Pierre-François Brevet

Optical Second Harmonic Generation in Plasmonic Nanostructures: From Fundamental Principles to Advanced Applications

Plasmonics has emerged as an important research field in nanoscience and nanotechnology. Recently, significant attention has been devoted to the observation and the understanding of nonlinear optical processes in plasmonic nanostructures, giving rise to the new research field called nonlinear plasmonics. This review provides a comprehensive insight into the physical mechanisms of one of these nonlinear optical processes, namely, second harmonic generation (SHG), with an emphasis on the main differences with the linear response of plasmonic nanostructures. The main applications, ranging from the nonlinear optical characterization of nanostructure shapes to the optimization of laser beams at the nanoscale, are summarized and discussed. Future directions and developments, made possible by the unique combination of SHG surface sensitivity and field enhancements associated with surface plasmon resonances, are also addressed.

Origin of optical second-harmonic generation in spherical gold nanoparticles: Local surface and nonlocal bulk contributions

The second-harmonic generation of 150 nm spherical gold nanoparticles is investigated both experimentally and theoretically. We demonstrate that the interference effects between dipolar and octupolar plasmons can be used as a fingerprint to discriminate the local surface and non-local bulk contributions to the second-harmonic generation. By fitting the experimental data with the electric fields computed with finite-element method (FEM) simulations, the Rudnick and Stern parameters weighting the relative nonlinear sources efficiencies are evaluated and the validity of the hydrodynamic model and the local density approximation approaches are discussed.

Multipolar second-harmonic generation in noble metal nanoparticles

Second-harmonic generation from noble metal nanoparticles with a noncentrosymmetrical shape is theoretically investigated by using finite element method simulations. The relative weight of the dipolar and quadrupolar responses is investigated in terms of both light polarization and size dependence of the harmonic scattered intensity. It is shown that, even for small deformations as compared with purely spherical particles, the dipolar response dominates and scales as the nanoparticle surface area squared. The difference between gold and silver metal nanoparticles is also addressed.

Sensing with Multipolar Second Harmonic Generation from Spherical Metallic Nanoparticles

We show that sensing in the nonlinear optical regime using multipolar surface plasmon resonances is more sensitive in comparison to sensing in the linear optical regime. Mie theory, and its extension to the second harmonic generation from a metallic nanosphere, is used to describe multipolar second harmonic generation from silver metallic nanoparticles. The standard figure of merit of a potential plasmonic sensor based on this principle is then calculated. We finally demonstrate that such a sensor is more sensitive to optical refraction index changes occurring in the vicinity of the metallic nanoparticle than its linear counterpart.

Mechanism and dynamics of methyl and ethyl orange transfer across the water/1,2-dichloroethane interface

The mechanism of methyl (MO) and ethyl orange (EO) transfer across the water/1,2-dichloroethane (DCE) interface was studied from thermodynamic and kinetic points of view. Ionic partition diagrams were constructed from the appropriate acid/base equilibrium and formal transfer potentials. In situ spectroscopic studies suggest that the transfer of the anionic species from water to DCE takes place mainly via the non-hydrogen bonded form. On the other hand, the transfer of the anions from DCE to water with a pH smaller than the pK(a) involves the protonation of MO during the transfer step. The rate of ion transfer was studied by chronoabsorptometry and potential modulated reflectance. It was confirmed that the transfer of both anions is a potential dependent process, giving a standard apparent rate constant of the order of 10(-2) cm s(-1) and a transfer coefficient of 0.5. Kinetic data are discussed within the framework of existing model for ion transfer at ITIES, Theoretical aspects of chronoabsorptometry for the study of ion transfer kinetics are also outlined

Cyclic voltammetry for the transfer of multiple charged ions at large ITIES: General computational methodology and application to simple and facilitated ion transfer reactions

General equations to model cyclic voltammetric experiments at a large ITIES are presented for multiple ion transfer. A method of integration of the mass balance equations is exposed and further applied to the case of an amphoteric molecule. This approach is based on the definition of total concentrations and on a Nicholson-Shain integration of the convolution integrals. This numerical method is validated by comparison with simulations performed for simple and facilitated ion transfer reactions and by comparison with experimental results.

