Reconstitution reveals how myosin-VI self-organises to generate a dynamic mechanism of membrane sculpting

One enigma in biology is the generation, sensing and maintenance of membrane curvature. Curvature-mediating proteins have been shown to induce specific membrane shapes by direct insertion and nanoscopic scaffolding, while the cytoskeletal motors exert forces indirectly through microtubule and actin networks. It remains unclear, whether the manifold direct motorprotein-lipid interactions themselves constitute another fundamental route to remodel the membrane shape. Here we show, combining super-resolution-fluorescence microscopy and membrane-reshaping nanoparticles, that curvature-dependent lipid interactions of myosin-VI on its own, remarkably remodel the membrane geometry into dynamic spatial patterns on the nano-to micrometer scale. We propose a quantitative theoretical model that explains this dynamic membrane sculpting mechanism. The emerging route of motorprotein-lipid interactions reshaping membrane morphology by a mechanism of feedback and instability opens up hitherto unexplored avenues of membrane remodelling and links cytoskeletal motors to early events in the sequence of membrane sculpting in eukaryotic cell biology

Local electrical excitation of plasmonic nanostructures with a scanning tunnelling microscope

Nous utilisons un microscope à effet tunnel (STM) associé à un microscope optique inversé pour l’excitation et la détection des plasmons de surface propagatifs et/ou localisés. L’excitation de ces plasmons est assurée par passage d’un courant tunnel inélastique entre la pointe du STM et la surface d’un film métallique mince (épaisseur de 50 nm) d’or ou d’argent déposé sur une lamelle de verre. Les fuites radiatives des plasmons de surface propagatifs et la lumière émise par les plasmons localisés dans le substrat de verre sont collectées par un microscope optique via un objectif à immersion. Il est alors possible de déterminer à la distribution spatiale et angulaire des émissions issues de ces plasmons de surface excités par STM, ainsi qu’à leur distribution en longueurs d’onde. Dans cette thèse, nous nous sommes intéressés au fonctionnement et à l’émission de lumière sous la pointe d’un microscope à effet tunnel fonctionnant à l’air. Nous montrons que la présence d’eau adsorbée au sein de la jonction tunnel, associée à la boucle d’asservissement du STM induit un mode de fonctionnement oscillant et périodique du STM sans lequel il serait difficile d’exciter les plasmons de surface. Ensuite, nous avons montré qu’il est possible de contrôler la directivité des plasmons de surface propagatifs excités par STM en excitant localement un nanofil d’or déposé sur le film d’or. L’étude détaillée de cette directivité nous a permis de démontrer que, contrairement au cas du nanofil d’or déposé sur verre, un nanofil d’or déposé sur film d’or ne se comporte pas comme un résonateur Fabry Pérot. Nous avons proposé un modèle simple dans lequel le nanofil est assimilé à un réseau linéaire d’antennes. Ce modèle permet de rendre compte des structurations spectrales et spatiales des plasmons de surface sur le film d’or résultant de l’ajout du nanofil d’or. Puis, nous avons étudié le couplage entre des nanofibres organiques fluorescentes (structures excitoniques) et les plasmons de surface propagatifs d’un film métallique d’or ou d’argent sur lequel ces nanofibres sont déposées. Nous avons ainsi montré que (i) la fluorescence de la nanofibre peut exciter des plasmons de surface à la surface du film d’or, (ii) la nanofibre organique agit comme un guide d’onde plasmonique et (iii) qu’il est possible d’injecter des plasmons de surface propagatifs du film excités par STM dans ces modes guidés par la nanofibre. D’autre part, en étudiant la figure d’interférences dans le plan de Fourier, nous avons pu confirmer que l’émission du dipôle sous la pointe STM et les plasmons de surface propagatifs excités par STM sont cohérents, donc issus du même événement tunnel. Enfin, nous discutons les effets du couplage entre des nanocristaux semiconducteurs (quantum dots) individuels et un monofeuillet de graphène. Nous montrons