Optical absorption and second harmonic generation in SiO 2 :DR1 sol-gel films as function of poling time

Abstract

Amorphous and nanostructured SiO₂:DR1 sol-gel films were prepared by dip-coating. X-ray diffraction studies were performed to determine the long-order structure obtained in the films. The optical absorption (AO) measurements were done in three different nanostructures of the SiO₂ network: lamellar, hexagonal and mixed. The AO measurements and the second harmonic generation (SHG) intensity were carried out at different orientation steps of the chromophores embedded in the films. These chromophore orientation distributions were obtained by means of the corona technique, and they depend on the corona poling time. We physically model the optical absorption and the second harmonic generation experimental results as function of the corona poling time, employing only one fitting parameter related to the matrix-chromophore interactions. The physical model and the experimental results were in an excellent agreement. The experimental results fitted by the model are shown in plots of order parameter against corona poling time and SHG intensity against corona poling time. The lamellar structure provides a larger order parameter values than those obtained for the other structures. A minimum value for the order parameter was detected by means of the optical absorption measurements at short poling times. For the SHG measurements, four different chromophore concentrations were used. As the concentration increases the measured SHG intensity increases too, but the increment is limited by the electrostatic interactions among the chromophores, which is also considered in our model.