Photonic crystals based on smart polymers. A new route for tunable devices.

The results of this Ph.D. thesis demonstrate the tunability of photonic platforms by introducing stimuli responsive polymers as constituents of the photonic structure itself or as thermally driven mechanical actuators. In particular, liquid crystalline networks (LCNs) were patterned by lithographic techniques, such as direct laser writing (DLW) and UV polymerization to develop and fabricate tunable photonic crystals for different applications, from tunable telecom filters to tunable structural colors and intelligent sensors, featuring good optical properties that can be controlled and modulated by multiple tuning mechanisms (e.g. temperature and light). In order to optimize the structure design and the tunability of LCN photonic devices, the refractive index and the tunable optical anisotropy (determined by the chemical composition of the material, the fabrication parameters, and the molecular ordering) have been precisely characterized. As first, it has been demonstrated, using a refractometer method, that optical properties of these new photonic materials can be tuned by adjusting mesogenic concentration both in LCN macro- and micro-structures. The tailored chemical formulation allows not only the determination of the shape changing properties of LCNs but also the modulation of the refractive indices and the optical anisotropy of liquid crystalline mixtures, which can be tuned at different temperatures or alternatively by laser light irradiation. Aiming to increase the fabricated structure resolution, a second result demonstrated how refined fabrication resolutions, never yet reached for liquid crystalline networks, can be achieved at low polymerization temperatures (5°C-10°C) using opportune writing parameters. The resulting 3D polymerizable unit, now comparable with the typical voxel of commercial resists, enlarges the application field of photo-responsive elastic materials without degradation of the patterned structure rigidity. Indeed, a spheroidal voxel would be the best polymerization unit to fabricate three-dimensional structures, especially in 3D photonic structures as woodpile photonic crystals for which isotropic voxel dimensions are needed. The best fabrication parameters using the DLW lithographic technique at controlled temperature enabled the fabrication of the first 3D woodpile photonic crystal made by LCN, having a geometric resolution and a light transmission attenuation at the stop band comparable with photonic crystal fabricated with commercial resists. This demonstrates the effectiveness of our previous study. Such photonic crystal has been characterized using temperature as an external stimulus to tune its optical properties in order to demonstrate its potential as a tunable filter at telecom wavelength. Finally, the first proof-of-concept of a smart millimetric optical sensor was developed during a six-month period in collaboration with Prof. Li and Prof. Keller group in Paris, at ChimieParis Tech. A temperature responsive actuator has been combined with the back side of the Morpho Menelaus wing, owing an optimized structural coloration due to its natural photonic crystal structure. Two different strategies have been proposed to control the visual sensor: a macroscopic deformation of the combined system induces an iridescence variation, whereas a nanoscale contraction generates a color shift through the lamellae interspacing variation, parameter that determines the structural coloration. In conclusion, this thesis focused on the material characterization of smart polymers and their nanopatterning for tunable photonic shows as the employ of smart LCNs can be extended from mechanical actuators and microrobotics to micrometric photonic structures for new multifunctional devices.