que la présence du graphène réduit d’un facteur ~10 la durée de vie de l’état excité des quantum dots déposés sur graphène par rapport aux quantum dots déposés sur verre. Pour les quantum dots déposés sur graphène, il résulte de cette réduction de la durée de vie de l’état excité, une baisse de l’intensité de fluorescence et une réduction du phénomène de scintillement avec un temps de résidence dans un état brillant globalement plus long que pour les quantum dots déposés sur verre. Les différents résultats obtenus au cours de cette thèse permettent de mieux comprendre l’excitation de plasmons de surface avec un microscope à effet tunnel, le couplage entre nanostructures plasmoniques et le couplage entre une structure plasmonique et une nanostructure excitonique. Ils ouvrent des perspectives intéressantes pour le développement de nanodispositifs hybrides plus complexes liants plasmons et excitons et contrôlés électriquementWe use a scanning tunnelling microscope (STM) to excite propagating and/or localised surface plasmons on a thin metallic film (50 nm thick) made of gold or silver deposited on a glass substrate. The leakage radiation of these STM-excited propagating surface plasmons, and the light emitted by localized plasmons into the glass substrate are collected by an inverted optical microscope equipped with an oil immersion objective. Using this setup, it is possible to image both the spatial and angular distribution of the light emitted into the glass substrate on a cooled-CCD. Sending this light to a spectrometer, it is also possible to obtain the wavelength distribution of these STM-excited plasmons. In this manuscript, we discuss the different operation modes of an STM in air. We show that the thin water layers adsorbed on both the STM tip and sample, along with the STM feedback loop, may give rise to an oscillatory mode of operation. Moreover, this mode turns out to be the most efficient one for plasmon excitation with a STM in air. We then show that, when the STM tip is used to locally excite plasmons on a gold nanowire deposited on a gold film, propagating surface plasmons may be preferentially launched along the nanowire axis. Precise understanding of this directivity allows us to demonstrate that, when deposited on a gold film, gold nanowires do not behave as Fabry-Perot resonators, but may be described quite accurately with a one dimensional antenna array model. With this model, it is thereby possible to explain the complex spatial and spectral characteristics of the STM-excited plasmons on the gold film after the addition of the nanowire. Next, we focus on the coupling between fluorescent organic nanofibres (excitonic nanostructures) and propagating surface plasmons on a metallic film (either gold or silver). We show that when the nanofibres are deposited on the metallic film, (i) their fluorescence can excite propagating surface plasmon, (ii) the nanofibre can act as a plasmonic waveguide, and (iii) it is possible to inject surface plasmons propagating onto the metallic film into the guided plasmonic modes of the nanofibre. Moreover, by studying Fourier space images, we confirmed that the vertical dipole localised under the STM tip and the STM-excited propagating surface plasmons are coherent. We finally study the coupling between individual semiconducting nanocrystals (quantum dots) and a graphene monolayer deposited on a glass substrate. We show that, when deposited on graphene, the fluorescence lifetime of the quantum dots is about 10 times shorter than for the quantum dots deposited on bare glass. This leads to a weaker fluorescence signal and reduced blinking behaviour with longer time spent into a bright state. These results improve our understanding of the STM excitation of surface plasmons. They also provide information on the coupling between plasmonic nanostructures and between plasmonic and excitonic entities. in particular, these results are a promising step toward the conception and the realisation of complex electrically driven hybrid plasmonic/excitonic nanodevice

Excitation électrique locale de nanostructures plasmoniques par la pointe d'un microscope à effet tunnel

