Etude du phénomène de la transition de spin dans les couches ultra-minces à l'aide des plasmons de surface

Depuis de nombreuses années, l'idée qu'une molécule ou un ensemble de molécules puissent servir comme élément actif dans un dispositif électronique ou photonique stimule de plus en plus l'activité scientifique des chercheurs à l'échelle mondiale. Certains complexes de métaux de transition présentent un phénomène de bistabilité moléculaire, ce sont les matériaux à transition de spin (TS). Le changement de l'état de spin s'accompagne d'une modification des propriétés physiques de la molécule telles que les propriétés magnétiques, optiques, électriques et mécaniques. Cependant, la détection de la transition de spin dans ces matériaux pose de grandes difficultés à l'échelle nanométrique (couches minces, nanoparticules, ...), en raison de la faible quantité de matière sondée d'une part, et d'autre part, par la résolution spatiale limitée des techniques utilisées. Pour palier ces difficultés, de nouvelles méthodes sont développées dans le cadre de cette thèse pour étudier ces matériaux à l'échelle nanométrique. Elles sont basées sur le phénomène de résonances des plasmons de surface localisés et des plasmons de surface délocalisés. Ces dispositifs plasmoniques, couches minces ou nano-objets d'or, nous ont permis de suivre la variation du changement d'indice de réfraction optique qui accompagne la TS. Ainsi, dans ce travail de thèse, nous avons pu, pour la première fois, détecter expérimentalement le phénomène de transition de spin dans des couches très minces (jusqu'à 15 nm) de différents matériaux mettant en évidence une variation de l'indice de réfraction de l'ordre de 10-1 - 10-2. De plus, nous avons montré que ces nano-objets hybrides métalliques/moléculaires peuvent être utilisés comme "dispositifs plasmoniques actifs" en modulant le signal plasmonique par un effet photo-thermique.Recently, nano-objects and thin films displaying molecular spin crossover phenomenon have attracted much attention for their possible application as an active element in electronic or photonic devices. The change of the spin state is accompanied by a change in various physical properties of this molecule such as magnetic, optical, electrical and mechanical properties. However, the detection of the spin crossover in these materials at the nanoscale (thin films, nanoparticles, ...) makes for great difficulties, due to the small amount of the probed material, as well as due to the limited spatial resolution of the usual detection methods. To overcome these problems new methods have been developed in this thesis to study these materials at the nanoscale. Our approach is based on the resonance phenomena of localised surface plasmons and surface plasmon polaritons. These techniques use thin noble metal layers or patterned nanorod arrays, which allowed us to detect the refractive index change accompanying the spin crossover. In this thesis work, for the first time, we have been able to detect the spin crossover phenomenon in nanometric layers (down to 15 nm) for different materials, highlighting a refractive index variation of 10-1 - 10-2. In addition, we have shown that the molecular spin state switching can be very efficiently triggered by a photo-thermal effect (plasmonic heating), which - in turn - allows for an active tuning of the plasmon resonance

Turbulence et dissipation à petite échelle dans le vent solaire

International audienceTurbulence at MagnetoHydroDynamics (MHD) scales of the solar wind has been studied for more than three decades, using data analyzes, theoretical and numerical modeling. However smaller scales have not been explored until very recently. Here, we review recent results on the first observation of cascade and dissipation of the solar wind turbulence at the electron scales. Thanks to the high resolution magnetic and electric field data of the Cluster spacecraft, we computed the spectra of turbulence up to 100 Hz (in the spaceraft reference frame) and found evidence of energy dissipation around the Doppler-shifted electron gyroscale fe . Before its dissipation, the energy is shown to undergo two cascades: a Kolmogorov-like cascade with a scaling f−1.6 above the proton gyroscale, and a new f−2.3 cascade at the sub-proton and electron gyroscales. Above fe the spectrum has a steeper power law f−4.1 down to the noise level of the instrument. Solving numerically the linear Maxwell-Vlasov equations combined with recent theoretical predictions of the Gyro-Kinetic theory, we show that the present results are consistent with a scenario of a quasi-two-dimensional cascade into Kinetic Alfv´en modes (KAW). New analyses of other data sets, where the Cluster separation ( 100 km) allowed us to explore the sub-proton scales using the k-filtering technique, and to confirm the 2-D nature of the turbulence at those scales

