Researchers from Institute of Physics in collaboration with Lomonosov Moscow State University elaborated novel photocontrollable photonic crystals based on porous silicon filled with photochromic liquid crystalline mixture. In their recent joint paper , whose figure highlighted the back cover of the November issue of Advanced Optical Materials, they demonstrated that these photonic crystals have great potential for creation of photoswitchable materials for photonics applications.
The photonic crystals based on porous silicon were prepared by electrochemical etching and filled with photochromic liquid crystalline (LC) nematic mixture formulated by 4-pentyl-4′-cyanobiphenyl and a specific azobenzene derivative. These molecules of LC mixture are aligned along the etched channels in silicon plate (see Figure 1). After an UV irradiation (375 nm), a shift of photonic band gap to longer wavelengths by ≈10 nm was observed. This phenomenon was explained by isothermal photoinduced nematic-isotropic phase transition associated with the bent-shaped Z isomer 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 (see Figure 1). Interestingly, this process between aligned and isotropic states was shown to be completely reversible under visible light action (428 nm) and can be repeated many times. Obtained photonic crystal with photochromic LC composites can be considered as new promising material for photonic applications.
 A. Bobrovsky, S. Svyakhovskiy, A. Bogdanov, V. Shibaev, M. Cigl, V. Hamplová, and A. Bubnov, Photocontrollable Photonic Crystals Based on Porous Silicon Filled with Photochromic Liquid Crystalline Mixture, Adv. Optical Mater. 8, 2001267 (2020).