In photopolymerizable materials, grating formation mechanism depends on the material composition and the permeability of the matrix which influence the diffusion of mobile species and the resultant refractive index modulation. In volume gratings, the refractive index modulation (Δn) is created by the impact of three mechanisms such as change in density, molar refraction and monomer concentration gradient driven diffusion as described in details further. Firstly, in bright fringe areas (illuminated areas) the density increases due to polymerization, and it has the minimum value in dark fringe areas (not illuminated areas). This leads to different refractive indexes in bright and dark areas, ndensity bright and ndensity dark , respectively, according to the distribution of the light intensity (Figure ). Secondly, the different degree of polymerization in bright and dark areas causes variation in polarizability due different degree of monomers' C=C bonds conversion which is shifted with respect to the modulation due to density variation by 180°. This provides variation in molar refraction and, as a result, in the refractive index too, such as Δnrefraction = nrefraction bright -nrefraction dark (Figure ). Thirdly, the concentration gradient that occurs during recording leads to the diffusion of monomers. If the diffusion is high then most of the monomers manage to reach bright areas, and the density variation is the main contributor to the refractive index modulation. In case of low diffusion, only a small fraction of monomer molecules reaches the bright area, and structural changes in the monomer molecules play a major role in the formation of refractive index modulation.