Surface Plasmons Reveal Spin Crossover in Nanometric Layers

International audienceNano-objects and thin films displaying molecular spin-crossover phenomena have recently attracted much attention. However, the investigation of spin crossover at reduced sizes is still a big challenge. Here we demonstrate that surface plasmon polariton waves propagating along the interface between a metal and a dielectric layer can be used to detect the spin-state changes in the latter with high sensitivity, even at the nanometer scale

Photonic gratings of the metal–organic framework {Fe(bpac)[Pt(CN)4]} with synergetic spin transition and host–guest properties

International audienceSurface-relief photonic gratings of the spin-crossover metal–organic framework {Fe(bpac)[Pt(CN)4]} (bpac = bis(4-pyridyl)acetylene) were elaborated by the combination of a sequential assembly process and lithographic methods. Optical diffraction, surface plasmon resonance spectroscopy and Raman micro-spectroscopy were used to investigate the temperature dependence of the spin state of the iron(II) ions and the concomitant change of the refractive index of the grating material. The refractive index change associated with the high spin (5T) to low spin (1A) transition was found to be as high as Δn = 0.08 ± 0.005, which was attributed to the pronounced mass density difference between the two spin states. While the grating thickness (15–90 nm) had no influence on the spin-crossover properties of the gratings, the adsorption of aromatic guest molecules was found to have a substantial effect both on the spin transition temperature and the completeness of the transition

Detection of molecular spin-state changes in ultrathin films by photonic methods

International audienceUltrathin films of molecular spin crossover materials exhibit very appealing properties for a variety of photonic applications because the spin-state switching is accompanied by a spectacular change of the complex refractive index in a wide spectral range. After examining different optical spectroscopic approaches for the detection of spin-state changes in nanometric films, we found that conventional light absorption measurements can be used down to the nanometer thickness if the oscillator strength of the transition is high, which is often the case for charge transfer transitions in the ultraviolet range. Methods based on fluorescence energy transfer provide a straightforward means for detecting spin-state changes in films in the visible wavelength range, even if photobleaching may be a problem for certain luminophores. Alternatively, changes in the refractive index accompanying the spin transition can be conveniently determined by surface plasmon resonance spectroscopy, which can also provide very accurate film thickness determination. Plasmonic effects were also used to investigate spin-crossover films by means of surface-enhanced Raman spectroscopy. We found that this technique can provide information not only on the spin state of the molecules in very thin layers, but also on their chemical composition and structure

Bistable photonic nanostructures based on molecular spin crossover complexes

International audienceIn this paper we discuss the elaboration, optical properties and possible applications of thin films and nano-patterns of molecular spin crossover complexes. These bistable nanostructures can respond reversibly with fast response times to various external stimuli, such as temperature changes, application of an external pressure, light irradiation or exposure to gas/vapor molecules. The response can be either transient (gating) or non-volatile (switching) depending on the experimental conditions. We show that these assets provide a very appealing scope for a variety of applications including tunable photonic devices, thermal imaging and chemical sensors. In particular, we discuss three photonic application principles based on fluorescence energy transfer, grating diffraction and guided plasmon-polariton waves

Synergistic switching of plasmonic resonances and molecular spin states

International audiencePlasmonic resonance properties of a series of lithographically patterned gold nanorod arrays, spin coated by thin films of an iron(II)–triazole type spin crossover complex, were investigated upon heating/cooling and also under 633 nm laser irradiation. In both cases a reversible shift of the localised surface plasmon resonance wavelength was observed and quantitatively linked to the refractive index change accompanying the spin transition. These results show that molecular spin state switching can be very efficiently triggered by the photo-thermal effect, which – in turn – allows for an active tuning of the plasmon resonance