Satoshi Hirono

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.

Increasing diffraction efficiency by heating phase gratings formed by femtosecond laser irradiation in poly(methyl methacrylate)

The diffraction efficiency of phase gratings formed by femtosecond laser irradiation in poly(methyl methacrylate) was increased by more than an order of magnitude by subsequent heating below the glass transition temperature (Tg). The first-order Bragg diffraction efficiency of a 10 μm period grating was 1.9% without heating, whereas it increased to 72% when heated at 70 °C for 500 h. This is because the induced refractive index changes (Δn) were increased by heating. From the Lorentz–Lorenz equation, one of the reasons why Δn increases by heating could be a slight volume contraction in the irradiated area.