5 pagesInternational audienceWe present an all-optical method to investigate the GHz dynamics of the elastic contact between a single metallic nanoparticle and a substrate. A resonant excitation mechanism driven by the 82-MHz Dirac comb of the femtosecond oscillator is associated with femtosecond pump-probe experiments performed in a transient reflectivity configuration. This scheme allows us not only to detect the known breathing mode of the nanoparticle but also to unravel the existence of an axial oscillation of the nanoparticle through an intrinsic common-path interferometer.We measured the eigenfrequency and the lifetime of this vertical motion, which are related to the contact stiffness and hysteresis, and to the acoustic leakage at the nanoparticle-substrate interface. A modeling of the axial oscillation in the framework of classical adhesion theories predicts a simple power law dependence of the axial eigenfrequency with respect to the breathing mode frequency. Measurements performed for single particles with radii ranging from 60 to 700 nm are in strong agreement with this prediction

We demonstrate that fluorophores coupled to plasmonic nanoparticles promote resonant excitation energy transfer processes leading to low-loss building block metamaterials. Experimental observations of Rayleigh scattering enhancement, accompanied by an increase in transmission as function of the gain, clearly reveal optical loss compensation effects. Fluorescence quenching is also observed in gain assisted nanoparticles owing to the increase in nonradiative decay rate triggered by plasmonic resonances. The gain induced transparency at optical frequencies is an unambiguous consequence of loss reduction in metamaterial subunits, representing a promising step to enable a wide range of electromagnetic properties of optical metamaterials.

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Owing to the competition between the radiative and non-radiative decay channels occurring in plasmonic assemblies, we show here how to conceive a long pass emission filter and actually design it. We report the synthesis of gold@silica nanoparticles grafted with dye molecules. The control of the thickness of the silica shell allows us to tune the distance between the metal core and the dye molecules. Assemblies of small number (1 to 7) of these core-shell (CS) particles, considered as multimers, have also been produced for the first time. We show that the shaping of the emission spectra of the multimers is drastically enhanced by comparison with the corresponding monomers. We also show a strong enhancement of the decay rates at the LSP resonance, dominated by the non-radiative energy tranfer from the active medium to the metal. The decay rates decrease as the detuning between the long wavelength emission and the LSP resonance increases.

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The following Acknowledgements sections should be included in the above publication. AcknowledgementsThis subject is supported by Korea Ministry of Environment as ''Converging technology project (202-101-009)''.

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