Second harmonic generation in SiO2 sol-gel films functionalized with Ethyl-[4-(4-nitro-phenylazo)-phenyl]-(2-oxiranylmethoxy-ethyl)-amine (ENPMA) molecules

High concentrations of Ethyl-[4-(4-nitro-phenylazo)-phenyl]-(2-oxiranylmethoxy-ethyl)-amine (ENPMA) chromophores were linked covalently to two different kinds of SiO2 matrices prepared by the sol–gel method. The matrices differed in their reticulation level, determined by the organometallic precursor used during the synthesis. The materials were deposited as films by spin-coating. The chromophores in the films were oriented in a non-centrosymmetric way by a modified Corona poling set-up, and the nonresonant second order non-linear optical coefficient d33(0) of the films was determined by means of the Maker fringe technique. It was found that there is an optimum concentration of chromophores for attaining the highest d33(0) value in these materials. Their maximum d33(0) value was found in the films with the least reticulated matrix, that value was equal to 5.2±0.5 pm/V. Besides, the d33(0) value was more stable in time in the less reticulated materials.