Single-monomer acrylate-based resin for three-dimensional photonic crystal fabrication

A single-monomer acrylate resin (SR348, Sartomer) with an aromatic ketone photoinitiator (Irgacure 369, Ciba) has been developed to produce photonic crystal structures by two-photon photopolymerisation. Compared with more complex acrylate-based resins, this one-component resin is easily prepared. The material is homogeneous before and after photocuring and has a high photosensitivity. SR348 (ethoxylated (2) bisphenol A dimethacrylate) is a difunctional monomer, which generates an insoluble crosslinked network by free radical polymerisation. The polymerised material is thermally stable up to 300 °C. Its refractive index jumps from 1.54 to 1.59 upon polymerisation. Scanning electron microscopy images of photonic crystal structures reveal a minimum structural element size of approximately 300 nm. The photonic crystals were fabricated in a 40 layer woodpile structure at a layer-spacing of 500 nm and an in-plane rod spacing of 1.5 μm. Fundamental stop gaps were observed in the stacking direction at wavelengths of 1.9–2.3 μm dependent on the rod size. Suppression of infrared light transmission of as much as 40% in the stop gap region and gap/midgap ratios of up to 0.15 demonstrate the high degree of correlation of structural elements in three dimensions.