Defects were deliberately induced in SiOx thin films during their deposition using a helicon plasma activated reactive evaporation technique. The films were thermally poled and the poling induced second-order optical nonlinearity was investigated by measuring the second harmonic generation of the samples. It was found that oxygen-rich SiOx thin films containing mainly peroxy radicals enabled a much larger optical nonlinearity than stoichiometric SiO2 films after poling, and their optical nonlinearity was long-time stable; while silicon-rich SiOx thin films containing mainly oxygen vacancy defects presented a smaller and unstable optical nonlinearity after poling. (c) 2007 Elsevier B.V. All rights reserved.