SHG from bulk and surface of nanoparticle composites

Abstract

Three wave mixing processes such as second harmonic generation (SHG) have proven to be a sensitive probe of buried interfaces. Recently SHG has been employed to monitor the surface of Si nanocrystals within a glass matrix. Due to the macroscopic homogeneity and centrosymmetry of the composite, its bulk SHG signal is of multipolar character, produced by the non-uniformity of the fundamental light beam, and can be enhanced several orders of magnitude by employing two interfering crossed beams. On the other hand, there are surface contributions to the SHG of the composite due to the relatively large inhomogeneities of the surface local field which act on the nanoparticles and to anisotropic particle shape fluctuations. These yield dipolar contributions which are therefore insensitive to the beam profile and are not enhanced in a two-beam geometry. We calculate the SHG from a thin nanocomposite material and compare the relative strength of its surface and bulk contributions.