Differential cyclic voltabsorptometry and chronoabsorptometry studies of ion transfer reactions at the water|1,2-dichloroethane interface

Abstract The combination of UV—visible absorption spectroscopy and conventional liquid|liquid electrochemistry is applied to study ion transfer at the liquid|liquid interface. Two separate examples of ion transfer are investigated. Results for Ru(bpy) 3 2+ and the dianionic form of Eosin B transfer from water to 1,2-dichloroethane, obtained using total internal reflection absorption spectroscopy, are presented. The techniques of differential cyclic voltabsorptometry and chronoabsorptometry are investigated and shown both to be specific to individual species and to produce responses analogous to conventional electrochemistry. A simple theory is presented and found to be an accurate description of the spectroscopic methodology.

Gold Metal Liquid-Like Droplets

Simple methods to self-assemble coatings and films encompassing nanoparticles are highly desirable in many practical scenarios, yet scarcely any examples of simple, robust approaches to coat macroscopic droplets with continuous, thick (multilayer), reflective and stable liquid nanoparticle films exist. Here, we introduce a facile and rapid one-step route to form films of reflective liquid-like gold that encase macroscopic droplets, and we denote these as gold metal liquid-like droplets (MeLLDs). The present approach takes advantage of the inherent self-assembly of gold nanoparticles at liquid-liquid interfaces and the increase in rates of nanoparticle aggregate trapping at the interface during emulsification. The ease of displacement of the stabilizing citrate ligands by appropriate redox active molecules that act as a lubricating molecular glue is key. Specifically, the heterogeneous interaction of citrate stabilized aqueous gold nanoparticles with the lipophilic electron donor tetrathiafulvalene under emulsified conditions produces gold MeLLDs. This methodology relies exclusively on electrochemical reactions, i.e., the oxidation of tetrathiafulvalene to its radical cation by the gold nanoparticle, and electrostatic interactions between the radical cation and nanoparticles. The gold MeLLDs are reversibly deformable upon compression and decompression and kinetically stable for extended periods of time in excess of a year.

Wavelength dependence of the hyper Rayleigh scattering response from gold nanoparticles

The wavelength dependence of the quadratic hyperpolarizability of 11 nm diam gold nanoparticles, is reported as measured by hyper Rayleigh scattering. An important photoluminescence background underlying the hyper Rayleigh signal is observed, a contribution attributed to radiative electron-hole recombinations following multiphoton excitation favored by adsorbed organic compound like citrate on the surface of the nanoparticles. The absolute value of the quadratic hyperpolarizability of gold spherical nanoparticles is determined and a strong enhancement is observed for harmonic frequencies in resonance with the dipolar surface plasmon excitation. No contribution of the interband transition is observed. The absolute values reported, beta(C)=5.1x10(-26) esu at the second harmonic energy 2.39 eV, have been measured with femtosecond long laser pulse, and are 1 order of magnitude weaker that the one previously reported with nanosecond long pulses. This difference can be related to similar measurements performed on the second order hyperpolarizability of gold nanoparticles and may be attributed to different electronic relaxation regimes. Finally, the spectrum of the quadratic hyperpolarizability is compared to the theoretically expected one.

Second harmonic generation study of myoglobin and hemoglobin and their protoporphyrin IX chromophore at the water/1,2-dichloroethane interface

Second harmonic generation (SHG) from the proteins myoglobin (Mb) and hemoglobin (Hb) adsorbed at the interface between 1,2-dichloroethane (DCE) and a pH 7 aqueous buffer solution is reported and compared to the response from the Mb and Hb chromophore Fe(III) protoporphyrin IX (PpIX) freely adsorbed at the interface. Resonant SHG experiments were performed probing the PpIX π–π* transition and large SHG signals were obtained for bulk aqueous concentrations of proteins as low as 2 μM for Mb and 350 nM for Hb. Both protein adsorption isotherms demonstrated interactions between the proteins at the interface. A Gibbs free energy of adsorption of −82 ± 8 kJ mol−1 was determined for Mb and of −121 ± 12 kJ mol−1 for Hb at high surface coverage. From a light polarization analysis of the SHG signal, the angle of orientation of the free PpIX chromophore at the water/DCE interface was found to be 43 ± 2° with respect to the interface normal direction, assuming a Dirac delta function for the angle distribution, and 55 ± 2° for Mb, independently of the protein surface coverage. The bulk aqueous buffer solution pH was varied and a clear modification in the Hb monolayer was observed at a pH of 4, possibly the protonation of the PpIX carboxylic groups.