Influence of interdiffusion on multilayered gradient refractive index (GRIN) lens materials

The interdiffusion of two materials used to fabricate polymeric gradient refractive index (GRIN) lenses was examined by varying contact time during multilayer films co-extrusion composed of alternating poly(methylmethacrylate) (PMMA) and poly(styrene-co-acrylonitrile) with 17 mol% acrylonitrile (SAN17). The model applied successfully described their interdiffusion, and a reasonable mutual diffusion coefficient of 7.0 × 10−13 m2/s was determined. Atomic force microscopy confirmed good agreement between modeled profiles and actual layer structure, and optical properties were investigated. Films with contact times of ≤160 s exhibited multiple refractive indices, while films with longer contact times, showed single refractive indices that followed an additive line. Though a single additive value requires layers ≤ a quarter-wavelength of light, these films exhibit such behavior with thicknesses 5× greater than expected, though layer resolution is still present. Using the model profiles, it was determined that only a 1% reduction in material purity is required to reduce the effective layer thickness.