Noëlle Lascoux

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.

Coupling between molecular rotations and OH⋯O motions in liquid water: Theory and experiment

A new theory is proposed to describe spectral effects of the coupling between molecular rotations and OH⋯O motions in liquid water. The correlation function approach is employed together with a special type of development in which the coupling energy of these two motions is the expansion parameter. The isotropy of the liquid medium plays an essential role in this study. Based on this theory, a new infrared pump–probe experiment is described permitting a visualization of molecular rotations at subpicosecond time scales. Full curves relating the mean squared rotational angle and time, and not only the rotational relaxation time, are measured by this experiment. However, very short times where the incident pulses overlap must be avoided in this analysis. The lifetime of OH⋯O bonds in water is rotation–limited.

A bottom-up approach to build the hyperpolarizability of peptides and proteins from their amino acids.

We experimentally demonstrate that some peptides and proteins lend themselves to an elementary analysis where their first hyperpolarizability can be decomposed into the coherent superposition of the first hyperpolarizability of their elementary units. We then show that those elementary units can be associated with the amino acids themselves in the case of nonaromatic amino acids and nonresonant second harmonic generation. As a case study, this work investigates the experimentally determined first hyperpolarizability of rat tail Type I collagen and compares it to that of the shorter peptide [(PPG)10]3, where P and G are the one-letter code for Proline and Glycine, respectively, and that of the triamino acid peptides PPG and GGG. An absolute value of (0.16 ± 0.01) × 10(-30) esu for the first hyperpolarizability of nonaromatic amino acids is then obtained by using the newly defined 0.087 × 10(-30) esu reference value for water. By using a collagen like model, the microscopic hyperpolarizability along the peptide bond can be evaluated at (0.7 ± 0.1) × 10(-30) esu.

Nonlinear Mie theory for the second harmonic generation in metallic nanoshells

In this work, Mie theory extended to the specific case of the optical second harmonic generation (SHG) from metallic nanoshells is described. Our model results from a combination of the Mie theory developed for the linear optical response of concentric nanospheres and the Mie theory developed for the SHG from nanospheres. This approach leads to a multipolar expansion of the second harmonic scattered electric fields. The total scattered intensity and the relative contribution of each multipole to the scattered wave are directly calculated within this framework. Our model is then applied to the calculation of the second harmonic cross section for nanoshells made of the most common metals used in plasmonics, namely gold and silver. Finally, the effect of the aspect ratio, i.e., the ratio between the inner and the outer radii of the metallic nanoshell, a parameter that is known to greatly impact the surface plasmon resonance properties of the system, is discussed notably in terms of the tunability of the optical SHG from metallic nanoshells.

Supramolecular chirality at the air/water interface [Invited]

Second Harmonic Generation (SHG) was used to study the non-linear optical properties of a two-dimensional film formed by the achiral amphiphilic compound 5-(octadecyloxy)-2-(2-thiazolylazo) phenol (TARC18) at the air-water interface. The S-polarized SHG intensity was measured as a function of the incident fundamental wave polarization angle during the monolayer compression. The method was applied to follow the emergence of chirality during the film compression and at constant surface pressure. The formation of molecular aggregates revealing supramolecular chirality was then demonstrated. It was shown furthermore that the origin of chirality was dominated by the magnetic contributions.

Effect of a thioalkane capping layer on the first hyperpolarizabilities of gold and silver nanoparticles

We have measured the first hyperpolarizabilities of thioalkane capped silver and gold metallic nanoparticles. The values found are β(AgC 12-10 nm) = (2.10 ± 0.23) × 10(-26) esu for 10 nm diameter silver nanoparticles and β(AuC 18-18 nm) = (3.37 ± 0.08) × 10(-26) esu for 18 nm diameter gold nanoparticles at the fundamental wavelength of 784 nm. By comparison to the corresponding values reported for citrate capped silver and gold metallic nanoparticles, after size corrections, decreases by factors of 4.3 and 6.5 respectively are observed. These decreases are tentatively attributed to the bonds formed between the gold and silver surface atoms and the sulfur atoms of the capping layer.

Nonlinear Fano profiles in the optical second-harmonic generation from silver nanoparticles

The resonance effects on the optical second harmonic generation from 140 nm silver nanoparticles is studied experimentally by hyper-Rayleigh scattering and numerically by finite element method calculations. We find that the interferences between the broad dipolar and narrow octupolar surface plasmon resonances leads to nonlinear Fano profiles that can be externally controlled by the incident polarization angle. These profiles are responsible for the nonlinear plasmon-induced transparency in the second harmonic generation.