Functional polymers by two-photon 3D lithography

In the presented work, two-photon 3D lithography and selective single-photon photopolymerization in a prefabricated polydimethylsiloxane matrix is presented as an approach with potential applicability of waveguide writing in 3D by two-photon polymerization. Photopolymers based on acrylate chemistry were used in order to evaluate the optical capabilities of the available two-photon system. Several photoinitiators, tailored for two-photon absorption, were tested in a mixture of trimethylolpropane triacrylate and ethoxylated trimethylolpropane triacrylate. Best results were obtained with a recently synthesized diynone-based photoinitiator. Feature resolutions in the range of 300 nm were achieved. Due to the cross-conjugated nature of that donor-π-acceptor-π-donor system a high two-photon absorption activity was achieved. Therefore, a resin mixture containing only 0.025 wt% of photoinitiator was practical for structuring by two-photon polymerization. The required initiator content was therefore a factor of 100 lower than in traditional one-photon lithography. The aim of the second part of this work was to fabricate optical waveguides by selectively irradiating a polymer network, which was swollen by a monomer. The monomer was polymerized by conventional single-photon polymerization and the uncured monomer was removed by evaporation at elevated temperatures. This treatment leads to a local change in refractive index. Refractive index changes in the range of Δ n = 0.01 (Δ n/ n = 0.7%) were achieved, which is sufficient for structuring waveguides for optoelectronic applications.