We use a scanning tunnelling microscope (STM) to excite propagating and/or localised surface plasmons on a thin metallic film (50 nm thick) made of gold or silver deposited on a glass substrate. The leakage radiation of these STM-excited propagating surface plasmons, and the light emitted by localized plasmons into the glass substrate are collected by an inverted optical microscope equipped with an oil immersion objective. Using this setup, it is possible to image both the spatial and angular distribution of the light emitted into the glass substrate on a cooled-CCD. Sending this light to a spectrometer, it is also possible to obtain the wavelength distribution of these STM-excited plasmons. In this manuscript, we discuss the different operation modes of an STM in air. We show that the thin water layers adsorbed on both the STM tip and sample, along with the STM feedback loop, may give rise to an oscillatory mode of operation. Moreover, this mode turns out to be the most efficient one for plasmon excitation with a STM in air. We then show that, when the STM tip is used to locally excite plasmons on a gold nanowire deposited on a gold film, propagating surface plasmons may be preferentially launched along the nanowire axis. Precise understanding of this directivity allows us to demonstrate that, when deposited on a gold film, gold nanowires do not behave as Fabry-Perot resonators, but may be described quite accurately with a one dimensional antenna array model. With this model, it is thereby possible to explain the complex spatial and spectral characteristics of the STM-excited plasmons on the gold film after the addition of the nanowire. Next, we focus on the coupling between fluorescent organic nanofibres (excitonic nanostructures) and propagating surface plasmons on a metallic film (either gold or silver). We show that when the nanofibres are deposited on the metallic film, (i) their fluorescence can excite propagating surface plasmon, (ii) the nanofibre can act as a plasmonic waveguide, and (iii) it is possible to inject surface plasmons propagating onto the metallic film into the guided plasmonic modes of the nanofibre. Moreover, by studying Fourier space images, we confirmed that the vertical dipole localised under the STM tip and the STM-excited propagating surface plasmons are coherent. We finally study the coupling between individual semiconducting nanocrystals (quantum dots) and a graphene monolayer deposited on a glass substrate. We show that, when deposited on graphene, the fluorescence lifetime of the quantum dots is about 10 times shorter than for the quantum dots deposited on bare glass. This leads to a weaker fluorescence signal and reduced blinking behaviour with longer time spent into a bright state. These results improve our understanding of the STM excitation of surface plasmons. They also provide information on the coupling between plasmonic nanostructures and between plasmonic and excitonic entities. in particular, these results are a promising step toward the conception and the realisation of complex electrically driven hybrid plasmonic/excitonic nanodevicesNous utilisons un microscope à effet tunnel (STM) associé à un microscope optique inversé pour l’excitation et la détection des plasmons de surface propagatifs et/ou localisés. L’excitation de ces plasmons est assurée par passage d’un courant tunnel inélastique entre la pointe du STM et la surface d’un film métallique mince (épaisseur de 50 nm) d’or ou d’argent déposé sur une lamelle de verre. Les fuites radiatives des plasmons de surface propagatifs et la lumière émise par les plasmons localisés dans le substrat de verre sont collectées par un microscope optique via un objectif à immersion. Il est alors possible de déterminer à la distribution spatiale et angulaire des émissions issues de ces plasmons de surface excités par STM, ainsi qu’à leur distribution en longueurs d’onde.Dans cette thèse, nous nous sommes intéressés au fonctionnement et à l’émission de lumière sous la pointe d’un microscope à effet tunnel fonctionnant à l’air. Nous montrons que la présence d’eau adsorbée au sein de la jonction tunnel, associée à la boucle d’asservissement du STM induit un mode de fonctionnement oscillant et périodique du STM sans lequel il serait difficile d’exciter les plasmons de surface.Ensuite, nous avons montré qu’il est possible de contrôler la directivité des plasmons de surface propagatifs excités par STM en excitant localement un nanofil d’or déposé sur le film d’or. L’étude détaillée de cette directivité nous a permis de démontrer que, contrairement au cas du nanofil d’or déposé sur verre, un nanofil d’or déposé sur film d’or ne se comporte pas comme un résonateur Fabry Pérot. Nous avons proposé un modèle simple dans lequel le nanofil est assimilé à un réseau linéaire d’antennes. Ce modèle permet de rendre compte des structurations spectrales et spatiales des plasmons de surface sur le film d’or résultant de l’ajout du nanofil d’or. Puis, nous avons étudié le couplage entre des nanofibres organiques fluorescentes (structures excitoniques) et les plasmons de surface propagatifs d’un film métallique d’or ou d’argent sur lequel ces nanofibres sont déposées. Nous avons ainsi montré que (i) la fluorescence de la nanofibre peut exciter des plasmons de surface à la surface du film d’or, (ii) la nanofibre organique agit comme un guide d’onde plasmonique et (iii) qu’il est possible d’injecter des plasmons de surface propagatifs du film excités par STM dans ces modes guidés par la nanofibre. D’autre part, en étudiant la figure d’interférences dans le plan de Fourier, nous avons pu confirmer que l’émission du dipôle sous la pointe STM et les plasmons de surface propagatifs excités par STM sont cohérents, donc issus du même événement tunnel. Enfin, nous discutons les effets du couplage entre des nanocristaux semiconducteurs (quantum dots) individuels et un monofeuillet de graphène. Nous montrons que la présence du graphène réduit d’un facteur ~10 la durée de vie de l’état excité des quantum dots déposés sur graphène par rapport aux quantum dots déposés sur verre. Pour les quantum dots déposés sur graphène, il résulte de cette réduction de la durée de vie de l’état excité, une baisse de l’intensité de fluorescence et une réduction du phénomène de scintillement avec un temps de résidence dans un état brillant globalement plus long que pour les quantum dots déposés sur verre. Les différents résultats obtenus au cours de cette thèse permettent de mieux comprendre l’excitation de plasmons de surface avec un microscope à effet tunnel, le couplage entre nanostructures plasmoniques et le couplage entre une structure plasmonique et une nanostructure excitonique. Ils ouvrent des perspectives intéressantes pour le développement de nanodispositifs hybrides plus complexes liants plasmons et excitons et contrôlés électriquemen

