproducible negative differential resistance related to electron tunneling through the QDiscs can be observed for temperatures up to 250 K. The demonstration of the resonant tunneling in GaN/AlN superlattices is of major importance for the development of nitride-based far-infrared quantum cascade lasers operating at high temperature.

SPR biosensing coupled to a digital microfluidic microstreaming system.

This article reports on a proof-of-concept system composed of a droplet based surface plasmon resonance (SPR) system coupled to a surface acoustic wave (SAW) microfluidic plateform. It is now well established that surface based binding analyses such as SPR are highly influenced by the transport of analyte to the sensing surface. Further, obtaining reliable equilibrium in flow cells to realize quantification studies is not straightforward. An original solution compared to generally used pressure driven flows is then proposed to favourably cope with these issues. Efficiency of SAW microstreaming coupled to SPR biosensing is considered, in order to improve the accuracy of kinetic parameter estimation in mass transport limited regime and to realize reliable quantification studies. First, the droplet based SPR technique and its advantages are presented. Then, the integration of the microstreaming on the system is discussed. Streptavidin binding is then monitored in static mode and under SAW streaming mode.

Energy harvesting efficiency in GaN nanowire-based nanogenerators: the critical influence of the Schottky nanocontact.

The performances of 1D-nanostructure based nanogenerators are governed by the ability of nanostructures to efficiently convert mechanical deformation into electrical energy, and by the efficiency with which this piezo-generated energy is harvested. In this paper, we highlight the crucial influence of the GaN nanowire-metal Schottky nanocontact on the energy harvesting efficiency. Three different metals, p-type doped diamond, PtSi and Pt/Ir, have been investigated. By using an atomic force microscope equipped with a Resiscope module, we demonstrate that the harvesting of piezo-generated energy is up to 2.4 times more efficient using a platinum-based Schottky nanocontact compared to a doped diamond-based nanocontact. In light of Schottky contact characteristics, we evidence that the conventional description of the Schottky diode cannot be applied. The contact is governed by its nanometer size. This specific behaviour induces notably a lowering of the Schottky barrier height, which gives rise to an enhanced conduction. We especially demonstrate that this effective thinning is directly correlated with the improvement of the energy harvesting efficiency, which is much pronounced for Pt-based Schottky diodes. These results constitute a building block to the overall improvement of NW-based nanogenerator devices.

Optically programmable excitonic traps.

With atomic systems, optically programmed trapping potentials have led to remarkable progress in quantum optics and quantum information science. Programmable trapping potentials could have a similar impact on studies of semiconductor quasi-particles, particularly excitons. However, engineering such potentials inside a semiconductor heterostructure remains an outstanding challenge and optical techniques have not yet achieved a high degree of control. Here, we synthesize optically programmable trapping potentials for indirect excitons of bilayer heterostructures. Our approach relies on the injection and spatial patterning of charges trapped in a field-effect device. We thereby imprint in-situ and on-demand electrostatic traps into which we optically inject cold and dense ensembles of excitons. This technique creates new opportunities to improve state-of-the-art technologies for the study of collective quantum behavior of excitons and also for the functionalisation of emerging exciton-based opto-electronic circuits.

Formation and control of Turing patterns in a coherent quantum fluid.

Nonequilibrium patterns in open systems are ubiquitous in nature, with examples as diverse as desert sand dunes, animal coat patterns such as zebra stripes, or geographic patterns in parasitic insect populations. A theoretical foundation that explains the basic features of a large class of patterns was given by Turing in the context of chemical reactions and the biological process of morphogenesis. Analogs of Turing patterns have also been studied in optical systems where diffusion of matter is replaced by diffraction of light. The unique features of polaritons in semiconductor microcavities allow us to go one step further and to study Turing patterns in an interacting coherent quantum fluid. We demonstrate formation and control of these patterns. We also demonstrate the promise of these quantum Turing patterns for applications, such as low-intensity ultra-fast all-optical switches.

