Photocontrollable Photonic Crystals Based on Porous Silicon Filled with Photochromic Liquid Crystalline Mixture

摘要

Novel photocontrollable photonic crystals based on porous silicon prepared by electrochemical etching are elaborated and their great potential for creation of photoswitchable materials is demonstrated. For this purpose, silicon-based photonic porous wafers are filled with photochromic liquid crystalline (LC) nematic mixture formulated by 4-pentyl-4 '-cyanobiphenyl and a specific azobenzene derivative. Results of electron paramagnetic resonance spectroscopy reveal that molecules of LC mixture are aligned along the etched channels in silicon plate, i.e., in normal direction. UV irradiation (375 nm) of the resulting composite causes a shift of photonic band gap to longer wavelengths by approximate to 10 nm. This phenomenon is explained by isothermal photoinduced nematic-isotropic phase transition associated with the bent-shapedZisomer formation of the azobenzene compound. During this transition, effective refractive index slightly increases as the initial homeotropic alignment of LC molecules transforms to the isotropic nonaligned state. This process is completely reversible under visible light action (428 nm) and can be repeated many times. Obtained photonic crystal photochromic LC composites can be considered as new promising material for photonic applications.