In this article, we have studied the evolution of the thermal diffusivity of SiO2-based sol–gel material containing various amounts of Mn. The thermal diffusivity was obtained using thermal lens spectroscopy. The thermal diffusivity measured as a function of time, from the precursor liquid solution to the solid glass, shows a behavior with three distinct regimes. These regimes correspond to the different stages of glass formation, the hydrolysis, gelation, and solidification stages. The materials show an optical memory effect, which is produced by the irradiation with a focused laser beam. The laser beam produces a gradient in the refractive index, which can be permanently stored in the material. This local change in the optical properties of the material can be read with a probe laser due to the strong diffraction pattern observed in the transmitted light.