Search

We describe the multipolar Second Harmonic (SH) response from metallic particles with sizes up to 150 nm. A particular emphasis is given to the light polarization and size dependence of the SH intensity collected to identify the different field mechanisms involved and quantify the different sources to the nonlinear polarization. It is shown that the dipolar, quadrupolar and octupolar modes can be observed depending on the input fundamental and output harmonic polarization configurations chosen and the size of the particles. We furthermore develop a careful analysis of the experimental results obtained for the largest size of particles investigated, e.g. 150 nm diameter gold particles, in conjunction with finite element simulations to compute the different sources to the nonlinear polarization, namely the surface local and the bulk nonlocal contributions. It is then shown that their relative weight can be determined. These results are recast within the general model initially proposed by Rudnick and Stern (J. Rudnick and E. A. Stern, Phys. Rev. B,4 (1971) 4274).

We report experiments where single metallic nanoparticle sensitivity is achieved with hyper Rayleigh scattering. To reach this level of sensitivity, long acquisition times and signal statistics were required. In particular, a careful study of the intensity fluctuations with time was undertaken and the first, second and third moments of the intensity distribution were determined. From this analysis, the hyperpolarizability of a silver metallic nanoparticle, the diameter of which is 80 nm, was measured and found to be β = (206 ± 20) × 10−25esu at 800 nm.

Surface second harmonic generation was used to study the nonlinear optical properties of a two-dimensional film of 4-(4-(dihexadecylamino)styryl)- N-methylpyridinium iodide (DiA) formed at the air−water interface in a Langmuir trough. The second harmonic intensity was measured as a function of the incident fundamental and outgoing harmonic wave polarization angles during the monolayer compression. Below a critical average density of 0.55 nmol/cm 2, the film is in a liquid-expanded state and the DiA molecules are strongly tilted on the interface. Above this critical average surface density, the film reveals chirality arising from the formation of molecular aggregates at the interface. It is demonstrated through a full analysis of the data that the origin of this chiral property of the film arises from the coupling of the electric and magnetic fields at the fundamental frequency.

Hyper Rayleigh scattering is used to investigate the second harmonic light collected from a liquid suspension of silver metallic particles, the diameter of which ranges from 20 nm up to 80 nm. From the dependence of the quadratic hyperpolarizability as a function of the particle size, it is inferred that retardation effects of the electromagnetic fields play a major role in the frequency conversion process. The hyper Rayleigh scattering intensity was also recorded as a function of the angle of polarization of the incident fundamental wave. For the particles with a diameter of 20 nm, the harmonic response is dominated by the dipolar contribution arising from the deviation of the particle shape from that of a perfect sphere. It is therefore concluded that the origin of the nonlinear polarization for the smaller silver particle sizes arises from the surface similarly to the case of the gold metallic particles. 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. The weighting parameter Vis used, equaling unity for a pure quadrupolar contribution and vanishing for a pure dipolar response, in order to quantify the relative magnitude of these two dipolar and quadrupolar contributions. The size dependence of this parameter obtained for the silver particles is compared to the dependence previously reported for gold metallic particles.

La génération de second harmonique par des nanoparticules d'or dont le diamètre varie de 20 nm à 100 nm a été étudiée par diffusion hyper Rayleigh en polarisation. Pour les petites nanoparticules, la réponse non linéaire est purement dipolaire électrique indiquant une brisure de la symétrie sphérique de la forme de la particule. Pour des nanoparticules de plus grand diamètre les effets retard ne sont plus négligeables et la contribution quadrupolaire électrique devient prépondérante.

La g?n?ration de second harmonique par des nanoparticules d'or dont le diam?tre varie de 20?nm ? 100?nm a ?t? ?tudi?e par diffusion hyper Rayleigh en polarisation. Pour les petites nanoparticules, la r?ponse non lin?aire est purement dipolaire ?lectrique indiquant une brisure de la sym?trie sph?rique de la forme de la particule. Pour des nanoparticules de plus grand diam?tre les effets retard ne sont plus n?gligeables et la contribution quadrupolaire ?lectrique devient pr?pond?rante.

Single metallic nanoparticle sensitivity in liquid solution was achieved with hyper Rayleigh scattering (HRS). The study of the HRS intensity fluctuations over time was performed for several dilutions of the nanoparticle solution. Histograms of the HRS intensity counts were then obtained. These histograms exhibit a Gaussian profile down to low concentrations before evolving towards Poisson distributions at very low concentrations, demonstrating that the single particle sensitivity is obtained. Based on these results, we were then able to achieve the three-dimensional mapping of immobile 150 nm gold metallic nanoparticles dispersed in a homogeneous transparent polyacrylamide matrix. Polarization resolved measurements were also performed allowing for a clear identification of the harmonic light generated by single gold metallic nanoparticles.