I. Russier-Antoine

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.

Multipolar second harmonic generation from metallic nanoparticles

A general framework is given to describe the multipolar Second Harmonic (SH) response from small metallic particles the size of which ranges between a few ten of nanometers and 150 nm. A particular emphasis is given to the input polarization dependence of the SH intensity and the separation of its different mechanisms. The nonlinear polarization is here reduced as a surface polarization term only, the latter stemming from the local breaking of the centrosymmetry at the surface of the particle. The results are applied to the experimental case of the incoherent scattering of the second harmonic light, i.e. hyper Rayleigh scattering, for non-interacting metallic particles dispersed in solution.

Hyper Rayleigh scattering from gold nanorods : The shape effect for centrosymmetric nanoparticles

Hyper Rayleigh Scattering (HRS) is used to determine the absolute first hyperpolarizability of gold nanorods with an aspect ratio of 2.2 and 2.7. Two different long axis lengths are used, namely 25.5 nm and 64 nm. This allows for a discussion of the size effect for these centrosymmetric nanoparticles. A comparison of the first hyperpolarizabilities obtained with that of spherical nanoparticles, also centrosymmetric particles, is then made to discuss the role of the shape of these particles on the origin of the response. For the smallest nanorods, a strong hyperpolarizability normalized per atom is determined underlining the role of the interface and the shape in determining such a large absolute value. For the larger nanorods, the first hyperpolarizability per atom is smaller than that of the smaller nanorods but remains larger than the one obtained for gold nanospheres with a similar volume indicating that the shape and the surface response still continue to play a major role. These results are in agreement with the multipolar theory for the first hyperpolarizability of centrosymmetric nanoparticles but show that for nanorods, the surface regime pertains over a longer size range.

Second Harmonic Generation from Tryptophan-Rich Short Peptides: WnKm and Gramicidin A

We report the first hyperpolarizability of a series of tryptophan-rich short peptides with the respective sequence KWK, KWWK, KWWWK, KWWKWWK, where W and K stand for tryptophan and lysine. The measurements were performed with the technique of hyper-Rayleigh scattering in the bulk of an aqueous Tris buffer solution at a pH of 8.5 and a salt concentration of 150 mM at the non-resonant fundamental wavelength of 784 nm. The first hyperpolarizability of the different peptides follows a simple additive model scaling with the number of tryptophan residues contained in the peptide. However, it appears that the first hyperpolarizability response of a single tryptophan residue in the peptide strongly differs from that of an isolated tryptophan. Hence, it is therefore demonstrated that the local environment of the tryptophan residues within the peptide strongly influences its nonlinear optical response. A comparison with the first hyperpolarizability of the natural peptide gramicidin A measured in trifluoroethanol (TFE) further confirms the key role of the local environment on the first hyperpolarizability of tryptophan residues in peptides.

Effect of the surface plasmon on second harmonic generation from tyrosine

The first hyperpolarizability of the aromatic amino acid Tyrosine has been obtained by Hyper Rayleigh Scattering (HRS) in aqueous environment and under non resonant conditions. The rather large value determined is in agreement with previous HRS studies and confirms that Tyrosine can be targeted as an endogenous molecular probe for the second harmonic studies of biological molecules. The same experiment was also performed in an aqueous environment containing 20 nm diameter gold metallic nanoparticles. The resulting first hyperpolarizability measured with the fundamental wavelengths of 815 nm and 1050 nm in these conditions for Tyrosine is reduced as compared to that obtained in absence of metallic nanoparticles.

Second harmonic generation from tyrosine containing peptides

The Second Harmonic Generation (SHG) response from Tyrosine-containing peptides at the air-water interface is presented. First, the quadratic hyperpolarizability of the aromatic amino acid Tyrosine obtained by Hyper Rayleigh Scattering is reported, demonstrating its potentiality as an endogenous molecular probe for SHG studies. Then, the single Tyrosine antimicrobial peptide Mycosubtilin is monitored at the air-water interface and compared to another peptide, Surfactin, lacking a Tyrosine residue. Adsorption kinetics and polarization analysis of the SHG intensity for the peptide monolayers clearly demonstrate that the SHG response from Mycosubtilin arises from Tyrosine. Besides, it confirms that indeed Tyrosine can be targeted as an endogenous molecular probe.