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 (?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 ?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.