Spin‐coated and solution‐cast films of poly[5,7‐dodecadiyn‐1,12‐diol‐bis(n‐butoxycarbonyl‐methyl‐urethane)], or briefly, poly‐4BCMU, irreversibly bleach when irradiated by light tuned within the absorption band of the polymer sample. Characteristic bleaching times have been measured at several wavelengths. The refractive index of poly‐4BCMU has been measured at 1.064 &mgr;m for various photobleaching energies. Refractive index changes as large as −0.08 have been measured, and the decrease in refractive index has been used to define waveguide channels and gratings.