Abstract
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.
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