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.
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.
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.
We report the use of Second Harmonic Generation (SHG) to investigate at the air-water interface molecular films of PL2(-) molecules, a chiral binaphthyl derivative. Under the compression of the monolayer film in a Langmuir trough, large fluctuations of the SHG intensity are observed. From the expressions of the SHG intensity with polarization control of the input fundamental and output harmonic beams, it appears that the intrinsic chirality of the PL2(-) molecule can be disentangled from the supramolecular chirality arising from PL2(-) molecular aggregates. With a careful polarization control of the input and output polarization configurations of the SHG optical set-up, it is then shown that the dominant origin of the observed SHG intensity fluctuations is the formation of PL2(-) molecular aggregates at the air-water interface. The proposed strategy is also suitable for the analysis of the fluctuations in the SHG intensity arising from molecular films at the air-water interface formed from achiral molecular compounds.
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 (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.
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