Modulation of Optical Properties in Liquid Crystalline Networks across Different Length Scales

Photopolymerization of customized materials became a well-established technique for micro- and nano-fabrication of photonic structures, and their optical properties as the refractive index should be precisely tailored to design specific photonic features. For this purpose, the refractive index determination in macroscopic samples is not exhaustive, and an in situ characterization is thus necessary at both the macro- and microscale to point out how different polymerization processes differently modulate the optical properties. In particular, we focus our attention on liquid crystalline networks (LCNs) that have been studied as birefringent materials whose tunable response is of interest for applications in different fields such as in robotics, biomedicine, and photonics. By tuning the molecular composition of LCN mixtures, e.g., modifying the cross-linker and dye amount inside the polymer network, the refractive index and the optical anisotropy of microscopic and macroscopic samples have been engineered and measured by a refractometer method under temperature variation or light actuation stimuli. Monitoring the refractive index at different length scales showed that two-photon polymerization increases the birefringence in microscopic structures, and the maximum variation of the optical anisotropy is achieved by a remote laser light stimulus.