Thermoplasmonics of metal layers and nanoholes

International audienceSince the early 2000s, the experimental and theoretical studies of photothermal effects in plasmonics have been mainly oriented toward systems composed of nanoparticles, mostly motivated by applications in biomedecine, and have overlooked the case of plasmonic resonances of nanoholes in metal layers (also called nanopores or nano-apertures). Yet, more and more applications based on plasmonic nanoholes have been reported these last years (e.g., optical trapping, molecular sensing, and surface-enhanced Raman scattering), and photothermal effects can be unexpectedly high for this kind of systems, mainly because of the very large amount of metal under illumination, compared with nanoparticle systems. Nanoholes in metal layers involve a fully different photothermodynamical picture, and few of what is known about nanoparticles can be applied with nanoholes. A plasmonic nanohole mixes localized and surfaces plasmons, along with heat transport in a two-dimensional highly conductive layer, making the underlying photothermodynamical physics particularly complex. This Tutorial is aimed to provide a comprehensive description of the photothermal effects in plasmonics when metal layers are involved, based on experimental, theoretical, and numerical results. Photothermal effects in metal layers (embedded or suspended) are first described in detail, followed by the study of nanoholes, where we revisit the concept of absorption cross section and discuss the influences of parameters such as layer thickness, layer composition, nanohole size and geometry, adhesion layer, thermal radiation, and illumination wavelength

Temperature Measurement in Plasmonic Nanoapertures Used for Optical Trapping

International audiencePlasmonic nanoapertures generate strong field gradients enabling efficient optical trapping of nano-objects. However, because the infrared laser used for trapping is also partly absorbed into the metal leading to Joule heating, plasmonic nano-optical tweezers face the issue of local temperature increase. Here, we develop three independent methods based on molecular fluorescence to quantify the temperature increase induced by a 1064 nm trapping beam focused on single and double nanoholes milled in gold films. We show that the temperature in the nanohole can be increased by 10°C even at the moderate intensities of 2 mW/µm² used for nano-optical trapping. The temperature gain is found to be largely governed by the Ohmic losses into the metal layer, independently of the aperture size, double-nanohole gap or laser polarization. The techniques developed therein can be readily extended to other structures to improve our understanding of nano-optical tweezers and explore heat-controlled chemical reactions in nanoapertures

