In this Letter, we demonstrate a reversible strong coupling regime between a dipolar surface plasmon resonance and a molecular excited state. This reversible state is experimentally observed on silver nanoparticle arrays embedded in a polymer film containing photochromic molecules. Extinction measurements reveal a clear Rabi splitting of 294 meV, corresponding to ~13% of the molecular transition energy. We derived an analytical model to confirm our observations, and we emphasize the importance of spectrally matching the polymer absorption with the plasmonic resonance to observe coupled states. Finally, the reversibility of this coupling is illustrated by cycling the photochromic molecules between their two isomeric forms.

We report on the quantitative characterization of the plasmonic optical near-field of a single silver nanoparticle. Our approach relies on nanoscale molecular molding of the confined electromagnetic field by photoactivated molecules. We were able to directly image the dipolar profile of the near-field distribution with a resolution better than 10 nm and to quantify the near-field depth and its enhancement factor. A single nanoparticle spectral signature was also assessed. This quantitative characterization constitutes a prerequisite for developing nanophotonic applications.

We study by femtosecond pump-probe microscopy the transient plasmonic response of individual gold nanoantennas fabricated by electron-beam lithography on a glass substrate. By exploiting the capability of the fabrication technique to control geometrical parameters at the nanoscale, we tuned the plasmonic resonance in a broad wavelength range, from the visible to the infrared. Numerical simulations based on a three-temperature model (3TM) for the electrons and lattice dynamics, combined with * To whom correspondence should be addressed full-wave numerical analysis and semiclassical theory of optical transitions in the solid state, are compared with the measurements on a single gold nanoantenna probed at different wavelengths. The agreement between the experiment and the prediction of the 3TM turns out to be comparable to that achievable with the more sophisticated Boltzmann equation formalism. We also investigate the influence of the plasmon detuning with respect to the pump and probe wavelengths on the nonlinear optical response using different nanoantennas. Quantitative comparison of the experimental data with the theoretical model also provides a disentanglement of the different contributions to the optical nonlinearity of gold giving rise to the complex features observed in the transient optical response. Our study provides a complete analysis of the physical mechanisms dominating the nonlinear plasmon dynamics of an individual nanoobject taking place on a few ps time scale.

International audienceSurface plasmon waveguides (SPW's) are metal ridges featuring widths in the micrometer range and thicknesses of a few tens of nanometers. A focused ion beam has been used to carve microscatterers into gold SPW's and the near-field distributions around these microstructures are observed by means of photon scanning tunneling microscopy (PSTM). On the basis of near-field images, we show that a finite length periodic arrangement of narrow slits can reflect a surface plasmon mode propagating along a SPW. The reflection efficiency of the micrograting is found to depend upon the number of slits, the period of the grating, and the incident wavelength. The optimum reflection efficiency is obtained for a period of the micrograting equal to half the incident wavelength in vacuum. The PSTM images of the plasmon mirrors taken at different wavelengths allow us to measure the experimental dispersion curve of the SPW in the near-infrared. From this dispersion curve, we found that, in analogy with a surface plasmon (SP) excited on extended thin films, the group velocity of a SPW mode is close to the speed of light. For a given frequency in the near-infrared, the effective index of the SP mode supported by a 2.5-mum-wide SPW is also found to be significantly larger than the effective index of an extended thin film SP. Finally, we show that the optical properties of microgratings engraved into a SPW can be qualitatively approached by a standard Bragg mirror model

The plasmonic properties of single Au triangular nanoprisms are investigated using photoemission electron microscopy with particular emphasis on polarization dependence. Two localized surface plasmon resonances (LSPRs) are studied, namely, the in-plane dipolar and quadrupolar plasmon excitations. Experimental maps of the near-field spatial distribution upon polarization and wavelength of the exciting field are interpreted in the framework of a group theory description and finite difference time domain simulations. This work demonstrates the selective excitations, by lifting of degeneracy, of the different LSPR eigenmodes at the single object level and opens ways for the active control of the angular radiation patterns of optical nanoantennas. This approach is general and applies to any nano-object, whatever its initial shape symmetry.

Optical antennas are elementary units used to direct optical radiation to the nanoscale. Here we demonstrate an active control over individual antenna performances by an external electrical trigger. We find that by an in-plane command of an anisotropic load medium, the electromagnetic interaction between individual elements constituting an optical antenna can be controlled, resulting in a strong polarization and tuning response. An active command of the antenna is a prerequisite for directing light wave through the utilization of such a device.

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The excitation of surface plasmon polaritons (SPP) by focusing a laser beam on single subwavelength holes opened in a thin gold film is studied both experimentally and theoretically. By means of leakage radiation microscopy, quantitative measurements of the light-SPP coupling efficiency are performed for holes with different sizes and shapes. The system is studied theoretically by using a modal expansion method to calculate the fraction of the incident energy which is scattered by the hole into a surface plasmon. We demonstrate that a single subwavelength hole can be used to generate SPP with an efficiency up to 28%.