Determination of the refractive index modulation of PQ:PMMA holographic phase gratings over a large spectral range through a two-level approximation of the electric susceptibility

Phenanthrenequinone doped poly(methyl methacrylate) is a well-known holographic polymer used in many applications. It is important to consider the refractive index modulation (Δ n ) when designing a phase grating, as it heavily influences the diffraction efficiency. However, due to the behavior of the electric susceptibility in this material, the Δ n will be different at varying reconstructed wavelengths. Here, we report on the observation of the difference in this modulation for various wavelengths. We develop a model for a two-level approximation of the electric susceptibility, based on the absorption spectrum of the material, to estimate the read wavelength dependence of the modulation for a given sample, and find our results to be in good agreement with this model.