Second-order optical nonlinearity of surface-capped US nanoparticles and effect of surface modification

Surface-capped US nanoparticles (5 nm mean diameter) with hexametaphosphate and CTAB (hexadecyltrimethyl-ammonium bromide) in chloroform were prepared by colloidal chemistry and phase transfer method. The second-order nonlinear optical response was measured by incoherent hyper-Rayleigh scattering (FIRS) technique. Results show that the surface-capped US nanoparticles have a large first-order hyperpolarizability beta value of 1.30 x 10(-26) esu per particle. Surface capping molecule modification was performed by adding pyridine to a chloroform sol of the surface-capped US nanoparticles where pyridine can be bound to the US nanoparticle surface through Cd-N chemical bonds. A reduction in HRS signal intensity was observed when increasing pyridine concentration. This is explained in terms of a two-level model approximation derived from molecular chromophores, when considering the influence of the formation of Cd-N coordinating bonds on the charge-transfer transition energy of surface molecule-like scatterers of Cd-S polar bonds. (C) 2003 Elsevier Science Ltd. All rights reserved.