The authors investigated the formation of periodic subwavelength structures, so-called nanogratings, in the volume of fused silica. These self-organized structures emerge upon irradiation with ultrashort laser pulses, undergoing three distinct stages of growth from randomly distributed nanostructures to extended domains with uniform periodicity. The experiments revealed that the cumulative action of subsequent laser pulses is mediated by dangling-bond type defects. On shorter time scales, transient self trapped excitons may significantly enhance the formation process. Nanogratings exhibit an extremely large temperature stability up to 1150 °C. In combination with the possibility to precisely tune their form birefringence, nanogratings provide a powerful tool to realize, thermally stable complex phase elements.