We report on multiphoton-induced periodic microstructures inside bulk polymers by multibeam interference of fermosecond laser pulses at a wavelength of 800 nm, which is out of the linear absorption range of the polymer sample. Two-dimensional structures and three-dimensional periodic microstructures were fabricated inside bulk azodye-doped polymethylmethacrylate by four-beam interference and five-beam interference, respectively. The Raman spectra measurements for the photomodified area of the sample before and after irradiation with the writing light showed that the periodic microstructures probably arose from photoinduced decomposition reaction of azodye molecules through multiphoton absorption. (C) 2004 Elsevier B.V. All rights reserved.