a composition for producing optical elements having a gradient structure , particularly for holographic applications , is formed by a refractive index gradient . the composition is produced from one or more polymerizable and / or polycondensable monomers and at least one biological polymer . a potential difference is generated for the directed diffusion of the monomers by inducing a local polymerization or polycondensation . the result is the formation of a refractive index gradient .