a method of creating an optical structure within a photosensitive light transmissive material comprising the steps of : exposing a selected region of the material to radiation at a wavelength selected to effect a refractive index change in the material ; terminating the exposure to the radiation at a selected fluence ; and afterwards exposing at least one portion of the selected region to uv radiation at a level sufficient to vary the refractive index of the material within the selected region to form the optical structure ; wherein the fluence is selected such that the optical structure is substantially thermally stable without a requirement for post - processing annealing .