Microscale Thermophoresis in Liquids Induced by Plasmonic Heating and Characterized by Phase and Fluorescence Microscopies

International audienceThermophoresis denotes the motion of particles along temperature gradients. Insignificant in most daily-life observations, this peculiar effect can become dominant in applications involving nano-and microscale heating in fluids. Recent studies in nanoplasmonics observed significant thermophoresis of molecules and particles, in particular in plasmonic trapping, SERS, and biosensing. Evidencing the presence of thermophoresis is not obvious and quantifying its magnitude is even less accessible considering existing techniques. In this article, we introduce a method capable of quantifying the thermophoresis of particles in the context of nanoplasmonic applications. A gold nanoparticle array under illumination is used to create microscale temperature gradients, and a dual fluorescence-phase microscopy technique is used to map both temperature and concentration in parallel. This association enables the determination of Soret coefficients for a wide range of temperatures from a single image acquisition. This metrology technique paves the way for broader fundamental research in microscale thermophoresis in liquids and better-controlled applications in nanophotonics involving thermoplasmonic effects

Enhanced Quantitative Wavefront Imaging for Nano-Object Characterization

International audienceQuantitative phase imaging enables precise and label-free characterizations of individual nano-objects within a large volume, without a priori knowledge of the sample or imaging system. While emerging common path implementations are simple enough to promise a broad dissemination, their phase sensitivity still falls short of precisely estimating the mass or polarizability of vesicles, viruses, or nanoparticles in single-shot acquisitions. In this paper, we revisit the Zernike filtering concept, originally crafted for intensity-only detectors, with the aim of adapting it to wavefront imaging. We demonstrate, through numerical simulation and experiments based on high-resolution wavefront sensing, that a simple Fourier-plane add-on can significantly enhance phase sensitivity for subdiffraction objects─achieving over an order of magnitude increase (×12)─while allowing the quantitative retrieval of both intensity and phase. This advancement allows for more precise nano-object detection and metrology

Thermodynamic properties of lizardite, determined by calorimetry.

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Engineering the emission of light from a scanning tunneling microscope using the plasmonic modes of a nanoparticle

International audienceThe inelastic tunnel current in the junction formed between the tip of a scanning tunneling microscope (STM) and the sample can electrically generate optical signals. This phenomenon is potentially of great importance for nano-optoelectronic devices. In practice, however, the properties of the emitted light are difficult to control because of the strong influence of the STM tip. In this work, we show both theoretically and experimentally that the sought-after, well-controlled emission of light from an STM tunnel junction may be achieved using a nonplasmonic STM tip and a plasmonic nanoparticle on a transparent substrate. We demonstrate that the native plasmon modes of the nanoparticle may be used to engineer the light emitted in the substrate. Both the angular distribution and intensity of the emitted light may be varied in a predictable way by choosing the excitation position of the STM tip on the particle

Edge scattering of surface plasmons excited by scanning tunneling microscopy

International audienceThe scattering of electrically excited surface plasmon polaritons (SPPs) into photons at the edges of gold metal stripes is investigated. The SPPs are locally generated by the inelastic tunneling current of a scanning tunneling microscope (STM). The majority of the collected light arising from the scattering of SPPs at the stripe edges is emitted in the forward direction and is collected at large angle (close to the air-glass critical angle, θ c). A much weaker isotropic component of the scattered light gives rise to an interference pattern in the Fourier plane images, demonstrating that plasmons may be scattered coherently. An analysis of the interference pattern as a function of excitation position on the stripe is used to determine a value of 1.42 ± 0.18 for the relative plasmon wave vector (k SPP /k 0) of the corresponding SPPs. From these results, we interpret the directional, large angle (θ~θ c) scattering to be mainly from plasmons on the air-gold interface, and the diffuse scattering forming interference fringes to be dominantly from plasmons on the gold-substrate interface