Extraordinary stability of anisotropic femtosecond direct-written structures embedded in silica glass

In this letter we report the different response to temperature displayed by isotropic femtosecond written structures (type I_fs), and anisotropic ones (type II_fs), which are characterized by the presence of a self-assembled subwavelength periodic structure within the irradiated volume. We observe that the anisotropic structures display an extraordinary annealing behavior, namely, their photoinduced change in refractive index increases with the annealing temperature. We explain our experimental results with a theoretical model.