Enhancement of biosensing performance in a droplet-based bioreactor by in situ microstreaming.

A droplet-based micro-total-analysis system involving biosensor performance enhancement by integrated surface-acoustic-wave (SAW) microstreaming is shown. The bioreactor consists of an encapsulated droplet with a biosensor on its periphery, with in situ streaming induced by SAW. This paper highlights the characterization by particle image tracking of the speed distribution inside the droplet. The analyte-biosensor interaction is then evaluated by finite element simulation with different streaming conditions. Calculation of the biosensing enhancement shows an optimum in the biosensor response. These results confirm that the evaluation of the Damköhler and Peclet numbers is of primary importance when designing biosensors enhanced by streaming.

Localized surface plasmon-enhanced fluorescence spectroscopy for highly-sensitive real-time detection of DNA hybridization.

Versatile and highly-sensitive detection of DNA hybridization is described using metal nanostructures-enhanced fluorescence (MEF) emission intensity when fluorescently-labeled DNA oligomers are covalently immobilized on a nanometer-thin amorphous silicon-carbon layer capping the metal nanostructures. The MEF structures are formed by thermal deposition of silver, gold or silver/gold thin films on glass surfaces and post-annealing at 500 degrees C. The choice of the metal film allows for tuning the optical properties of the interface. The metallic nanostructures are subsequently coated with an amorphous thin silicon-carbon alloy (a-Si(0.80)C(0.20): H) layer deposited by PECVD. Carboxydecyl groups are attached on these surfaces through hydrosilylation then reacted with amine-terminated single-stranded DNA oligomers, forming a covalent link. The immobilized DNA is hybridized with its complementary strand carrying a fluorescent label. Through optimization of the thickness of the a-Si(0.80)C(0.20): H alloy overlayer and by working close to resonance conditions for plasmon and fluorophore excitation, the hybridization of very dilute oligomers (5 fM) is easily detected, and the hybridization kinetics can be monitored in situ and in real-time.

Amorphous silicon-carbon alloys for efficient localized surface plasmon resonance sensing.

This paper describes a novel platform for preparing localized surface plasmon resonance (LSPR) sensing surfaces. It is based on the coating of gold nanostructures deposited on glass with an amorphous silicon-carbon alloy overcoating. The interest in coating the Au NSs with an amorphous silicon-carbon alloy resides in the possibility of incorporating carboxyl functions directly onto the surface via Si-C covalent bonds. This permits the use of hyrdosilylation reactions to modify the sensor surface. The use of this multilayer structure for the detection of hybridization events is discussed.

Silicon nanowires coated with silver nanostructures as ultrasensitive interfaces for surface-enhanced Raman spectroscopy.

Silver nanoparticles (Ag NPs) were chemically deposited on silicon nanowires (SiNWs), prepared using the vapor-liquid-solid (VLS) growth mechanism, using an in situ electroless metal deposition technique. The resulting SiNWs/Ag NPs composite interfaces showed large Raman scattering enhancement for rhodamine 6G (R6G) with a detection limit of 10(-14) M and an enhancement factor of 2.3 x 10(8). This large enhancement factor was attributed to the presence of "hot" spots on the SiNWs/Ag NPs substrate.

Reversible electrowetting on superhydrophobic silicon nanowires.

This paper reports for the first time on the reversible electrowetting of liquid droplets in air and oil environments on superhydrophobic silicon nanowires (SiNWs). The silicon nanowires were grown on Si/SiO2 substrates using the vapor-liquid-solid (VLS) mechanism, electrically insulated using 300 nm SiO2, and hydrophobized by coating with a fluoropolymer C4F8. The resulting surfaces displayed liquid contact angle (Theta) around 160 degrees for a saline solution (100 mM KCl) in air with almost no hysteresis. Electrowetting induced a maximum reversible decrease of the contact angle of 23 degrees at 150 VTRMS in air.