Density characterization of femtosecond laser modification in polymers

Bragg-type gratings were prepared by femtosecond laser irradiation in a series of optical polymers. The diffraction efficiency of polymethylpentene (PMP) was an order of magnitude higher than those of other polymers. Repeated scanning irradiation with femtosecond laser light formed gratings by refractive index changes inside the polymers. In PMP, whose density was the lowest among the polymers examined, large volume contraction by femtosecond laser irradiation was observed by transmission electron microscopy. The larger refractive index change of PMP was attributed to its large volume contraction based on its low density.