No abstract

Long-range orientational correlations in liquids have received recent renewed interest, in particular for the neat water case. These long-range orientational correlations, exceeding several tens of nanometers, originate from the presence of the strong permanent water dipolar moment. However, the exact dependence with the dipolar moment and the role of other local forces like steric hindrance has never been addressed. In this work, we experimentally measure long-range correlations for a set of liquids differing by their molecular weight and dipolar moment, in order to reveal the origin of their long-range organization. Hence, we show that the dipolar moment of a solvent molecule is not the unique feature determining the orientational correlation. Steric hindrance significantly helps to structure the liquids as well. In order to quantify these long-range correlations, we also derive theoretically the polarization resolved second harmonic scattering intensity as a function of the rotational invariants describing the dipolar and octupolar interaction.

In the search for efficient building blocks for nonlinear metamaterials, plasmonic nanoparticles have received considerable attention. Their quadratic nonlinear optical properties result from a complex interplay between the nature of the bulk material, the object shape, surface defects and retardation effects. This versatility can be used to tailor the properties of the resulting material. However, despite extensive investigation, separating and controlling these contributions is still a challenge. In this report, we control the morphology of colloidal gold nanoparticles with tetrahedral symmetry and explore their second harmonic scattering response. The material itself is centrosymmetric but not the shape, conferring to the first hyperpolarizability an almost purely octupolar symmetry. Gradually reshaping the nanoparticles into spheres (and thus decreasing their asymmetry) reduces their nonlinear response, showing that it is is controlled by the morphology. These results open the way towards new kinds of nonlinear plasmonics platforms and imaging probes.