Optical properties of hybrid nanoparticles consisting of zinc oxide (ZnO) nanoparticles coupled to gold (Au) nanoparticles, were examined both theoretically and experimentally. This work was performed to establish if confined light fields in the form of the localised surface plasmon resonance of the gold component of nanoparticle hybrids could enhance nonlinear emission by several orders of magnitude from the Au-ZnO nanoparticle system, which required experimental synthetic advances. A distinct gold seed size-effect was discovered during synthesis of hybrid nanoparticles made by crystallising ZnO from gold-nanoparticle seeds attached at an interface formed between the Au (111) crystal facets possessing hexagonal symmetry, and the (000 ) basal plane of the seeded, wurzite ZnO nanocrytals. Au particle size played a crucial role in determining the structure and morphology of newly forming crystallites. z-Scan nonlinear absorption measurements were performed on the Au-ZnO hybrid nanoparticle systems to determine if Au nanoparticles significantly enhance optical nonlinearity in the hybrid nanoparticles through the intense electric fields generated by the localised surface plasmon absorption at the Au nanoparticle interface. it was found that a significant improvement in 2-photon absorption (2PA) properties of ZnO nanoparticles occurred when attached to plasmonic Au nanoparticles. Au-ZnO hybrids exhibited an enhancement in the 2PA coefficient () of up to 3-orders-of-magniturde. The 2PA absorption appeared to be independent of the size and the number of Au nanoparticles (NPs) attached to the ZnO NPs. An increase in saturable absorption was also observed when the ZnO NPs were covered with small Au NPs this was attributed to a greater overall surface area coverage by the small Au NPs, compared to larger Au NPs, at the same mass fraction of Au.