E. Benichou

Second harmonic generation from small gold metallic particles : From the dipolar to the quadrupolar response

Hyper Raleigh scattering, a common technique to investigate the second harmonic light scattered from a liquid suspension of molecular compounds and to determine their quadratic hyperpolarizability, has been used for aqueous suspensions of gold nanoparticles, the diameter of which ranges from 20 up to 150 nm. The hyper Rayleigh signal intensity was recorded as a function of the angle of polarization of the incident fundamental wave. For the particles with a diameter smaller than 50 nm, the response is dominated by the dipolar contribution arising from the deviation of the particle shape from that of a perfect sphere. For larger diameter particles, retardation effects in the interaction of the electromagnetic fields with the particles cannot be neglected any longer and the response deviates from the pure dipolar response, exhibiting a strong quadrupolar contribution. It is then shown that in order to quantify the relative magnitude of these two dipolar and quadrupolar contributions, a weighting parameter zeta(V) which equals unity for a pure quadrupolar contribution and vanishes for a pure dipolar response, can be introduced.

First Hyperpolarizability of the Natural Aromatic Amino Acids Tryptophan, Tyrosine, and Phenylalanine and the Tripeptide Lysine−Tryptophan−Lysine Determined by Hyper-Rayleigh Scattering

We report the first hyperpolarizability of tryptophan (Trp) and tyrosine (Tyr) and an upper limit for that of phenylalanine (Phe), three natural aromatic amino acids. The measurements were performed with hyper-Rayleigh scattering in an aqueous Tris buffer solution at a pH of 8.5 and 150 mM salt concentration with a fundamental wavelength of 780 nm. A value of (4.7 ± 0.7) × 10(-30) esu is found for Trp and (4.1 ± 0.7) × 10(-30) esu for Tyr whereas the upper limit of 1.4 × 10(-30) esu is found for that of Phe due to its limited solubility. The influence of the presence of lysine (Lys) in close vicinity of Trp is investigated with a measurement of the first hyperpolarizabilty of Trp in an excess of Lys and compared to the first hyperpolarizability obtained for the tripeptide Lys-Trp-Lys. The clear decrease of the values measured in these two cases indicates that the first hyperpolarizabilty of Trp is very sensitive to its local environment.

Chirality in molecular films at the air-water interface

Second Harmonic Generation was used to study the optical properties of molecular films formed at the air-water interface. The technique was first applied to a two-dimensional film of 4-(4-dihexadecylaminostyryl)-N-methylpyridinium iodide (DiA) formed at the air-water interface in a Langmuir trough. The SHG intensity was measured as a function of the incident fundamental and outgoing harmonic wave polarization angles at different states of the monolayer compression. The film revealed chirality arising from the formation of molecular aggregates and it was demonstrated that this chirality property arose from the coupling of the electric and magnetic fields at the fundamental frequency. In a second stage, we applied the SHG technique to follow the emergence of chirality during the compression of a molecular film of the amphiphilic compound 5-(octadecyloxy)-2-(2-thiazolylazo) phenol (TARC18). A similar behavior was observed and it was shown that the same origin was responsible for the appearance of chirality. Polarization resolved intensity measurements demonstrated that the magnetic dipole contribution was the dominant one.

Second harmonic generation at liquid interface: molecular organization, supramolecular assemblies, and chirality

Second Harmonic Generation (SHG) was used to study the optical properties of molecular films formed at liquid interfaces (air-water or dodecane-water interfaces). The technique was applied to a two-dimensional film of hemicyanine molecules: dye (4-(4-dihexadecylaminostyryl)-N-methylpyridinium iodide) (DiA) and 4-(4-(diethylamino)styryl)-Nmethylpyridinium iodide (sDiA). First, at the air-water interface, the SHG intensity was measured as a function of the incident fundamental and outgoing harmonic wave polarization. The orientation of the molecules was determined by the analysis of the polarization plots. Secondly, the adsorption of sDiA at the dodecane-water interface was characterized by the SHG technique. Finally, in the case of DiA molecules, the formation of aggregates at the air-water interface was monitored by the analysis of SHG intensity fluctuations.

