Analysis of holographic photopolymers for integrated optical systems via quantitative phase microscopy

Optically-driven diffusion of high refractive index molecules within a transparent thermoset polymer matrix is a promising platform for hybrid optics that combines a wide range of optical structures from large scale holograms to micron-scale gradient index waveguides in a single integrated optical system. Design of such a system requires characterization of the optical response of the material at a wide range of spatial scales and intensities. While holographic analysis of the photopolymers is appropriate to probe the smaller spatial scales and lower intensity optical response, quantitative phase mapping of isolated structures is needed to probe the response to the higher intensities and lower spatial frequencies used in direct write lithography of waveguides. We apply the transport of intensity equation (TIE) to demonstrate quantitative refractive index measurements of 10 μm-scale localized gradient index structures written into diffusive photopolymer materials using both single- and two-photon polymerization. These quantitative measurements allow us to study the effect of different exposure conditions and material parameters such as writing beam power, exposure time, and wt% loading of the writing monomer on the overall profile of the refractive index structure. We use these measurements to probe the time scales over which diffusion is significant, and take advantage of the diffusion of monomer with a multiple-write scheme that achieves a peak refractive index contrast of 0.025.