Materials for the fabrication of optical waveguides with two photon photopolymerization

Two photon photopolymerization (2PP) is an innovative method for 3D-structuring structural and functional materials. 2PP allows the fabrication of sub-micron structures from a photopolymerizable resin. By the use of near-infrared (NIR) lasers it is possible to produce 3D structures with a spatial feature resolution as good as 200 nm. This technique can be used in polymer-based photonic and micro-electromechanical systems (MEMS), for 3D optical data storage or for the inscription of optical waveguides into materials based on a local refractive index change upon laser exposure. Since the 2PP only takes place inside the focus of the laser beam, complex 3D-structures can be inscribed into a suitable matrix material. In the presented work, 2PP is used to write optical waveguides into a prefabricated mechanically flexible polydimethylsiloxane matrix. The waveguides were structured by selectively irradiating a polymer network, which was swollen by a monomer mixture. The monomer was polymerized by two photon photopolymerization and the uncured monomer was removed by evaporation at elevated temperatures. This treatment led to a local change in refractive index in the order of Δn = 0.02, which was significantly above the industrial requirement of Δn = 0.003. The measured optical losses were around 2.3dB/cm. Since all unreacted monomers were removed by evaporation, the final waveguide was stable up to temperatures of more than 200°C. Since commercially available one-photon-photoinitiators are only of limited use for 2PP, a new class of photoinitiators (PIs) has been developed. These PIs exhibit a large 2PP-crossection and therefore contribute to a process with higher throughput. The PIs were evaluated using UV-Vis and fluorescence measurements. The TPA-cross-section was determined by the use an open aperture Z-scan technique. In 2PP structuring tests the ideal building parameters for each PI were determined, indicating that the presented PIs can be used at very low concentrations (0.05 wt%).