Hyper Rayleigh and hyper Raman from neat water

Hyper Rayleigh and Hyper Raman spectra of neat water have been recorded in two different geometrical configurations where the incident fundamental beam is impinging on the cell at right angle from the collection direction and in retroreflection. A polarization analysis is also performed for the hyper Rayleigh experiment to underlines the differences in measuring some elements of the first hyperpolarizability tensor of the molecules present in the liquid solution under study. These data, beyond the comparison of the cross sections for the different bands observed, allows for a better insight into the quadratic nonlinear optical response from neat water.

Quadratic optical nonlinearity of gold nanoparticles

We investigate the quadratic nonlinear optical response from metallic gold nanoparticles homogeneously dispersed in a medium or deposited on glass substrates. The nanoparticles are prepared by the wet chemistry method in solution used afterwards. The diameter of the gold nanoparticle is 80 nm. In a homogeneous medium, hyper Rayleigh scattering, effectively incoherent second harmonic generation is used to determine the origin of the response. It is shown that for 80 nm diameter gold nanoparticles, the overall response stems from the deviation of the shape of the nanoparticle from that of a perfect sphere and from retardation effects with a similar weight. The latter retardation effects occur because the diameter of the nanoparticle is no longer vanishing before the wavelength of the incoming electromagnetic field. For deposited nanoparticles, the sample is illuminated through the transparent glass slide and the light at the harmonic frequency, produced through the second harmonic generation phenomenon, is observed in the retro-reflection. From the collected SHG images, it can be unambiguously concluded that the origin of the nonlinearity in 80 nm diameter gold nanoparticles stems from the substrate influence normal to the interface. It can also be concluded that the gold nanoparticles can be used to map out the electromagnetic field in the focal volume.

Symmetry breaking and birth of chirality in molecular film at the air/water interface: an approach with nonlinear optics

Second Harmonic Generation (SHG) was used to investigate the two enantiomers of a chiral bridged binaphthol derivative 1(+) and 1(-) at the air-water interface under lateral compression in a Langmuir trough. For each enantiomer, surface pressure and SH intensity were measured simultaneously during compression and decompression cycle of the molecular film. S polarized output SHG intensity as a function of the input polarization angle of the fundamental beam demonstrated the supramolecular origin for the chirality. The formation of 1(+) and 1(-) aggregates in the film was deduced from the non-vanishing SHG intensities collected for the 90° input and S output polarization angles.

Molecular organization and phase transition at the air-water interface investigated by second-harmonic generation

Second Harmonic Generation (SHG) was employed to study phase transition in molecular films formed at the air/water interface. Studies were performed at biomimetic lipid monolayers by simultaneous surface tension and SHG measurements. Light polarization analysis of the SHG intensities was performed with different lipid interfaces. A comparison between these interfaces provided lipid condensed state-dependent data. An optical signature was observed in the polarization plots for the transition from the Liquid-Expanded (LE) to the Liquid-Condensed (LC) state.

SHG techniques to investigate the surface and the bulk of aqueous solutions

Second Harmonic Generation (SHG) a nonlinear optical process sensitive to medium structure deviation from centrosymmetry has been used to investigate the bulk and the surface of an aqueous phase. Using the combination of incoherent SHG, also named Hyper Rayleigh Scattering (HRS), and interface coherent SHG, we have investigated the neat air-water interface. In this paper, we report an analysis where the experimental conditions have been investigated to have the best contrast between the surface and the volume contributions. Our data are described within a simple model allowing us to normalize the surface contribution to the volume one.

Coherent and incoherent second harmonic generation in liquids

In this paper, the Second Harmonic light intensity scattered off a liquid solution upon illumination by an incident fundamental frequency beam is written within a general framework in order to describe its coherent and incoherent contributions. It is shown that this formulation requires the introduction of a correlation function in time, position and orientation. We discuss this framework in light of recent experiments where the interface and the bulk of liquid solutions can be investigated simultaneously. We apply here this analysis to a neat water solution to compare the bulk volume and the interface correlation functions.