Ultrafast Dynamics of Excited Electronic States in Nitrobenzene Measured by Ultrafast Transient Polarization Spectroscopy.

We investigate ultrafast dynamics of the lowest singlet excited electronic state in liquid nitrobenzene using ultrafast transient polarization spectroscopy, extending the well-known technique of optical Kerr effect spectroscopy to excited electronic states. The third-order nonlinear response of the excited molecular ensemble is measured using a pair of femtosecond pulses following a third femtosecond pulse that populates the S1 excited state. By measuring this response, which is highly sensitive to details of the excited state character and structure, as a function of time delays between the three pulses involved, we extract the dephasing time of the wave packet on the excited state. The dephasing time, measured as a function of time delay after pump excitation, shows oscillations indicating oscillatory wave packet dynamics on the excited state. From the experimental measurements and supporting theoretical calculations, we deduce that the wave packet completely leaves the S1 state potential energy surface after three traversals of the intersystem crossing between the singlet S1 and triplet T2 states.

Symmetry Breaking in Oligomer Surface Plasmon Lattice Resonances.

We describe a novel plasmonic-mode engineering, enabled by the structural symmetry of a plasmonic crystal with a metallic oligomer as unit cell. We show how the oligomer symmetry can tailor the scattering directions to spatially overlap with the diffractive orders directions of a plasmonic array. Applied to the color generation field, the presented approach enables the challenging achievement of a broad spectrum of angle-dependent colors since smooth and continuous generation of transmitted vibrant colors, covering both the cyan-magenta-yellow and the red-green-blue color spaces, is demonstrated by scattering angle- and polarization-dependent optical response. The addition of a symmetry driven level of control multiplies the possibility of optical information storage, being of potential interest for secured optical information encoding but also for nanophotonic applications, from demultiplexers or signal processing devices to on-chip optical nanocircuitry.

Laser induced mechanisms controlling the size distribution of metallic nanoparticles.

This paper describes a model to simulate changes in the size distribution of metallic nanoparticles (NPs) in TiO2 films upon continuous wave light excitation. Interrelated laser induced physical and chemical processes initiated directly by photon absorption or by plasmon induced thermal heating are considered. Namely the model takes into account the NP coalescence, Ostwald ripening, the reduction of silver ions and the oxidation of metallic NPs, competitive mechanisms that can lead to counter-intuitive behaviors depending on the exposure conditions. Theoretical predictions are compared successfully to the experimental results deduced from a thorough analysis of scanning transmission electron microscopy (STEM) pictures of Ag:TiO2 films processed with a scanning visible laser beam at different speeds. Ag:TiO2 systems are considered for many applications in solar energy conversion, photocatalysis or secured data printing. Numerical investigations of such a system provide a better understanding of light induced growth and shrinking processes and open up prospects for designing more efficient photocatalytic devices based on metal NP doped TiO2 or for improving the size homogeneity in self-organized metallic NP patterns, for instance.

Fano-like resonance emerging from magnetic and electric plasmon mode coupling in small arrays of gold particles.

In this work we theoretically and experimentally analyze the resonant behavior of individual 3 × 3 gold particle oligomers illuminated under normal and oblique incidence. While this structure hosts both dipolar and quadrupolar electric and magnetic delocalized modes, only dipolar electric and quadrupolar magnetic modes remain at normal incidence. These modes couple into a strongly asymmetric spectral response typical of a Fano-like resonance. In the basis of the coupled mode theory, an analytical representation of the optical extinction in terms of singular functions is used to identify the hybrid modes emerging from the electric and magnetic mode coupling and to interpret the asymmetric line profiles. Especially, we demonstrate that the characteristic Fano line shape results from the spectral interference of a broad hybrid mode with a sharp one. This structure presents a special feature in which the electric field intensity is confined on different lines of the oligomer depending on the illumination wavelength relative to the Fano dip. This Fano-type resonance is experimentally observed performing extinction cross section measurements on arrays of gold nano-disks. The vanishing of the Fano dip when increasing the incidence angle is also experimentally observed in accordance with numerical simulations.