A New Class of Organic−Inorganic Sol−Gel Materials for Second-Order Nonlinear Optics

A new class of organic/inorganic sol−gel second-order nonlinear optical (NLO) materials based on the prepolymer of hexakis(methoxymethyl)melamine (HMM) and an alkoxysilane dye (ASD) has been investigated. The NLO-active alkoxysilane dye containing nitroazobenzene was incorporated into the melamine matrixes with a weight ratio of 1:1. This organic−inorganic composite (HMM/ASD) was formed via sol−gel reaction of the respective components. The optically clear (HMM/ASD) sample exhibits large second-order optical nonlinearity ( d 33 = 40 pm/V) after poling and curing at 220 °C for 30 min. The poled/cured HMM/ASD sample has better temporal stability at 100 °C compared to that of the poled/cured ASD, due to its higher cross-linking density. This is confirmed by dielectric relaxation results indicating that the molecular mobility associated with the glass transition was suppressed remarkably for the cured HMM/ASD sample. Furthermore, the second harmonic generation (SHG) relaxations of these NLO-active sol−gel materials were studied via biexponential function curve-fitting.