Electrical and Raman characterization of silicon and germanium-filled microstructured optical fibersEnhanced photosensitivity using thermal treatment has been observed on several silicate optical fibers. The effect of the treatment on fibers with different dopants has been tested via Bragg grating inscription. The presence of Ge or Sn atom has been established to be fundamental for the effect to occur. To explain the main features a model involving defect dynamics is proposed.